Milligrams are a measurement of weight, and teaspoons are a measurement of volume, so it is not possible to directly convert an amount between them. It is necessary to know the density of the material being measured to convert its amount in milligrams to teaspoons. For example, the density of water is 1 gram per milliliter, and 0.2 grams of water equals 0.2 milliliters, which is 0.04 teaspoons of water.

There are 1,000 milligrams in a gram, so 200 milligrams equals 0.2 grams. The equation for density is mass divided by volume. To convert mass (weight) to volume, divide the mass by the density of the material. For example, the density of butter is about 0.9 grams per milliliter, so 0.2 grams of butter divided by 0.9 grams per milliliter equals 0.22 milliliters of butter. Therefore, 200 milligrams of butter has a volume of 0.22 milliliters. This result still needs to be converted from the metric system to the imperial system. One teaspoon is equal to 5 milliliters. To convert milliliters to teaspoons, divide the number of milliliters by 5; therefore, 0.22 milliliters of butter divided by 5 equals 0.04 teaspoons of butter. The result means that 200 milligrams of butter equal 0.04 teaspoons of butter.