How to Read a Ruler: Inches, Centimeters, and the Small Lines Between

A ruler looks simple until you're staring at a cluster of unlabeled lines between the numbers and have to guess which one means what.

Inch rulers

On an inch ruler, the numbered lines mark whole inches. Between each pair of numbers, the lines are usually divided into eighths (1/8 in) or sixteenths (1/16 in). The longest line between two whole numbers marks the half-inch. The next-longest lines mark quarters, then eighths, then sixteenths — each level is a little shorter than the one before it.

Centimeter rulers

Centimeter rulers are more forgiving because they're base-10. Each numbered line is one centimeter, and the small lines in between mark millimeters — there are exactly 10 of them per centimeter. Usually the 5 mm mark (the middle one) is drawn slightly taller so you can find the halfway point at a glance.

Reading a measurement

Line up the zero mark exactly with one edge of what you're measuring, then read the line closest to the other edge. It sounds obvious, but the most common ruler-reading mistake is starting from the edge of the ruler itself rather than the printed zero line — on many physical rulers there's a small margin before the "0" that will throw off your reading if you ignore it.

If you don't have a physical ruler handy, our online ruler shows both inch and centimeter scales directly on your screen.