API gravity is a system developed by the American Petroleum Institute to measure how light or heavy a petroleum-based liquid is in comparison to water. An API gravity of 10 means the petroleum-based liquid being measured has about the same density (mass per unit volume) as water. API gravity can be calculated using specific gravity, which is a ratio of the density of the sample liquid in comparison to a reference liquid, usually water.
Using the scale, determine the weight of the sample liquid. Be sure to account for the weight of the container holding the substance--weigh it dry first and then subtract that weight from the total weight of the substance in its container.
Divide the substance's weight by the volume of the substance being measured to determine its density. For example, if four cubic centimeters of a substance weigh two grams, its density is 2/4 = 0.5 grams per cubic centimeter.
Determine the substance's specific gravity by dividing its density by the density of water (1 gram/cubic centimeter). For the exemplar substance, its specific gravity would be 0.5/1 = 0.5.
Using the specific gravity of the substance, calculate its API gravity using the following formula: (141.5/specific gravity) - 131.5. For the exemplar substance, its API gravity would be (141.5/0.5) - 131.5 = 151.5.
Make sure all your calculations use the same measurement system--don't mix metric with imperial values.