Can You Use a Thermometer on Shabbat? Halachic Guidance from Rabbi Ovadia Yosef

Is it permissible to use a thermometer on Shabbat?

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It is forbidden to use a digital thermometer on Shabbat, as activating the device and displaying numbers involves electrical activity, which is prohibited.

However, there is room to discuss the use of a non-digital (mercury) thermometer that contains liquid mercury inside a glass tube.

Measurement on Shabbat

The Shulchan Aruch (Orach Chaim 306) rules that one may not engage in commercial measurements on Shabbat — such as weighing, counting, or measuring for business or personal convenience.

However, when measurement is done for a mitzvah or for medical need, it is permitted. For example, one may measure the amount of water in a mikveh to ensure it contains forty se’ah, or measure for a sick person to know how to treat him.

Thus, measuring a person’s temperature to determine proper medical care is allowed, since it serves the needs of the ill.

Is It Considered “Measuring” by Hand?

Rabbi Ovadia Yosef writes in Yabia Omer (Vol. 9, p. 261) that the issue of “measurement” does not apply fully here. He cites the Maharikash, who discussed a clock running on Shabbat — and ruled that since the clock measures time automatically, without direct manual measurement, it is not considered an act of forbidden measuring.

By analogy, a mercury thermometer also measures automatically — we do not measure with our hands, we simply observe the result.
Therefore, it is permitted to use such a thermometer on Shabbat.

This leniency is supported by other great halachic authorities, including Rabbi Moshe Feinstein (Igrot Moshe) and Rabbi Shlomo Zalman Auerbach (Meorei Esh), who both permit using a non-electric thermometer for medical purposes.

Shaking Down the Thermometer

Rabbi Shmuel HaLevi Wosner raised a concern: After measuring, one shakes the thermometer to reset the mercury for future use. This action, he argued, might be considered “fixing a vessel” (metaken kli), which is a rabbinic prohibition on Shabbat.

However, Rabbi Ovadia Yosef responded that this concern does not apply. Shaking the thermometer is a simple, routine motion, done frequently and without intent to repair — similar to closing or opening a lid. The Magen Avraham already ruled that such everyday actions do not constitute “repair.”

Therefore, one may shake down the thermometer on Shabbat to prepare it for another use. This view is shared by Rabbi Eliezer Yehuda Waldenberg (Tzitz Eliezer) and other leading halachic authorities.

It is permissible to use a non-electric thermometer on Shabbat to check a patient’s temperature, and one may shake it down afterward. However, electronic thermometers may not be used under any circumstances.

Based on the rulings of Rabbi Ovadia Yosef