Question: On Yom Tov when cooking is permissible, may one use a mechanical scale (not an electronic one) to weigh the ingredients one needs for cooking?
Answer: Our Sages prohibited measuring on Shabbat or Yom Tov, for this is considered a “mundane act”, i.e. an action performed specifically during weekdays, such as while purchasing produce and the like.
For instance, on Yom Tov (Pesach, Shavuot, Sukkot, and Rosh Hashanah), one may take a piece of meat and cook it. Nevertheless, the Mishnah (Shabbat 28a) states that one may not weight the piece of meat before cooking it; rather, one may only take a piece based on how much it appears to the individual that is needed, but one may not weigh it.
Nevertheless, the Tur (Chapter 500) writes in the name of the Rashba that it seems from the Talmud Yerushalmi that our Sages only forbade weighing meat on Shabbat or Yom Tov when one takes meat from the butcher shop, for this very much resembles a “mundane act.” However, weighing things within one’s own home on Shabbat or Yom Tov is permissible.
On the other hand, the Rashba explains in his commentary on the Talmud that although at first he believed that weighing food items in one’s home should be permissible on Shabbat and Yom Tov, he later changed his mind and ruled that this is forbidden because it seems from the Talmud Bavli (Babylonian Talmud) that there is no distinction between weighing in the butcher shop or at home and our Sages prohibited any sort of measuring on Shabbat. The Poskim and Maran Ha’Shulchan Aruch rule likewise.
For this reason, it is forbidden to weigh food items on a mechanical scale on Shabbat or Yom Tov. It is likewise forbidden to place liquids in special measuring cups in order to determine their weight or volume, for this constitutes measuring on Shabbat and Yom Tov.
Summary: One may not measure the weight of foods or beverages on Shabbat or Yom Tov, whether this is being done for the purpose of cooking or for any other reason. In the next Halacha, we shall discuss several other details regarding this law.