API Gravity Calculator

Created by Rahul Dhari
Reviewed by Steven Wooding
Last updated: Aug 16, 2021

The API gravity calculator estimates the API gravity of petroleum liquids. API gravity is extensively used as a measure of the grade of crude oil product. The API gravity is a function of the relative density or specific gravity of the crude oil liquid. This calculator will help you answer questions like, what is the API gravity of gasoline or petrol? Read on to understand more on specific gravity to API conversion formula.

What is API gravity and how it is measured?

The API gravity is a measure of the heaviness of crude oil product when compared with water. The term API in API gravity stands for American Petroleum Institute, with reference to the institution that developed it. The API gravity is calculated based on the specific gravity of the liquid. To find the API gravity of a liquid, initially, the relative density is calculated using a hydrometer or an oscillating U-tube conforming to industry standards ASTM D1298 or D4052. The specific gravity is then used in the API gravity formula. The API gravity of a crude product is related to the specific gravity using the equation:

Specific Gravity = 141.5 / (131.5 + API)

Rearranging the terms, we get:

API = (141.5 / Specific Gravity) - 131.5

The above equation can also be called the specific gravity to the API conversion formula. The API gravity parameter is dimensionless, based on the equation. However, it is often used with a degree symbol, such as the degree of API gravity (°API). The specific gravity of the liquid can be estimated from their density using the equation below:

Specific gravity = ρLiquid / ρwater

where, density of water is taken as 999 kg/m3. Therefore, one can directly calculate API gravity from density.

The API gravity is often compared with water such that, the relative density of heavy oil at 60 °F is taken as 1.0, the same value as for pure water.
The API gravity for pure water in that case will be:

API = (141.5 / 1) - 131.5 = 10 °API

The behavior of the crude products in water is judged based on this parameter. If the API gravity of a liquid is greater than 10° API, the liquid will float on water. However, if the value of API gravity is less than 10, it implies that the liquid is heavier than water and will sink.

How to calculate API gravity from specific gravity?

To determine the API gravity of a petroleum liquid:

  1. Enter the density of the liquid.
  2. The calculator will determine the relative density of the liquid.
  3. The API gravity calculator will calculate the API gravity from density.

Note: You can also directly choose liquid from the pre-existing list of alternatives, available in the Advanced mode of the calculator.

Example: Using the API gravity calculator

Question: What is the API gravity of gasoline? Take density of gasoline as 750 kg/m3?

To determine the API gravity of gasoline:

  1. Enter the density of the gasoline as 750 kg/m3.
  2. You can now see the relative density of the gasoline as 0.7508.
  3. Using the API to specific gravity conversion calculator:
API = (141.5 / 0.7508) - 131.5 = 56.98 °API

Classification of API gravity

The API gravity can be related to its value such that, the high API are lighter crude oil products and are more valuable as they can yield more high-value products when processed. However, crude products having API values beyond 45° are less valuable as they are considered extra light. The degree of API for most crude products are between 15-45° with 35-45° being most valuable.

Degree of API gravity
Crude types
Extra heavy crude
Heavy crude
Medium crude
Light crude
Extra light crude


What is API gravity?

API gravity is defined as the heaviness or lightness property of the liquid with respect to water. This parameter is specifically derived for petroleum products.

How to estimate API gravity?

API gravity can be estimated by the following method:

  1. Divide 141.5 by the specific gravity of the liquid.
  2. Subtract 131.5 from the resultant to obtain the API gravity.

How is specific gravity of a liquid measured?

The relative density of the liquid can be measured using a hydrometer or an oscillating U-tube by following the procedure as per ASTM standards D1298 or D4052.

What is API gravity of kerosene?

API gravity of kerosene is 50.53 °API. Considering density of kerosene as 775 kg/m3:

API = (141.5 / 0.7758) - 131.5 = 50.9 °API
Rahul Dhari
Density of crude liquid
lb/cu ft
Specific gravity of liquid
Degree of API gravity
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