Ruler Facts for Kids

A ruler is a tool used for measuring how long things are. It can also help you draw straight lines. Most rulers have numbers and small lines along their edge to show different measurements.

Rulers are made from many materials like plastic, wood, and metal. They also come in different lengths. For example, you can find rulers that are 1 metre long, or shorter ones like 30 centimetres, 20 centimetres, or 15 centimetres. These are called metric rulers. You can also find rulers that measure in imperial units, like 1 yard, 18 inches, or 6 inches.

Different Kinds of Rulers
Rulers in Geometry
History of Rulers
Images for kids
See also

Different Kinds of Rulers

A gilded bronze ruler from ancient China.

A bronze ruler from the Han dynasty.

Rulers have been made for a very long time, using different materials and in many sizes. Wooden rulers are common. Plastic rulers are also popular because they can be easily shaped with measurement marks. Metal rulers are very strong and are often used in workshops. Sometimes, a metal edge is added to a wooden ruler to protect it when cutting with a blade.

A ruler that is about 30 centimetres (12 inches) long is good for a desk, helping with drawing. Smaller rulers fit easily in a pocket. Longer rulers, like those 45 centimetres (18 inches) long, are needed for bigger tasks. There are also rigid wooden or plastic yardsticks (1 yard long) and meter sticks (1 meter long) for measuring larger distances. For very large projects, people now use tape measures or laser tools instead of long measuring rods.

Most rulers used at a desk have three main jobs: to measure things, to help draw straight lines, and to guide a blade for cutting.

A line gauge is a special ruler used in the printing world. It can be made of metal or clear plastic. It measures in units like inches, agate, picas, and points.

Some measuring tools can be folded or rolled up to make them easy to carry. A carpenter's folding rule, for example, can fold down to about 25 centimetres (10 inches) to fit in a pocket. A metal tape measure rolls up into a small case. When you pull them out, they become straight like a ruler.

A tailor's fabric tape measure is flexible and can bend. It's used to measure around curved things, like a person's waist, or for straight measurements like the length of a leg. It rolls up small when not in use.

A contraction rule (also called a shrinkage or shrink rule) is made with slightly larger divisions than a normal ruler. This helps when making metal castings, because metal shrinks a little as it cools.

You can also find ruler software programs that measure pixels on a computer screen or phone. These are often called screen rulers.

Rulers in Geometry

In geometry, a ruler that has no marks on it (called a straightedge) can only be used to draw straight lines between two points. A straightedge is also helpful for drawing accurate graphs and tables.

A ruler and compass construction means drawing shapes using only an unmarked ruler and a compass. For example, you can use these tools to divide an angle into two equal parts. However, it's impossible to divide an angle into three equal parts using only a compass and an unmarked straightedge. This is a famous problem called angle trisection. If you are allowed to put two marks on the ruler, then it becomes possible!

History of Rulers

A wooden carpenter's rule and other tools found on the 16th-century ship Mary Rose.

Throughout history, people used many different units to measure distance. These units were often based on body parts, like the cubit (from elbow to fingertip), hand, or foot. These measurements changed depending on the time and place.

In the late 1700s, the metric system was created. It uses units like meters and centimetres, and it is now used in most countries around the world.

The oldest known measuring rod is a copper-alloy bar from about 2650 BCE. It was found in Nippur. Rulers made of Ivory were used by the Indus Valley Civilization before 1500 BCE. One such ruler found in Lothal (from 2400 BCE) was marked with very small divisions, about 1.6 millimetres (1/16 inch). Another ruler from Mohenjo-Daro was divided into units of about 33.5 millimetres (1.32 inches) with amazing accuracy.

Anton Ullrich invented the folding ruler in 1851. Later, Frank Hunt created the flexible ruler in 1902.