API stands for American Petroleum Institute, an organisation that develops technical standards and recommended practices for the oil and natural gas industry. It also gives its name to API gravity, a measurement used to describe how light or heavy petroleum liquids are compared with water. Higher API gravity indicates lighter oil, while lower values indicate heavier oil.
The oil and gas industry uses numerous technical abbreviations to describe petroleum properties, engineering standards and operational requirements. Among these, API is one of the most widely recognised. You may encounter it when reading about crude oil quality, refinery operations, pipeline construction, storage tanks or industrial lubricants. Although these applications appear quite different, they share a connection to the American Petroleum Institute.
Understanding what API stands for in oil and gas is particularly important because the abbreviation has two closely related uses. It identifies an influential industry organisation responsible for developing technical standards, and it appears in the term API gravity, a measurement used to describe the relative density of petroleum liquids.
For example, an engineer might refer to an API standard when inspecting a storage tank, while a refinery operator might discuss the API gravity of an incoming crude oil shipment. These are not interchangeable meanings, even though both originate from the same organisation.
This guide explains the full meaning of API, the organisation’s responsibilities, how API gravity works and why these standards and measurements matter throughout petroleum production, processing, transportation and storage.
What Is the American Petroleum Institute?
The American Petroleum Institute is a US-based trade association representing businesses involved in the oil and natural gas industry. Established in 1919, it brings together expertise from petroleum exploration, production, processing, transportation, refining and related services.
One of its most important functions is the development of technical standards and recommended practices. These documents provide requirements or guidance for particular equipment, materials, inspection procedures and operational activities.
Although the organisation is based in the United States, many API standards are recognised and used internationally. Their application can help businesses establish consistent engineering practices, assess equipment suitability and communicate technical expectations across different operations.
The Institute’s main activities include:
- Developing technical standards: Creating specifications and recommended practices for equipment, materials and operating systems.
- Supporting industry safety: Providing guidance addressing risks associated with petroleum operations.
- Certification programmes: Administering programmes that assess conformity with specified requirements.
- Research and technical information: Publishing information on industry developments and operational practices.
- Industry representation: Presenting the interests and positions of its members on public policy matters.
- Professional development: Supporting technical knowledge and competency through relevant programmes.
API standards are not automatically laws. Their legal significance depends on whether they are incorporated into regulations, contracts, permits or other enforceable requirements.
Why Is API Important in the Oil and Gas Industry?
Oil and gas operations involve complex machinery, pressurised systems, hazardous substances and large volumes of valuable materials. Equipment failures can lead to injuries, environmental damage, operational downtime and substantial financial losses.
Technical standards help organisations establish consistent expectations for the design, manufacture, inspection and maintenance of equipment. They also support communication between manufacturers, engineering contractors, operators and inspectors.
Consider an industrial storage tank. Its safe operation depends on appropriate design, compatible materials, correct construction and suitable inspection procedures. An applicable API document may help engineers evaluate these requirements, although local legislation and other relevant standards must also be considered.
API is important because it provides a shared technical framework across different parts of the petroleum industry.
Common Areas Where API Standards Are Used
Industry area | Typical application | Why it matters |
Oil exploration and production | Well equipment and associated systems | Supports equipment specifications and operational consistency |
Petroleum storage | Tank design, inspection and maintenance | Helps address structural integrity and operational risks |
Refining | Pressure equipment and processing systems | Supports inspection and equipment management |
Pipelines | Equipment and pipeline-related practices | Contributes to reliable transportation infrastructure |
Fuel and lubricant products | Product classifications and performance requirements | Helps communicate technical quality expectations |
Equipment manufacturing | Product specifications and certification | Supports consistent manufacturing requirements |
Not every petroleum operation uses the same API documents. The relevant requirements depend on the equipment, product, application and jurisdiction.
What Does API Gravity Mean in Oil and Gas?
API gravity is a measurement used to describe the relative density of petroleum liquids. It is expressed in degrees API and helps distinguish lighter oils from heavier ones.
Unlike ordinary density, where a larger value indicates more mass within a particular volume, API gravity uses an inverse scale. A higher API gravity value indicates a petroleum liquid with a lower density relative to water.
For example, crude oil with an API gravity of 40° is lighter than crude oil with an API gravity of 25°.
This measurement is important because crude oil varies considerably between producing fields. Some grades are relatively light and flow easily, while others contain heavier hydrocarbon components and may require more complex processing.
API gravity provides a convenient way to compare these characteristics, although it does not describe every aspect of crude oil quality.
Understanding High and Low API Gravity
- Higher API gravity: Indicates a lighter petroleum liquid with a lower relative density.
- Lower API gravity: Indicates a heavier petroleum liquid with a higher relative density.
- 10° API: Corresponds to a liquid with the same specific gravity as water at the reference conditions.
- Above 10° API: Indicates a liquid less dense than water at those conditions.
- Below 10° API: Indicates a liquid denser than water at those conditions.
API gravity is a density-related measurement, not a direct measure of viscosity, sulphur content, combustion performance or overall fuel quality.
How Is API Gravity Calculated?
API gravity is calculated using the specific gravity of the petroleum liquid at a standard reference temperature.
Specific gravity compares the density of a substance with the density of water under specified conditions. In conventional petroleum calculations, the reference temperature is 60°F, approximately 15.6°C.
The standard formula is:
- API Gravity = (141.5 ÷ Specific Gravity at 60°F/60°F) − 131.5
The calculation converts the relative density into degrees API, making it easier to compare different petroleum liquids.
Example of an API Gravity Calculation
Imagine a crude oil sample has a specific gravity of 0.85 at the standard reference conditions.
The calculation would be:
- API Gravity = (141.5 ÷ 0.85) − 131.5
- API Gravity = 34.97°
The resulting value is approximately 35° API, indicating a relatively light crude oil.
Now consider a heavier petroleum liquid with a specific gravity of 0.95.
- API Gravity = (141.5 ÷ 0.95) − 131.5
- API Gravity = 17.45°
The second sample has a much lower API gravity and is therefore denser than the first.
API Gravity Calculation Examples
Specific gravity | Approximate API gravity | Relative description |
0.75 | 57.2° | Very light petroleum liquid |
0.80 | 45.4° | Light petroleum liquid |
0.85 | 35.0° | Relatively light crude |
0.90 | 25.7° | Intermediate-density crude |
0.95 | 17.4° | Heavy crude |
1.00 | 10.0° | Equal in density to reference water |
These figures illustrate the mathematical relationship rather than establishing universal commercial classifications. Actual crude oil categorisation may also depend on the reporting organisation, market and producing region.
What Are the Different API Gravity Ranges?
Crude oil is often described as light, medium or heavy according to its density. API gravity provides one way of making this distinction.
However, there is no single classification range used identically by every organisation. Different statistical agencies and petroleum markets may apply slightly different thresholds.
For general reference, the US Energy Information Administration uses a classification in which light crude commonly exceeds 38° API, intermediate crude lies between approximately 22° and 38°, and heavy crude is around 22° API or below.
Light Crude Oil
Light crude oil has a relatively low density and often contains a greater proportion of lighter hydrocarbon components. These characteristics can make it attractive to refineries seeking certain petroleum fractions.
However, light crude does not automatically produce better results in every refinery. The value and suitability of a crude grade depend on its complete composition and the processing equipment available.
Medium Crude Oil
Medium crude occupies an intermediate density range between light and heavy grades. It can contain a broad mixture of lighter and heavier hydrocarbons.
Many refineries process medium crude as part of their regular feedstock, sometimes blending it with other grades to achieve suitable processing characteristics.
Heavy Crude Oil
Heavy crude oil has a higher density and often contains substantial quantities of larger hydrocarbon molecules. Some grades are also relatively viscous, which may introduce additional transportation and processing requirements.
Refining heavy crude can require specialised conversion equipment to produce valuable lighter products. However, density alone does not reveal its sulphur content or the complete complexity of the feedstock.
Why Does API Gravity Matter to Oil Refineries?
Refineries receive crude oil from different producing regions, and the composition of these supplies can vary considerably. API gravity is one of the first measurements used to describe an incoming crude stream.
It helps refinery personnel understand the relative density of a feedstock and contributes to decisions about blending, processing and commercial value.
However, engineers also need detailed information about sulphur, acidity, metals, viscosity and the distribution of hydrocarbon components.
Crude Oil Selection
Refineries are designed to process particular ranges of crude oil properties. A facility with limited conversion equipment may prefer certain lighter feedstocks, while a more complex refinery may be capable of processing heavier grades.
API gravity provides useful initial information, but it does not determine refinery suitability on its own.
Distillation Performance
Crude oil contains hydrocarbons with different boiling points. During refining, these components are separated into fractions that may undergo further treatment or conversion.
Although API gravity does not directly measure boiling-point distribution, it can offer a broad indication of feedstock density. Detailed laboratory analysis is still required to predict the quantities and qualities of refinery products.
For a deeper explanation of these processing stages, understanding what is refining in oil and gas can help clarify how crude oil becomes usable petroleum products.
Blending Decisions
Refineries and petroleum terminals may blend different streams to achieve specified properties.
API gravity can help describe the density of the final mixture, but blending calculations must account for actual volumes, temperatures and other physical characteristics.
It should not be assumed that API gravity values can simply be averaged without considering the proper calculation method.
Commercial Valuation
Crude oil prices are influenced partly by quality characteristics, including density. However, higher API gravity does not automatically mean a crude grade will sell for more.
Market demand, sulphur content, transportation costs, refinery capacity and product yields all influence commercial value.
API Gravity vs Specific Gravity vs Viscosity
API gravity, specific gravity and viscosity are frequently discussed together because they describe the physical characteristics of petroleum liquids. However, they measure different properties.
Confusing these terms can lead to misunderstandings when comparing crude oils, lubricants or finished fuels.
Measurement | What it describes | Common relevance |
API gravity | Relative density expressed on a petroleum-specific scale | Crude oil and condensate classification |
Specific gravity | Density relative to a reference substance, usually water for liquids | Liquid comparison and density calculations |
Density | Mass per unit volume | Product specification, storage and quantity calculations |
Viscosity | Resistance of a fluid to flow | Pumping, lubrication and handling |
Flash point | Lowest test temperature at which sufficient vapour forms to ignite under specified conditions | Fuel handling and safety |
Sulphur content | Quantity of sulphur present | Fuel specification, emissions and processing |
API Gravity Is Not the Same as Viscosity
A lighter petroleum liquid may often have lower viscosity than a heavier one, but this relationship is not universal.
Two oils with similar API gravity can have different viscosity values because their chemical compositions differ. Temperature also has a substantial effect on viscosity.
This distinction is important for industrial lubricants, where the correct viscosity grade and equipment specifications are essential. API gravity alone cannot determine whether a lubricant is suitable for a machine.
API Gravity Is Not a Fuel Quality Rating
A high API gravity does not automatically mean that petroleum is cleaner, safer or more efficient.
For example, crude oil can be relatively light but still contain significant amounts of sulphur or other unwanted substances. Similarly, a finished fuel must meet several technical requirements beyond density.
API gravity should therefore be treated as one measurement within a broader quality assessment.
What Are API Standards in Oil and Gas?
API standards are technical documents developed to support particular engineering and operational activities.
They may include specifications, recommended practices, inspection codes and other guidance addressing equipment, materials, processes and performance requirements.
The distinction between different document types is important. A specification may define requirements for a product, while a recommended practice provides guidance for an activity or system.
Common API Standards Used in Petroleum Operations
API document | Main subject | Typical application |
API 650 | Welded tanks for oil storage | Design and construction of applicable storage tanks |
API 653 | Tank inspection, repair, alteration and reconstruction | Integrity management of applicable storage tanks |
API 510 | Pressure vessel inspection | Inspection and management of in-service pressure vessels |
API 570 | Piping inspection | Inspection and maintenance of applicable in-service piping |
API 1104 | Welding of pipelines and related facilities | Applicable pipeline welding requirements |
API 610 | Centrifugal pumps for petroleum and related industries | Pump equipment specification |
API 682 | Shaft sealing systems for centrifugal and rotary pumps | Mechanical sealing arrangements |
These descriptions are simplified. Each standard has a defined scope, applicable edition and technical conditions that must be checked before use.
Using a particular API standard does not automatically establish compliance with every relevant UK legal requirement.
How Are API Standards Used in the UK?
Although API originated in the United States, its standards are frequently recognised internationally, including within petroleum engineering and industrial equipment supply chains serving the UK.
A UK business may encounter API requirements when purchasing storage tanks, inspecting pressure equipment, commissioning pumps or working with international engineering contractors.
However, API standards do not replace British legislation or applicable regulatory obligations.
UK operators must consider requirements such as relevant health and safety legislation, environmental permits, equipment regulations and recognised technical guidance.
Other standards developed by organisations such as the British Standards Institution, ISO and European standards bodies may also apply.
API Standards and UK Compliance
An organisation may use an API document as part of its engineering approach when the document is appropriate for the equipment and operating circumstances.
The relevant specification should be selected by competent personnel who understand the design conditions, statutory requirements and intended application.
For example, an industrial tank inspection may involve recognised technical standards alongside site-specific risk assessments and applicable UK safety obligations.
This is particularly relevant to tank inspection, where identifying the condition of storage equipment can help businesses make informed maintenance decisions.
What Does API Mean in Motor Oil and Lubricants?
The abbreviation API also appears on engine oil packaging, where it identifies performance classifications associated with the American Petroleum Institute’s engine oil programmes.
These classifications are different from API gravity.
For example, an API service category may indicate that an engine oil meets a particular performance specification for a type or generation of engine.
Engine oil categories can change as engine technology, emissions systems and performance requirements develop. Therefore, the correct specification should always be checked against the vehicle or equipment manufacturer’s recommendations.
API Oil Classification vs API Gravity
Feature | API oil classification | API gravity |
Main purpose | Communicates engine oil performance requirements | Describes relative density |
Typical application | Engine lubricants | Crude oil, condensate and petroleum liquids |
Measurement or designation | Service category or performance marking | Numerical value in degrees API |
Main consideration | Engine compatibility and performance | Liquid density relative to water |
Does it determine suitability alone? | Manufacturer requirements must also be checked | No, other product properties remain important |
A lubricant’s API performance category cannot be used to determine its API gravity. Likewise, knowing its API gravity does not establish whether it provides the required engine protection.
What Is the Relationship Between API Gravity and Condensate?
Gas condensate is a mixture of relatively light hydrocarbons recovered during natural gas production and processing.
Many condensate streams have high API gravity because they are less dense than typical crude oils. However, composition and physical properties vary between producing fields.
API gravity helps characterise the recovered liquid and may be considered during transportation, storage and processing.
It does not indicate that the condensate is ready for direct use as petrol, diesel or heating oil.
Understanding what is condensate in oil and gas provides additional context about how these liquids form, why they are separated and how they enter the petroleum supply chain.
How Is API Gravity Measured in Practice?
In industrial settings, API gravity is determined from density or specific gravity measurements made using suitable instruments and standardised procedures.
The exact method depends on the sample, equipment and testing standard.
Sample Collection
A representative petroleum sample is collected using an appropriate method. Sampling is important because contamination, temperature differences and product separation can affect the result.
Industrial sampling procedures must account for the characteristics of the fluid, including its volatility and potential hazards.
Density Measurement
A laboratory may use a suitable hydrometer or digital density meter to determine the density or relative density of the sample.
The selected method must be appropriate for the petroleum liquid being tested.
Temperature Correction
Petroleum density changes with temperature. For meaningful comparisons, measurements must be expressed at the required reference conditions.
The API gravity scale conventionally uses a reference temperature of 60°F.
Calculation and Reporting
The corrected specific gravity is converted into degrees API using the standard relationship.
The reported result may form part of a wider petroleum quality analysis containing information about density, sulphur, viscosity, water and other properties.
For stored fuels, fuel testing can help identify quality concerns such as contamination or unsuitable physical characteristics. The exact parameters tested depend on the product and the purpose of the analysis.
Why API Gravity Matters During Petroleum Transportation
Crude oil and petroleum liquids move through an extensive network of pipelines, terminals, tankers and storage facilities.
Their physical properties influence how they are measured, handled and transported.
API gravity is useful for describing relative density, but operators must also consider viscosity, temperature, vapour pressure and other safety-related properties.
Pipeline Transportation
The density and viscosity of a petroleum stream can influence pumping requirements and operating conditions. However, pipeline design depends on a complete engineering assessment rather than API gravity alone.
Storage Facilities
Storage operators need reliable information about the liquids being handled.
Density contributes to quantity calculations and equipment considerations, while other properties determine requirements for vapour control, fire protection and material compatibility.
Marine Transportation
Petroleum shipments are traded and transported internationally, requiring consistent product measurements and specifications.
API gravity may be included in cargo documentation, but it forms only part of the information needed to evaluate a shipment.
Product Quality Control
Changes in measured density can sometimes suggest differences between batches or possible mixing of products.
However, density alone cannot identify the cause of contamination. Additional laboratory tests may be required.
The importance of properly controlled storage and measurement also extends to downstream fuel management, where product quality, stock monitoring and suitable handling practices support reliable operations.
Common Misunderstandings About API in Oil and Gas
Because API appears in several technical contexts, the abbreviation can be misunderstood.
Recognising the differences helps make industry information easier to interpret.
API Does Not Always Mean API Gravity
When an engineer refers to an API standard, they are discussing a technical document associated with the American Petroleum Institute.
When a petroleum analyst mentions 35° API, they are describing a liquid’s relative density.
The context determines which meaning applies.
Higher API Gravity Does Not Always Mean Better Oil
Lighter crude oil can be desirable for particular refining processes, but commercial quality depends on much more than density.
Sulphur content, acidity, contaminants, processing requirements and market demand all influence its value.
API Standards Are Not Automatically Legal Requirements
API standards may become contractually or legally relevant when incorporated into applicable requirements. However, their existence does not automatically make them mandatory for every petroleum business.
Operators must establish which documents and regulations apply to their particular activities.
API Gravity Does Not Measure Oil Thickness Directly
Viscosity describes resistance to flow, while API gravity describes relative density.
Although the two properties can be related, one cannot reliably replace the other.
API Classification Is Not the Same as Oil Viscosity Grade
Engine oil packaging may show an API service category alongside an SAE viscosity grade.
These markings describe different aspects of the lubricant. Both should be considered according to the equipment manufacturer’s specifications.
How API Relates to the Wider Petroleum Supply Chain
API standards and measurements are relevant throughout the oil and gas industry, from the production of raw hydrocarbons to the handling of finished petroleum products.
The specific application changes according to the stage of the supply chain.
Upstream Operations
Upstream activities involve exploration, drilling and hydrocarbon production.
API standards may apply to selected drilling equipment and well-related systems, while API gravity can help characterise produced crude oil and condensate.
Midstream Operations
Midstream activities involve the gathering, processing, storage and transportation of petroleum and natural gas.
Pipeline equipment, storage facilities and measurement systems may rely on applicable technical standards. Product density is also relevant to transportation and quantity calculations.
Downstream Operations
Downstream activities include refining, fuel distribution and the supply of finished petroleum products.
Refineries use crude oil property data to assess incoming feedstocks, while distributors rely on suitable product specifications and quality-control practices.
For UK customers requiring commercial fuels, heating oil or associated services, 123 Oil operates within this downstream supply environment. The role of product specifications and responsible handling remains important, although an API standard or gravity measurement should never be confused with a guarantee of overall product quality.
How Do API Standards Support Safer Fuel Storage?
Fuel storage presents risks associated with leakage, equipment deterioration, fire, contamination and environmental damage.
Suitable technical standards can support engineering decisions, but the correct documents must be chosen for the specific tank, material and operating conditions.
A comprehensive storage management approach may involve:
- Appropriate tank design: Selecting equipment suitable for the stored product and installation conditions.
- Inspection and maintenance: Identifying deterioration before it develops into a significant problem.
- Leak prevention: Maintaining compatible fittings, pipework and containment systems.
- Product monitoring: Checking for unwanted water, sediment and other contamination.
- Emergency preparedness: Establishing suitable procedures for spills and equipment failures.
- Competent personnel: Ensuring inspections and maintenance are performed by appropriately qualified individuals.
The effectiveness of these measures depends on their implementation and suitability for the installation.
Why Technical Standards and Fuel Testing Serve Different Purposes
Technical standards may describe how equipment should be designed, inspected or maintained. Fuel testing examines the properties or condition of the material stored within that equipment.
Both can contribute to reliable operations, but they are not interchangeable.
For example, a storage tank could be structurally sound while containing contaminated diesel. Conversely, fuel could meet its specification while being stored in equipment that requires repair.
Understanding these separate responsibilities is essential for sound petroleum storage management.
Conclusion
API stands for the American Petroleum Institute, an organisation recognised for developing technical standards and recommended practices across the oil and natural gas industry. Its name is also associated with API gravity, a measurement used to compare the relative density of petroleum liquids. Understanding both meanings is important because they serve different purposes. API standards support engineering and operational consistency, while API gravity helps describe crude oil and condensate properties. Together, they contribute to the technical language used throughout petroleum production, refining, transportation and storage.
Frequently Asked Questions
API stands for American Petroleum Institute, a US-based industry association that develops technical standards and recommended practices for petroleum and natural gas operations. The abbreviation is also used in API gravity, a measurement describing the relative density of petroleum liquids.
A value of 40° API indicates a relatively light crude oil. Because the API gravity scale is inversely related to density, a crude oil with 40° API is less dense than one with 25° API. However, the measurement does not establish sulphur content, viscosity or overall refining quality.
Not necessarily. Higher API gravity indicates lighter oil, which may be desirable for certain refining processes. However, crude oil value also depends on sulphur content, chemical composition, processing requirements and market demand. A higher API gravity does not guarantee better commercial quality.
API stands for the American Petroleum Institute, the organisation responsible for developing numerous petroleum industry standards. API gravity is a density-related measurement named after the Institute. One refers to an organisation, while the other describes a physical property of petroleum liquids.
API 650 is a standard covering the design and construction of certain welded tanks for oil storage. It establishes technical requirements within its specified scope. Its applicability depends on tank design, operating conditions, the relevant edition and any additional legal or contractual requirements.
API gravity is principally used for liquid petroleum products such as crude oil and condensate. Natural gas is normally characterised using other measurements, including gas composition, density, specific gravity and heating value. API gravity should not be treated as the standard method for classifying gaseous natural gas.
API standards are not automatically mandatory throughout the UK. They may become relevant through legislation, contracts, engineering specifications or recognised industry practice. Businesses must assess applicable UK legal requirements and select suitable standards for their equipment and activities.