API Gravity Calculator

About the API Gravity Calculator

API gravity is the petroleum industry's standard measure of how heavy or light a crude oil is compared to water. Developed by the American Petroleum Institute, the API gravity scale is inversely related to specific gravity: lighter oils have higher API values and heavier oils have lower values. Water has an API gravity of exactly 10°.

This calculator converts between API gravity and specific gravity using the standard formula: API = 141.5/SG − 131.5. It also classifies oils into industry-standard categories (light, medium, heavy, extra heavy), computes density in multiple units, and determines whether the oil floats or sinks in water. The volume/mass calculation helps engineers estimate the weight of oil shipments.

Preset buttons for common crude benchmarks (WTI, Brent, Heavy) and a comprehensive reference table of major crude oils and refined products provide context for the API gravity values used throughout the petroleum and refining industries. That context is especially useful when comparing blend specs or estimating how a cargo will behave during storage and transport.

When This Page Helps

API gravity conversions are used daily in the petroleum industry but the inverse formula can be confusing. This calculator eliminates errors by handling the conversion both ways, providing instant density values in multiple units, and classifying the oil automatically.

The reference table of common crude oils provides valuable context for interpreting API gravity values, whether you are trading crude oil futures, designing refinery processes, or studying petroleum engineering.

How to Use the Inputs

1. Select whether to solve for API gravity or specific gravity.
2. Enter the known value — specific gravity or API gravity.
3. Use preset buttons for common crude oil benchmarks.
4. Optionally enter a volume to calculate the corresponding mass.
5. Read the API gravity, density, classification, and buoyancy from the outputs.
6. Compare with common crude oils in the reference table.

Example Calculation

Tips & Best Practices

• API > 31.1° = Light crude (most valuable, easiest to refine).
• API 22.3-31.1° = Medium crude.
• API 10-22.3° = Heavy crude (needs more complex refining).
• API < 10° = Extra heavy / bitumen (may sink in water).
• Temperature affects density — always reference 60°F standard.

The API Gravity Scale

The American Petroleum Institute adopted this gravity scale in 1921 to provide a standardized measure for petroleum density. Before API gravity, different companies and countries used inconsistent density measurements, making trade difficult. The scale ranges from negative values (very heavy) to about 70° (very light fuels like natural gasoline).

Economic Significance

The price differential between light and heavy crude can exceed $10-20 per barrel. Light sweet crude (high API, low sulfur) commands premium prices because it produces more valuable products with simpler refining. The WTI-Maya price spread is a key metric in oil market economics.

Environmental Considerations

Heavier crude oils (lower API) are more difficult to clean up after spills because they are more viscous and may sink. The Deepwater Horizon spill involved medium crude (API ≈ 35°), while oil sands bitumen (API ≈ 8°) poses different environmental challenges. API gravity helps spill response teams predict oil behavior in water.

Frequently Asked Questions

• What does API gravity measure?

  API gravity measures the density of petroleum liquids relative to water. Higher API means lighter (less dense) oil. It was developed by the American Petroleum Institute for consistent industry-wide density reporting.

• Why is the API gravity scale inverted?

  The API scale is designed so that lighter, more valuable crude oils have higher numbers. This is because the formula involves dividing by specific gravity, creating an inverse relationship.

• What is the API gravity of water?

  Water has an API gravity of exactly 10°. Oils with API > 10 float on water; oils with API ≤ 10 sink.

• Why does API gravity matter for oil prices?

  Lighter crudes (higher API) are generally more valuable because they yield more gasoline and diesel per barrel during refining. Heavy crudes require more processing and energy to refine.

• What is the standard temperature for API gravity?

  API gravity is measured at 60°F (15.56°C). Petroleum volume and density change with temperature, so the standard keeps measurements comparable.

• How do refineries use API gravity?

  Refineries use API gravity to estimate the yield of different products (gasoline, diesel, fuel oil) from a crude oil. It helps pricing, blending, and process optimization.