The grain is a measure of mass that's rarely used today, though it has a long history. The grain was the earliest unit of mass and is the smallest unit in systems of measurements used at one point in the United Kingdom, United States and northeastern Europe, according to the National Institute of Standards and Technology. Dealers in precious metals once used actual grains of wheat or barleycorn to balance their scales. Today, those who still measure in grains have defined the grain as a specific weight, so you can use a regular household gram scale to calculate weight in grains.
Place the object on the gram scale.
Read its weight in grams.
Multiply the weight by 15.432 to calculate the weight in grains. To make calculations easier when measuring out medicine, either 15 or 16 grains can be used, according to the book Math and Meds for Nurses. Both numbers will produce an answer in the 10 percent range that the medicine field deems acceptable when converting one type of measurement to another.