How accurate is an online ruler?

After a careful calibration with a card or coin it is accurate to about 1 mm. Lay a flat object on the screen and read it from straight above for the best result.

Do I have to calibrate the screen ruler?

Yes, for real size. Pixels differ in size between screens, so the browser cannot know how long 1 cm is until you match it once to a card or coin of known size.

What can I calibrate with if I have no card?

A coin, a banknote, a standard object such as A4 paper or an SD card, the screen's diagonal size, or a ruler of your own. On phones a coin or banknote works better, because the exact screen size is hard to know.

How do I measure cm with my phone?

Calibrate the screen, press "Lock the measuring area", lay the object on the screen and put cursors A and B on its ends. The distance is shown in mm, cm and inches.

How many cm is 1 inch?

One inch is 2.54 cm (25.4 mm). Choose "cm and inch" to see both scales side by side.

How do I measure something longer than my screen?

Move cursor B to the end of the screen, mark that spot on the object, press "Add this segment", then move the mark to cursor A and repeat. The total is added up for you.

Why does the 0 mark not start at the edge of the screen?

The screen frame and rounded corners make the very edge hard to measure from, so 0 sits 8 mm inside. Line objects up with the 0 mark, not the edge of the screen.

Do I need to calibrate every time?

No. The calibration is saved in this browser on this device and used again next time. Another device or browser needs its own calibration.

Can I use an actual size ruler on a computer monitor?

Yes. Hold a card against the screen to calibrate, or enter the monitor's diagonal in inches and its resolution in the calibration window.

Online Ruler — actual size cm and inch

An actual size cm, mm and inch ruler for your phone or monitor. Calibrate with a bank card or coin, measure between two cursors and add up long objects

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An online ruler that shows an actual size cm and inch scale on your phone or monitor. It is made for measuring small things such as screws, buttons, cards and photos when no ruler is at hand. Calibrate once with a bank card or a coin and the marks match real size; the distance between two cursors is read in mm, cm and inches at once, and objects longer than the screen can be measured in segments.

How to use

Press "Calibrate screen" in the bar at the top, pick a bank card, coin or banknote you have, match the outline on screen to the real thing and press "Save this size".
Under "Scale", choose "cm · mm", "inch" or "cm and inch", and under "Direction", choose "Horizontal" or "Vertical". On a phone, "Vertical" gives the longer ruler.
If the object will lie on the screen, press "Lock the measuring area" first, so touching the screen does not move the cursors.
Line one end of the object up with the 0 mark, or drag cursors A and B to its two ends. While locked, move the cursors with the − / + buttons or the mm number boxes.
The distance between the cursors appears in large type, with cm, inches and the nearest 1/16 inch fraction underneath.
For something longer than the screen, open "Measure something longer than the screen", press "Add this segment" for each part and read the total.
Press "Reset cursors" to put the cursors back where they started, or "Full screen" to use the whole display.

How it works

Marks are 1 mm apart (longer at every 5 and 10 mm) and 1/16 inch apart on the inch scale; the cursors move in steps of 0.1 mm. After selecting a cursor, the arrow keys move it by 0.1 mm, or 1 mm with Shift.
The 0 mark sits 8 mm inside the edge, away from the screen frame. An object pushed against the edge of the screen reads too long, so line it up with 0 or a cursor instead.
Without calibration the ruler uses the browser default (1 inch = 96 CSS pixels), which may not match your screen. Page zoom is corrected automatically.
The calibration and settings are kept only in this browser on this device and are not sent anywhere. The site's other measuring tools, such as the protractor and ring sizer, use the same calibration.
Under "Compare with a known object", choosing a card or coin draws its length from the 0 mark, so you can hold the real one against it and check the calibration at a glance.

Why 1 cm on an online ruler is not always 1 cm

Web standards define 1 inch as 96 CSS pixels, so a web page always draws 1 cm as about 37.8 CSS pixels. How long one CSS pixel really is depends on the screen size, its resolution and the scaling setting of the operating system. That is why a ruler drawn in CSS centimetres comes out at a different length on every device.

Take a 15.6-inch laptop at 1920×1080: its pixel density is about 141 ppi. At 100 % scaling, 96 CSS pixels are about 0.68 inch, so 1 cm on screen is really about 6.8 mm; at 150 % scaling it is about 10.2 mm. Matching the screen once to an object of exactly known size, such as a bank card (85.60 × 53.98 mm) or a coin, removes this difference.

How to measure with your phone screen and keep the error small

Small flat objects are most accurate when laid directly on the screen. Press "Lock the measuring area" so the object cannot push the cursors, put one end on the 0 mark or cursor A and move cursor B to the other end. Read with your eye straight above the screen; viewed at an angle, the ends of a thick object look shifted.

With a careful calibration the error is about 1 mm. Calibrate with something long, such as a card or a banknote, rather than a small coin: the same 0.5 mm misalignment is a much smaller share of a long object. Holding the phone upright and choosing "Vertical" runs the ruler along the long side of the screen, so more fits in one go.

Converting cm to inches and reading 1/16 inch marks

One inch is defined as exactly 25.4 mm (2.54 cm). The inch scale is divided into sixteenths, each about 1.59 mm, and the 1/2, 1/4 and 1/8 inch marks get progressively longer so they are easy to find.

The ruler shows decimals and fractions together. A gap of 30 mm reads 1.181 in, with the nearest sixteenth, about 1 3/16 in (1.1875 in), shown next to it. When you compare US drawings or product sizes from overseas shops, choose "cm and inch" to see the cm scale on top and the inch scale below it, side by side.

Worked example: measuring something longer than the screen

Say you measure the short side of an A4 sheet (210 mm) on a phone. Put the edge of the paper at cursor A and move cursor B as far as the screen allows, for example to 120.0 mm. Mark that spot on the paper with a pencil and press "Add this segment". Move the paper so the mark sits at cursor A, put cursor B on the far edge and it reads 90.0 mm; the total shows 210.0 mm.

The last segment counts toward the total without being added. Remove a mistaken segment with "Remove last segment", or start over with "Clear total". Every mark adds a little error, so use as few segments as you can, and reach for a tape measure for anything longer than about a metre.

Questions and answers

How accurate is an online ruler?: After a careful calibration with a card or coin it is accurate to about 1 mm. Lay a flat object on the screen and read it from straight above for the best result.
Do I have to calibrate the screen ruler?: Yes, for real size. Pixels differ in size between screens, so the browser cannot know how long 1 cm is until you match it once to a card or coin of known size.
What can I calibrate with if I have no card?: A coin, a banknote, a standard object such as A4 paper or an SD card, the screen's diagonal size, or a ruler of your own. On phones a coin or banknote works better, because the exact screen size is hard to know.
How do I measure cm with my phone?: Calibrate the screen, press "Lock the measuring area", lay the object on the screen and put cursors A and B on its ends. The distance is shown in mm, cm and inches.
How many cm is 1 inch?: One inch is 2.54 cm (25.4 mm). Choose "cm and inch" to see both scales side by side.
How do I measure something longer than my screen?: Move cursor B to the end of the screen, mark that spot on the object, press "Add this segment", then move the mark to cursor A and repeat. The total is added up for you.
Why does the 0 mark not start at the edge of the screen?: The screen frame and rounded corners make the very edge hard to measure from, so 0 sits 8 mm inside. Line objects up with the 0 mark, not the edge of the screen.
Do I need to calibrate every time?: No. The calibration is saved in this browser on this device and used again next time. Another device or browser needs its own calibration.
Can I use an actual size ruler on a computer monitor?: Yes. Hold a card against the screen to calibrate, or enter the monitor's diagonal in inches and its resolution in the calibration window.