API gravity is a scale created by the American Petroleum Institute to measure the density of petroleum liquids relative to water. Water has an API gravity of 10. Liquids lighter than water have API > 10; liquids heavier than water have API < 10.

What is the formula for API gravity?

API gravity = (141.5 ÷ Specific Gravity at 60°F) − 131.5. The inverse is: Specific Gravity = 141.5 ÷ (API Gravity + 131.5). Both formulas use specific gravity measured at 60°F (15.6°C).

What specific gravity corresponds to water?

Water has a specific gravity of 1.0 and an API gravity of exactly 10. Crude oils typically range from SG 0.79 (very light, ~45° API) to SG 1.0 (extra-heavy, ≤10° API).

How is crude oil classified by API gravity?

Light crude: API > 31.1°; Medium crude: API 22.3°–31.1°; Heavy crude: API 10°–22.3°; Extra-heavy crude: API < 10°. Light crude is generally more valuable because it yields more gasoline and diesel.

Why is API gravity measured at 60°F?

Petroleum density varies with temperature. The API standard specifies 60°F (15.6°C) as the reference temperature to ensure consistent, comparable measurements across labs and field conditions.

Can API gravity be negative?

Yes. Bitumen and very heavy petroleum residues can have API gravity below 0°. This occurs when the specific gravity exceeds approximately 1.076, meaning the liquid is denser than water.

API Gravity Calculator

Convert between API gravity and specific gravity to classify petroleum products per American Petroleum Institute standards.

Input

Enter either value — the other is calculated automatically.

Specific Gravity (SG) at 60°F

dimensionless

API Gravity °API

°API

Quick Examples

Enter a value above to see results

API Formulas

SG → API gravity

°API = (141.5 ÷ SG) − 131.5

API gravity → SG

SG = 141.5 ÷ (°API + 131.5)

Crude Oil Classification

Grade	API Gravity
Light crude	> 31.1°
Medium crude	22.3° – 31.1°
Heavy crude	10° – 22.3°
Extra-heavy crude	< 10°

Copied to clipboard

Summary

API gravity is a measure of how heavy or light petroleum liquid is compared to water, expressed on the American Petroleum Institute scale. This calculator converts between API gravity and specific gravity (SG) using the standard formulas defined by the API. Results are automatically classified into crude oil grades (light, medium, heavy, extra-heavy) so you can instantly identify the petroleum type.

How it works

Enter a specific gravity value (dimensionless ratio relative to water at 60°F) or an API gravity value.
The calculator applies the API formula: API = (141.5 / SG) − 131.5.
To convert back, the inverse formula is used: SG = 141.5 / (API + 131.5).
Results are rounded to two decimal places for practical field use.
The petroleum grade classification (light, medium, heavy, extra-heavy crude) is determined from the computed API gravity.

Use cases

Determining crude oil grade before purchase or refining decisions.
Converting laboratory specific gravity readings to API gravity for reporting.
Classifying petroleum products (gasoline, diesel, kerosene, fuel oil) by API gravity.
Checking whether a crude oil meets pipeline or refinery intake specifications.
Educational use in petroleum engineering courses covering fluid properties.
Field calculations when appraising oil wells or production streams.

Frequently Asked Questions

Last updated: 2026-06-19 · Reviewed by Nham Vu