A teaspoonful is the amount of something that fits in a teaspoon, which is about one-third of a tablespoon or roughly five milliliters. When a recipe calls for a teaspoonful of vanilla extract, you fill a teaspoon and add that amount to your batter. When you're sick and the medicine bottle says to take two teaspoonfuls, you measure out two level teaspoons of the liquid.

The word is often shortened to “teaspoon” in recipes, but teaspoonful emphasizes the measurement itself rather than the utensil.

In baking and cooking, precise measurements matter. A teaspoonful of salt versus a tablespoonful makes an enormous difference in how your food tastes. Too much baking soda, even by a teaspoonful, can ruin a batch of cookies. This small measurement, though it seems simple, helps cooks around the world create consistent, delicious results every time they follow a recipe.