Jewish law is full of measurements. How big must a sukkah be? How much water must be in a mikveh? How much grain do you have to bring to the Temple? These were pressing concerns when the Talmud was written, but the rabbis didn’t have the metric system to standardize their rulings. Instead, they used measurements based on the human body, and objects that were easily accessible. An amah, or a cubit, was the length from the elbow to the tip of one’s middle finger. A tefach was the length of a fist, from bottom of pinky to top of index finger. A beitzah is a unit of volume equal to the size of an egg.
Over time different rabbis have attempted to provide conversions of the traditional measurements to more modern systems. The conversions vary a little, with notable competing theories brought forth by Avraham Chaim Naeh and the Chazon Ish, both 20th century Orthodox rabbis who lived in pre-State Palestine.
But there’s a new authority now, with its own opinion. Google has gotten into Jewish measurements. If you Google the question “How much is one cubit?” Google responds that 1 cubit = 45.72 centimeters. This differs from conclusions drawn by both Rabbi Naeh and the Chazon Ish (48 cm and 57.6 cm respectively). It’s hard to know where Google got their standard for cubits, but the more pressing question is, when will they have a standard for the tefach?