The Shulhan Aruch (306:6) discusses the prohibition of "Medidah"-measuring on Shabbat. Different reasons are given as to the reason for this prohibition. The Rambam learns that it is out of concern that one may record the measurements by writing. The Tosafot understand that it is "Uvdin D’hol"-a mundane weekday activity. Applications of this prohibition include standing on a scale to measure weight or to measure one’s height.
Nevertheless, Maran states that "Medidah Shel Misva"-measuring for the purpose of a Misva is permissible. For example, a person is allowed to use a (non-electric) food scale to measure the amount of bread to fulfill the minimum requirement for the Shabbat meals or the minimum measure of Massa and Marror for the Seder night. Likewise, it is permissible to use a measuring cup to measure the volume of wine contained in a Kiddush cup for Kiddush on Shabbat or the four cups of the Seder night.
An additional example of measuring for a Misva is a case where a small of amount milk fell into a meat dish. It is permissible to measure and calculate whether the milk constitutes less than 1/60 of the mixture. It is also permissible to measure a Mikveh to determine whether it contains the requisite 40 Se’ah of water. This is only permitted if one intends to use the Mikveh on Shabbat.
The Halacha also permits measuring for the benefit of someone who is ill. Therefore, it is permitted to take someone’s temperature with a non-digital thermometer on Shabbat. It is even permitted to shake down the mercury to reset the thermometer. Similarly, it is permitted to measure blood pressure, with a non-electric apparatus. The Sis Eliezer (R. Eliezer Yehuda Waldenberg, 1915-2006, Jerusalem, Vol. 11:37) permits a woman undergoing fertility treatments to take her temperature on Shabbat to determine the time of ovulation.
It is permissible to measure a baby’s milk in the bottle, if the doctor advised tracking and adjusting the baby’s food intake. However, if there is no medical reason, one should not use the measuring lines on the bottle when pouring the milk; rather he should ignore the lines and approximate. Likewise, one may use a food scale to measure his food if he is on a medical diet, but not if it is a regular diet.
SUMMARY
It is prohibited to measure on Shabbat, unless for the purpose of Misva or someone who is ill.
