API Gravity Calculator

What Is API Gravity?

API gravity is a petroleum industry scale for crude oil density. Higher API values mean lighter oil. It is defined relative to water at 60°F, where water has API gravity of 10° and specific gravity of 1.0.

API Gravity and Specific Gravity

$$\text{API} = \frac{141.5}{SG} - 131.5$$ $$SG = \frac{141.5}{\text{API} + 131.5}$$

Density in kg/m³ at the 60°F reference is:

$$\rho = SG \times 1000\,\text{kg/m}^3$$

Example: oil with API 35 has $SG = 141.5/(35+131.5) \approx 0.8498$ and density about $850\,\text{kg/m}^3$.

Using API Gravity in Practice

Refineries classify crude as light, medium, or heavy by API range. Light crude (high API) flows more easily and often yields more gasoline. Heavy crude (low API) is denser and may need more processing. Converting between API, SG, and density is routine in tank gauging and custody transfer.

Frequently Asked Questions

What API gravity does water have?

Fresh water at the 60°F reference has API gravity of 10° and specific gravity of 1.000.

Is higher API gravity lighter or heavier?

Higher API means lighter oil. Lower API means denser, heavier crude.

Why is 60°F used as the reference?

API gravity is defined at 60°F (15.6°C) by industry convention, matching common petroleum measurement standards.

How do I convert API to density?

First compute SG with $SG = 141.5/(\text{API}+131.5)$, then multiply by 1000 kg/m³.

Can API gravity be negative?

Yes. Very dense liquids can have API below 10°. The formula still applies for valid specific gravity values.

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What Is API Gravity?

API Gravity and Specific Gravity

Using API Gravity in Practice

Frequently Asked Questions

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