The number of milligrams in 1 teaspoon varies according to the density of the substance being measured. Milligrams are a unit of weight; the teaspoon is a unit of volume equivalent to 5 milliliters. These measurements are part of the modern metric system established by the International System of Units.
Dry unit measurements are generally used in baking. One cup of white flour, whole wheat flour, bread flour and cake flour weighs 123 grams, 140 grams, 130 grams and 114 grams respectively. Liquid measurements remain consistent, but accurate conversions are important. One cup of water is equivalent to 8 ounces. One tablespoon is equal to about 15 milliliters. One quart is equal to 4 cups or 32 ounces.