Weighing and Measuring on Shabbat

Friday night. You're preparing the Shabbat meal. You're making a salad and want to add seasonings exactly according to the recipe. Half a teaspoon of salt, a tablespoon of soy sauce... You take out your special measuring spoon and want to measure and add, exactly as in the recipe.

Stop!

Measuring with measuring tools is forbidden on Shabbat, as it's considered prohibited work. It's only permitted for the purpose of a mitzvah or in specific circumstances (such as measuring out medicine for a baby).

Furthermore, one may not use a measuring spoon or measuring cup for any purpose, unless there are no alternatives -- and even then one must ensure not to measure precisely.

Is there a difference between a special measuring spoon and a regular spoon?

Yes, there is a difference.

A regular teaspoon, tablespoon, regular cup -- these are utensils designed for eating rather than measuring, and it's permissible to use them (for eating, and also to approximately measure out a teaspoon, for example) because they are defined as food utensils, not measuring tools.

When is it permitted to use a precision measuring tool on Shabbat?

For the purpose of an obligatory mitzvah - such as measuring a "k'zayit" (olive-sized portion) or a measure of liquid.

Ideally, one should measure before Shabbat. But if one didn't manage to, it's permitted to measure even on Shabbat if necessary to fulfill a mitzvah.

Regarding weighing items for the purpose of fulfilling a mitzvah, this is only permitted with a mechanical scale and not an electrical one.

In summary:

It is forbidden to use measuring tools on Shabbat, unless one has no regular utensil and is careful not to use it with precision. 

It is permitted to measure with regular utensils (such as a teaspoon), which are not explicitly designated for measuring, but are used for regular eating and drinking. One may also use a measuring tool to fulfill a mitzvah.