One teaspoon is about equal to 5 milliliters. Technically a teaspoon is equal to about 4.93 milliliters, so 5 milliliters equals 1.01 teaspoons. However, for the nutritional labeling on food packaging in the United States, it is defined as exactly 5 milliliters.
A teaspoon is an imperial unit of volume, while a milliliter is a metric unit of volume. One teaspoon is defined as 1/3 tablespoon. There are 16 tablespoons in 1 cup, so there are 48 teaspoons in 1 cup. One cup is equal to 237 milliliters. Dividing the number of milliliters in 1 cup by the number of teaspoons in 1 cup reveals the number of milliliters in a teaspoon: 237 / 48 = 4.93.