The Delicate Balance of Measuring on Shabbos

With the winter months fast approaching and the flu season starting, a common scenario that presents itself is if one is allowed to take their or their children’s temperature on Shabbos.

What is the nature of the prohibition of measuring on Shabbos?

One is not allowed to measure things on Shabbos, and according to the majority of the Rishonim the reason being that its Uvdah D’Chol, or an action that is not befitting of the holy Shabbos and is reserved for the weekdays (Rashi Beitzah 29a). Following this reasoning if there a necessity to measure in order to perform a Mitzvah one would be allowed to do so (Tosfos Shabbos 126b).

What is considered measuring for a Mitzvah?

One of the prime examples presented by the Shulchan Aruch (Orach Chaim 306:7) where it is allowed to measure is when a sick person needs to measure a belt (as was commonly done by women). Similarly, the modern poskim (Tzitz Eliezer 11:38) discuss permitting the use of a (nonelectric) thermometer on Shabbos for a sick individual (and not for somebody who just wants to measure his temperature). The Shevet Haleivi (1:61) goes one step further and applies this leniency to a woman who is having difficulty conceiving and wants to measure her temperature in order to evaluate whether she is ovulating (the body temperature is higher during ovulation), since this too is a measuring for a Mitzvah.

Is measuring temperature even a problem of Uvdah D’Chol?

Rav Moshe (Orach Hachim 1:128) is of the opinion that taking ones temperature is not even included in the prohibition of measuring on Shabbos, and therefore its only a stringency to not use a thermometer, and therefore we would not object to somebody who wants to be lenient even if he is not sick.

Would the same leniency apply to weighing something for a Mitzvah (not using electricity)?

Most of the poskim are of the opinion that the same leniency would apply to weighing things for a mitzvah (not just to check if your diet is going well). However, some poskim think that weighing something is more stringent and would Not be permitted even for a Mitzvah (Kaf Hachaim). Therefore, even if something needs to be weighed for medical reasons, one should try to do this before Shabbos if possible and if this is not achievable one can do so on Shabbos.

What about measuring a formula or the like for a child?

A child is considered a choleh shein bo sakanah, and therefore measuring formula (ex. the correct amount of formula to water ratio) for a baby’s feeds is by definition allowed because it is for medical reasons.

[1] Adapted and translated from Rabbi Yossi Sprung on medicalhalacha.org