The size of a grain originally derived from the weight of barleycorn. Another Imperial weight unit, the pound, contains exactly 7,000 grains. To calculate how many grains are in a volume of a substance, you need to know its density. The common scientific unit for density is grams per cubic centimeter, and a gram contains 15.43 grains.
Things You'll Need
 Calculator

Look up the material's density, measured in grams per cubic centimeter. For example, you might want to calculate the mass of table salt, which has a density of 2.16 grams per cubic centimeter.

Multiply the substance's volume by its density. If you are converting the mass of 2 cubic centimeters of salt, you would multiply 2 by 2.16 to get 4.32. This is the material's mass, measured in grams.

Multiply this answer by 15.43. So you would calculate 4.32 x 15.43 = 66.7. This is the material's mass, measured in grains.
References
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