The conversion of 1 teaspoonful to 5 milliliters is a general approximation used for convenience in cooking and measuring small amounts of liquid.
Teaspoons and milliliters are both units of volume measurement, but they are not directly equivalent.
A teaspoon is a unit of volume commonly used in cooking and baking. It is approximately equal to 4.92892 milliliters. However, for simplicity and ease of measurement, it is often rounded to 5 milliliters.
So, 1 teaspoonful is considered to be approximately equal to 5 milliliters for practical purposes.