About the Density to API Gravity Calculator
This tool converts a petroleum liquid’s density into specific gravity and API gravity using the standard API equation. It is useful for engineers, students, lab personnel, and operators who need a quick educational estimate from density values in common units.
How to use this calculator
- Enter a positive density value in the Density field.
- Select the matching density unit: kg/m³, g/cm³, lb/US gal, or lb/ft³.
- Use a density measured or corrected to 60°F when possible.
- Read the calculated API gravity, specific gravity, density in kg/m³, and water comparison.
- Check the displayed crude classification such as light, medium, heavy, or extra heavy.
How it works
The calculator first converts the entered density into specific gravity by dividing it by the density of water in the same unit. The water reference values used are 999.016 kg/m³, 0.999016 g/cm³, 8.337 lb/US gal, and 62.366 lb/ft³, corresponding to the 60°F reference condition.
API gravity is then calculated with the standard formula: API gravity = 141.5 ÷ specific gravity − 131.5. A lower specific gravity gives a higher API gravity, meaning the liquid is lighter relative to water.
The calculator also converts the entered density to kg/m³ for display and compares the specific gravity to 1. Values below 1 are lighter than water, values above 1 are heavier than water, and exactly 1 is equal to water.
The crude classification is based on the API result: 50 or more is very light, 31.1 to under 50 is light crude, 22.3 to under 31.1 is medium crude, 10 to under 22.3 is heavy crude, and below 10 is extra heavy. Results are educational estimates; lab reporting and custody-transfer work should use calibrated measurements and the applicable ASTM/API standard.
Example calculation
Suppose the density is 850 kg/m³. The calculator divides 850 by the water reference density 999.016 kg/m³ to get a specific gravity of about 0.85084. It then applies API = 141.5 ÷ 0.85084 − 131.5, giving 34.79 °API. Because this is above 31.1 °API, the liquid is classified as light crude and is lighter than water.
Frequently asked questions
What density should I enter for API gravity?
Enter the liquid density measured or corrected to the 60°F reference condition when possible, using one of the available units.
What is the formula for converting density to API gravity?
The calculator converts density to specific gravity, then uses API gravity = 141.5 ÷ specific gravity − 131.5.
Is API gravity the same as specific gravity?
No. Specific gravity is the liquid density relative to water, while API gravity is a petroleum scale derived from specific gravity.
Does a higher API gravity mean a lighter liquid?
Yes. Higher API gravity corresponds to lower specific gravity, so the petroleum liquid is lighter relative to water.
Why does temperature matter for API gravity?
Density changes with temperature, so API gravity is standardized to 60°F/60°F for consistent petroleum comparisons.