Online Ruler: Actual Size in cm, mm and Inches

Quick start in 3 steps

1. Calibrate. Press Calibrate at the top right of the page (on a phone, it sits above the ruler). Hold a credit card against the screen and match its edges to the outline. This takes about 20 seconds.
2. Pick a unit. Choose cm, mm, inches or pixels. You can also show cm and inches together.
3. Measure. Put your object against the ruler, with one end at the zero mark in the corner. Pull a guide out of the ruler to mark the other end. The length appears at the bottom of the ruler.

This online ruler works in the browser on a phone, a tablet and a computer. There is nothing to install.

Why most online rulers are wrong

A web page cannot see how big your screen is. It sees pixels. A CSS pixel is not a fixed length. The web standard says 96 CSS pixels make one inch, but that is a reference value, not a measurement of your hardware.

Real screens differ. A phone packs many pixels into a small panel. A laptop may scale its display to make text easier to read. A monitor may be set to a lower resolution than its panel supports. Each of these changes how long one pixel is in real life.

Many online rulers still draw at 96 pixels per inch and call it done. On a typical laptop that makes the ruler too short, and on a phone it can be far off. An actual size ruler needs one fact from you: how big something real looks on your screen. That is what calibration does.

Until you calibrate, this ruler shows a labeled estimate. It never says "calibrated" for a guess.

Pick a calibration method

You can calibrate in four ways. The credit card method is the one we suggest, because a card has a fixed, standard size and almost everyone has one.

How to read cm, mm and inches

On a metric ruler, the numbered marks are centimeters. The small marks between them are millimeters. There are 10 mm in 1 cm. To read a length, find the last numbered mark, then count the small marks past it.

An inch ruler is split into halves, quarters, eighths and sixteenths. The longer the mark, the larger the fraction. Count the small marks past the last whole inch, then reduce: 8 sixteenths is 1/2, and 10 sixteenths is 5/8. The chart below shows each step with its decimal and millimeter value.

When you pull two guides out of the ruler, the tool shows the span in your chosen unit and the other units too. For inches it gives the nearest 1/16 as a fraction, for example "3 5/8 in", next to the decimal value. See the pages for cm, mm and inches for more on each scale.

Check your scale with everyday objects

After you calibrate, the tool asks you to check the result. It draws a second object at true size. Lay a coin or card on the screen. If it fits, the calibration is good. If it does not, calibrate again.

You can also check at any time with something you own. A credit card is 85.60 by 53.98 mm. A sheet of US Letter paper is 215.9 mm wide, and A4 is 210 mm wide. The coin sizes below come from the US Mint and the European Commission.

What people measure with a screen ruler

A screen ruler suits small, flat things that you can hold against the glass. People use it for:

• Screws, bolts, washers and other small parts, to match a size.
• Beads, gemstones and jewelry parts, in millimeters.
• Photos, prints and stamps, to check the size before ordering a frame.
• Label and sticker sizes.
• Spacing on a design, when you want to see a layout at real size.

Hold the object flat and close to the glass. If it sits high above the screen, the angle of view makes it look larger or smaller than it is. Look straight at the edge you are reading.

A worked example: measuring a screw

Say you want the length of a wood screw. Here is the full routine, from start to finish.

1. Calibrate with a card and run the check step. The status chip should read "Calibrated, checked".
2. Choose millimeters. Screw sizes are often given in mm, such as 4 x 40.
3. Lay the screw flat on the glass, below the top ruler. Put the flat of the head against the left ruler, or against a guide set at zero.
4. Press Add guide, then drag the guide to the tip of the screw. The readout shows the length, for example "40 mm = 4 cm = 1 9/16 in".
5. Look straight down at the tip while you place the guide. If you look from the side, the guide appears to move.

Measure twice, and take the middle value if the two differ. A screen shows whole pixels, so a reading is only as fine as the pixel grid on your screen. On most phones the grid is finer than a tenth of a millimeter. On a standard monitor one pixel is about a quarter of a millimeter.

Common mistakes to avoid

• Skipping the calibration. The first screen is an estimate. Use the card step before you rely on a number.
• Changing the zoom afterwards. Chrome, Edge and Firefox keep the scale when you zoom the page, but Safari and display scaling changes can break it. If in doubt, calibrate again.
• Using a bent or worn card. A card with a warped edge is hard to line up. Pick a flat one. A coin is a good second check, because its edge is a clean circle.
• Looking from an angle. The glass has a small gap above the pixels. A sideways view moves the object relative to the marks. Look straight down.
• Measuring something thick. A tall object casts the edge away from the screen. Measure flat things, or measure at the base.
• Trusting a stretched screen. If your monitor is set to a resolution with a different shape than the panel, everything is stretched. Use the native resolution, or calibrate with the card, which shows the true size of what you see.

Which pages to use

All the pages share one tool and one saved calibration. Pick the page that matches the scale you read most. The cm ruler and the mm ruler suit metric work. The inch ruler reads fractions down to 1/16. The actual size ruler opens the calibration first and explains the method in detail. On a phone, open the phone ruler, which puts the ruler down the long edge of the screen.

Limits you should know

• Length. The ruler can only be as long as your screen. A 6.1 inch phone has a screen about 14 cm tall, so it fits small objects. A laptop screen is wider, from about 29 to 34 cm. For long objects, use a real ruler or a tape measure.
• Edges. The rulers touch the real glass edge only in full screen. Press F, or the Full screen button, so the rulers use the whole display. Phones in Safari may not offer full screen.
• Zoom. The ruler stores the physical size of a screen pixel, so page zoom is designed not to change the real size in Chrome, Edge and Firefox. We have not tested every browser and screen. In Safari, keep zoom at 100 percent (press Cmd and 0).
• More than one screen. A calibration belongs to one screen. If you move the window to a second monitor, the tool tells you and asks you to calibrate it or use a saved one.
• Curved glass. A thick bezel or curved edge can hide a little of the screen at the sides.

Privacy

Everything runs in your browser. The tool does not send your measurements or your calibration anywhere. Your calibration is saved in your browser, one small record for each screen, so you do not have to do it again. You can remove it with Reset calibration in the Options. If your browser blocks storage, the ruler still works, and you calibrate again next time.