A millimeter is a tiny unit of measurement in the metric system, equal to one-thousandth of a meter. To picture how small that is, look at a ruler: those smallest lines marking the spaces between centimeters show millimeters. Ten millimeters make one centimeter, and it takes 1,000 millimeters to equal one meter.

Millimeters are perfect for measuring small, precise things. A pencil's thickness is about 7 millimeters. The width of your pinky fingernail is roughly 10 millimeters. Scientists use millimeters when exact measurements matter: a doctor measuring the size of a mole, an engineer designing tiny gears, or a jeweler crafting a ring.

You'll often see millimeters abbreviated as mm. Rainfall gets measured in millimeters because even small amounts matter: 25 millimeters of rain is quite a lot, enough to soak the ground thoroughly. Camera lenses, screws, and computer screens all get measured in millimeters when precision counts.

While Americans often use inches for everyday measurements, most of the world relies on millimeters and the metric system. Even in America, doctors, scientists, and engineers typically work in millimeters because the metric system makes calculations simpler and more accurate.