It’s forbidden to measure precisely on Shabbat as it’s considered Uvda DeChol, a weekday activity. [1]
It’s permissible to measure for the purposes of a mitzvah such as measuring Bitul BeShishim, 40 Seah for a Mikveh, or the temperature of someone sick. [2]
References
↑ Mishna Shabbat (157a) writes that measuring for a mitzvah is permitted implying that there’s a prohibition of measuring on Shabbat if not for the purpose of a mitzvah. So rules Tur and S”A 306:7 and Mishna Brurah 306:34.
