Teaspoons are a measurement of volume, and milligrams are a measurement of mass, so before one can calculate the teaspoons that are equivalent to 25 milligrams of a substance, one must know the density of that substance and divide 25 milligrams by the density in milligrams per teaspoon. Since milligrams per teaspoon is not a standard form of measurement for density, as milligrams are part of the metric system and teaspoons are part of the imperial system, some simple conversions must take place.

If one is calculating the number of teaspoons in 25 milligrams of lead, for example, the density is 11.34 grams per cubic centimeter. There are 1,000 milligrams in a gram and 4.93 cubic centimeters in a teaspoon, so the density is 2,300.2 milligrams per teaspoon. Dividing the 25 milligrams of lead by the 2,300.2 milligrams per teaspoon provides a result of roughly 0.1 teaspoon.