According to tables published by the National Institute of Standards and Technology of the U.S. Department of Commerce, a grain equals exactly 64.79891 mg, or 0.06479891 g. Therefore, the conversion between the two units is only a matter of one multiplication or division (or a combination of the two), with the result being the exact equivalent of the original value. However, rounding the results can help you avoid long decimals.
Multiply by 64.79891 to obtain the equivalent of your grains in milligrams. For example, 34 grains become 34 x 64.79891 = 2203.16294 mg. Multiply grains by milligrams instead of grams to avoid the initial zeros of the grams equivalence. Divide the value of milligrams by 1,000 to calculate their equivalent in grams, which translates to moving the decimal point three times to the left. Therefore, 2203.16294 mg becomes 2.20316294 g.
To convert grams to grains through divisions, repeat the steps of the previous section in reverse order. Multiply the grams value by 1,000 (move the decimal point three times to the right) to obtain the equivalent in milligrams. Divide milligrams by 64.79891, and you have mass expressed in grains. For example, 35.64 g becomes 35640 mg, which in turn equals 35640 / 64.79891 = approximately 550 grains.
A simpler method to convert grams to grains involves a single multiplication. A gram equals approximately 15.43 grains. Therefore, on the above example, 35.64 g equals 35.64 x 15.43 = 549.9. As the equivalence on this occasion is only an approximation, the result can be slightly off the mark. However, it's a quicker and simpler conversion method if absolute precision is not your main concern.