API Gravity Calculator

Created by Gabriela Diaz
Last updated: Aug 01, 2022

With the API gravity calculator, you can determine the API gravity of any liquid petroleum product by simply indicating its density. You can also use this tool to convert specific gravity to API gravity and vice versa.

Keep on reading to learn:

  • What is API gravity?;
  • The API gravity formula;
  • How to convert API to specific gravity (API to SG); and
  • The crude oil classification by API gravity 🛢

What is API gravity?

The API gravity is a relative density or heaviness measure exclusively used for crude oils and other petroleum liquid derivatives. Crude oil from wells, and in general any liquid petroleum product, can be classified according to its specific gravity or relative density to water 💧🛢 This is, how much lighter or heavier it's compared to water, or whether it sinks or floats in water. The API gravity may be used to make this distinction.

This gravity scale was adopted in 1921 by the American Petroleum Institute, hence its name. Before the API appeared, the French Braumé scale, developed in the late 1760s, was used. And after some variations based on this scale, the API scale appeared.

Although the API gravity measures the relative density of crude oil and water, its primary application is to identify and classify crude oil's quality by employing a simple numerical scale expressed in API degrees (°API) — The API gravity is used to indicate the quality of crude oil.

The API gravity of a crude oil or any liquid crude product is given in terms of the liquid's specific gravity. This is the API gravity formula:

API=141.5SG131.5\small \text{API} = \cfrac{141.5}{\text{SG}} - 131.5

where:

  • API\text{API} — API gravity, this value is dimensionless or given in °API; and
  • SG\text{SG} — Liquid's specific gravity, this is dimensionless.

We can see from the equation that the API gravity parameter is dimensionless. However, it is often used with a degree symbol, such as the degree of API gravity (°API). Also note from the equation that the lower a liquid's specific gravity is, the higher its API gravity is — Higher API values indicate a lighter crude oil.

By rearranging the previous formula, it is possible to get a simple expression for converting from API gravity to specific gravity:

SG=141.5API +131.5\small \text{SG} = \cfrac{141.5}{\text{API} \ + 131.5}

It's also worth noting that because density is a temperature-dependent property, specific gravity and API gravity are influenced by the liquid's temperature. At higher temperatures, a crude oil's density and specific gravity diminish while its API gravity increases. The standard temperature for measuring densities is 60 °F (15.6 °C).

💡 You can learn more about density and find out if a material floats or sinks in water with the density calculator!

The values of water properties are relevant in this subject. Water density at 60°F is 999 kg/m3, and its specific gravity is 1.0. With this last value, we can calculate its API gravity by substituting into the corresponding formula:

API=141.51.0131.5=10 °API\small \text{API} = \cfrac{141.5}{1.0} - 131.5 = 10 \ \text{°API}

This means that the API gravity of water at 60 °F is 10 °API. Implying that liquids with API gravities above this value will float on water, while those with values below 10 °API are heavier and will sink. This is the API gravity number used as a reference to describe crude oil's heaviness and quality, with 10 °API being the heaviest oil, known as bitumen.

Crude oil chart by API gravity

As mentioned, the API classification of crude oils is not only used to measure its relative density but, more than that, to identify its quality and thus its value. Crude oil is classified as light, medium, or heavy according to its measured API gravity. The API of crude oil is generally between 15 and 45 degrees. This categorization is shown in the table below:

API gravity [°API]

Crude oil type

10

Bitumen

< 15

Extra heavy crude

15 - 25

Heavy crude

25 - 35

Medium crude

35 - 45

Light crude

45

Extra light crude

In general, oil with an API gravity of 40 to 45° is the most valuable. Crudes lighter than 45° are classified as extra-light crude or condensates and are less valuable than light crude since they include a lot of light ends such as propane and butane 🛢

How to use the API gravity calculator

This API gravity calculator can assist you in calculating a crude oil's API gravity and specific gravity. It will also help determine if a particular substance will float or sink in water. To use this tool:

  1. Simply indicate the value of Density of crude oil.

  2. The calculator will display the Specific gravity of liquid.

  3. In the row below, Degree of API gravity, the calculator will show the API gravity.

  4. The calculator will also indicate if the substance floats or sinks on water based on the API gravity.

You may also use this tool as API gravity and specific gravity converter. By just indicating the API gravity of the crude oil, the calculator will instantly determine its corresponding specific gravity and density.

🙋 In the Advanced mode of the calculator, you'll find a list of pre-load liquids along with their corresponding values of density, specific gravity, and API gravity.

Gabriela Diaz
Density of crude liquid
lb/cu ft
Specific gravity of liquid
Degree of API gravity
° API
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