In the previous Halacha we have mentioned that our Sages have prohibited any kind of measuring on Shabbat or Yom Tov. For instance, one may not weigh various foods items or beverages on Shabbat. Although the scale is mechanical and not electronic, this is likewise a rabbinic prohibition.
Measuring for the Purpose of a Mitzvah
Nevertheless, the Mishnah (Shabbat 157a) states that our Sages permitted measuring for the purpose of a Mitzvah. For instance, if it is necessary to use a “measuring tape” to measure the dimensions of a Mikveh in or to determine whether or not it is valid for immersion, this is permissible on Shabbat. Maran Ha’Shulchan Aruch (Chapter 306, Section 7) rule likewise.
The Opinion of the Peri Megadim
The Peri Megadim explains that the reason for this because the original reason why our Sages forbade measuring on Shabbat is because this appears to be a mundane act performed only on weekdays, for one measures while purchasing produce, meat, and the like. However, measuring for the purpose of a Mitzvah, such as measuring a Mikveh, does not appear like a mundane act and is permissible.
One Who Must Eat on Yom Kippur
Similarly, when one is ill and must eat on Yom Kippur, the individual is fed in quantities less than the size of a date and time intervals that are less than that for which one would be liable for the “Karet” punishment for eating on Yom Kippur. The Sefer Ha’Chinuch (Mitzvah 313) rules that one may measure the amount of food the ill individual must eat so that they are less than the prescribed amount. The Poskim (including the Mishnah Berura, Chapter 306) explain that this is because it is preferable to be concerned with the more severe Torah prohibition of eating on Yom Kippur as opposed to the rabbinic prohibition of measuring on Shabbat and Yom Tov; thus, this is also considered measuring for the purpose of a Mitzvah.
The Ruling of Maran zt”l
Based on this, Maran Rabbeinu Ovadia Yosef zt”l rules (in his Chazon Ovadia-Shabbat, Part 6, page 26, among other places) that on the night of the Pesach Seder, one may measure the Kezayit amounts of Matzah and Maror, for this is also considered measuring for the purpose of a Mitzvah and this does not resemble a mundane act.
It is nevertheless clear that although cooking on Yom Tov for the holiday meals is permissible and is a Mitzvah of enjoying and honoring the holiday, measuring or weighing food items for the purpose of cooking or baking is not considered measuring for the purpose of a Mitzvah, for only when something is completely recognizable as being done for the purpose of a Mitzvah, such as measuring Matzah and Maror or measuring a Mikveh, is this permissible. However, measuring for the purpose of cooking on Yom Tov is forbidden.
It is likewise permissible to prepare formula and the like in a baby bottle for a baby on Shabbat although doing so entails measuring the water and formula.
Summary: Although measuring on Shabbat or Yom Tov is forbidden, nevertheless, measuring for the purpose of a Mitzvah, such as measuring the amounts of Matzah and Maror, measuring the size of a Mikveh, or taking an ill person’s temperature etc., is permissible on Shabbat and Yom Tov.