But for quick-and-dirty estimates when you don’t have a standard measuring device handy, they can’t be beat

Is That a Ruler in Your Pocket?
At some point years ago, I picked up the seemingly useless piece of information that an ancient unit of measurement called the cubit was the distance from the elbow to the tip of the middle finger. For an average adult male (at least, average as of a couple of millennia ago), a cubit works out to about 18 inches (45.7cm). Cubits were a standard unit of length in Sumeria, Egypt, and other parts of the Middle East long before anyone dreamed of an arbitrary, decimal-based measuring system. Cubits were most often used in the context of building; if you have an English Bible published before the mid-20th century, it probably lists the measurements of Noah’s ark (among other things) in cubits.

Not long after learning this tidbit, I began discovering how useful such a built-in measuring device could be. I don’t always carry a tape measure with me, but I frequently need to estimate whether, for example, a piece of furniture will fit in a certain room. Knowing the approximate length of my arm is surprisingly handy, because it’s extremely easy to use for rough measurements.Hand, Hand, Fingers, Thumb
The cubit was just one of numerous units of measurement based on the typical size of body parts. Here are a few more:

foot: It probably goes without saying that the unit foot was based on the length of a man’s foot.
span: Stretch out your hand so that the tip of your thumb is as far away as possible from the tip of your pinky. That distance is called a “span,” which for most people is almost exactly half a cubit.
handbreadth: The width of your four fingers where they meet the palm—usually about 4 inches—is a handbreadth or sometimes just a “hand.” The height of horses is usually expressed in hands.
digit: The width of a finger, which tends to be about 2cm (about 13/16 of an inch).
thumb: The width of a thumb, which was later used as the basis for the inch.

fathom: If you stretch out your arms to either side of your body as far as they’ll go, the distance between the tips of your middle fingers will be very close to your height, or about six feet—your own feet, that is—a length also known as a fathom.
handful: Although we normally use the word handful in the informal sense of “just a little bit,” your hand can serve as a fairly repeatable measure of volume for dry goods such as grains, beans, and seeds.

The reason units of measurement like these fell out of favor is that they vary from one person to the next, so if you need accuracy or repeatability, they’re not the best choice. (It turns out, for example, that my personal “cubit” is 18.375 inches (46.67cm). I always was an overachiever.) But for quick-and-dirty estimates when you don’t have a standard measuring device handy, they can’t be beat. —Joe Kissell