Is teaspoon customary or metric

Answer 1

The teaspoon is a traditional customary measurement that has been standardized in manufacturing practice and in food service / food labeling to equal an exact metric equivalent.

In the United States one teaspoon as a unit of customary culinary measure is 1⁄3 tablespoon, that is, 4.92892159375 mL; it is exactly 1⁄3 US fluid drams, 1⁄6 US fl oz, 1⁄48 US cup, and 1⁄768 US liquid gallon and approximately 1⁄3 cubic inch.

For nutritional labeling on food packages and in food service in the US, the teaspoon is defined as precisely 5 mL, the tablespoon as 15 mL.

Almost ALL less expensive teaspoon measures sold in the US since the late 1950's have held an exact 5mL (overseas manufacturing is metric and they simply rounded up) - so you have been likely using the metric system for many years thinking you were using a customary measure without knowing it!