How to Read a Ruler

Directly above view of green ruler on a peach-colored background. — Whether you're measuring inches or centimeters, reading a ruler is a pretty straightforward task. Javier Zayas Photography / Getty Images

A ruler — a short length of wood, plastic or metal — is used for measuring things (and it's also a handy straight edge for drawing a straight line in math class). But do you know how to read a ruler correctly?

Different Types of Rulers

In the United States, most rulers are 12 inches (30.48 centimeters) long, featuring big marks to show inches and smaller marks for fractions of an inch.

Most other parts of the world use the metric system, which measures distance in units of millimeters, centimeters and meters. In fact, the U.S., Liberia and Myanmar are the only countries that use the imperial system by default.

The United Kingdom uses both systems of measurement and many English rulers have inches on one side and centimeters on the other side.

How to Read a 12-Inch Ruler

Reading a ruler is easy. Here's how to use and read a U.S. or English ruler.

Place the end of the object you want to measure at the zero mark to the left side of the first inch mark.
The longest lines on the ruler are the inch lines. These will be numbered. If your object ends directly on one of the inch marks, your object is that many inches long. For example, an object that falls on the first inch mark is one inch long.
If your object falls in between two long lines, you'll need to count fractions of an inch.
The second longest lines on an inch ruler represent half an inch and are exactly halfway between each inch line. Between the inch line and the half-inch line is a shorter line: the quarter-inch line. The smallest lines on a standard ruler represent 1/16 inch.

How to Read a Metric Ruler

Countries and industries that rely on the metric system use metric rulers, which express measurement in centimeters and millimeters. Reading a metric ruler is similar to reading an inch ruler, but instead of fractions of an inch marked by different length markings, the smaller lines represent millimeters.

Place the end of the object you want to measure at the zero mark, typically labeled "0 cm."
The longest lines on the ruler are the centimeter marks. The length between each numbered line is one centimeter.
The shortest lines on the ruler are the millimeter markings. There are 10 millimeters in one centimeter, so the fifth line between two centimeter marks represents half a centimeter or 5 millimeters.
The 5-millimeter mark may be longer than the other millimeter marks; this makes it easier to count. Instead of counting six millimeter marks, you can count five plus one.

Ruler measurements are great for small, flat items, but if you need to take precise measurements of something larger, you'll want a tape measure. Tape measures are flexible measuring tools that read the same way that you read a ruler. Because they roll up, they can measure larger items.