API Gravity Calculator

API gravity is an inverse measure of a petroleum liquid's density relative to water, established by the American Petroleum Institute. The fundamental relationship linking API gravity to specific gravity ($sp\_gr$) at 60 °F is defined by the standard formula:

API = (141.5 / sp\_gr) - 131.5

Conversely, if you know the API gravity and need to find the specific gravity, the formula rearranges to:

sp\_gr = 141.5 / (131.5 + API)

To relate this further to absolute liquid density ($dens\_liquid$), specific gravity compares the density of the fluid to the density of pure water at standard conditions (approximately 997 kg/m³), giving us the equation sp\_gr = dens\_liquid / 997.

Let us walk through a practical calculation for a petroleum sample with a known API gravity of 32.5. We want to determine its specific gravity and its absolute density in kilograms per cubic meter.

Step 1: Calculate Specific Gravity
Using the rearranged formula, we insert our API value: sp\_gr = 141.5 / (131.5 + 32.5).
Simplifying the denominator gives: 141.5 / 164.0 = 0.8628.
Thus, the specific gravity of the liquid is approximately 0.863.

Step 2: Calculate Absolute Liquid Density
Using the relationship between specific gravity and density (where water density is 997 kg/m³):
dens\_liquid = 0.8628 × 997 = 860.2 kg/m³.
This tells us the liquid is lighter than water and has a density of 860.2 kg/m³, categorizing it as a medium crude oil.

Temperature Correction Matters: API gravity values change significantly with temperature. Always ensure your hydrometer readings or density measurements are standardized to 60 °F (15.56 °C) for accurate classification.

Understand the Scale: Remember that API gravity is an inverse scale. A higher API number means a lighter, less dense fluid (such as gasoline), while a lower number indicates a heavier, denser oil (such as heavy crude).