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.