Online ruler, actual size

Is it really actual size?

Screens differ, so no website can know the true size of your pixels on its own. That is why this ruler asks you to set it once. Hold a bank card, a library card or a driving licence against the screen and drag the slider until the box matches the card's long edge. These cards all share one international size, 85.6 mm by 54 mm, so the ruler can work out exactly how many pixels make a centimeter on your screen. If you do not have a card to hand, type your screen's diagonal size instead; that is usually close, but the card is more exact.

The setting is saved in this browser only, so it is ready next time. Keep the browser zoom at 100%, because zooming in or out stretches the ruler. On a phone, turn it sideways for a longer ruler.

How to measure with it

• Lay the object flat against the screen with one end on the purple marker.
• Drag the orange marker to the other end, or use the arrow buttons to move it one millimeter at a time.
• Read the length under the ruler in inches (to the nearest sixteenth), in decimal inches, and in centimeters and millimeters.
• Press Start again to put the markers back at the left.

Reading inches and centimeters

The inch side has a long mark for each whole inch, a slightly shorter one for each half, then quarters, eighths and the shortest marks for sixteenths. A length of 2 and 5 small spaces past the 2 is 2 5/16 inches. The centimeter side has 10 equal millimeter spaces in every centimeter, with a middle mark at 5 mm, so 4 centimeters and 7 small spaces is 4.7 cm or 47 mm. One inch is exactly 2.54 cm.

For children learning to measure

Children learning to measure for the first time do better with the practice ruler, which gives them objects to measure, checks the answer, and explains what went wrong, such as starting at 1 instead of 0. This actual-size ruler is the everyday tool for measuring something real: a stamp for a collection, a screw, a photo print, or a drawing for a school project. Reading fractions of an inch is also good fraction practice, since every mark is a point on a number line.