API Gravity Calculator: Convert Between API Gravity, Specific Gravity and Density of Oil

Oil gravity in API degrees, specific gravity and density.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose whether you know the API gravity or the specific gravity.

Enter the value.

The calculator gives the other gravity, the density and the class of oil.

Formulas used

  • SG = \frac{141.5}{API + 131.5}
  • API = \frac{141.5}{SG} - 131.5

Worked example

With the default numbers in the calculator above:

  1. Specific gravity at 60 F: 0.84985
  2. API gravity: 35 degrees API
  3. Density at 15.6 C (60 F): 849.01 kg/m3
  4. Class of oil: Light oil (31.1 API or more)

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: What do you know? The specific gravity (60 F / 60 F).

  1. Specific gravity at 60 F: 0.85
  2. API gravity: 34.971 degrees API
  3. Density at 15.6 C (60 F): 849.16 kg/m3
  4. Class of oil: Light oil (31.1 API or more)

Common questions

What is API gravity?

A scale from the American Petroleum Institute for how heavy an oil is compared with water. Water is 10 degrees API.

Is a higher API number heavier or lighter?

Lighter. A light crude is above 31 API and a heavy crude is below 22 API.

What API gravity floats on water?

More than 10.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any petroleum or fuels text (API gravity).

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