Online ruler in centimeters and inches
Centimeters along the top edge, inches in sixteenths along the bottom. Calibrate it once with a bank card and the marks match real millimeters on your screen.
Tap or drag along the ruler to place a marker and read the length in both units.
Calibrate with a bank card
Hold a bank card flat against the screen, over the dashed outline. Move the slider until the outline matches the card’s edges exactly, then save. Turn the card over first: only its size matters.
Reviewed on by the Skala Convert team
Why does an on-screen ruler need calibrating?
Because your browser does not know how big its pixels are. The W3C’s CSS Values and Units specification fixes one inch at exactly 96 CSS pixels and one centimeter at 96 ÷ 2.54 of them — a convention tied to a reference viewing distance, not to the glass in front of you. On a typical desktop monitor that lands close to a real centimeter. On a phone, where pixels are much smaller, an uncalibrated “centimeter” can come out around six millimeters long.
Calibrating replaces the convention with a measurement. You compare an object of known size with the screen, and the ruler redraws itself at your display’s real density.
Why a bank card?
Its size is fixed by an international standard. ISO/IEC 7810 defines the ID-1 format as 85.60 × 53.98 mm with 3.18 mm rounded corners, and bank cards everywhere are made to it. The standard allows only fractions of a millimeter of tolerance, which is finer than you can judge by eye against a screen.
Match the outline to the card’s long edge on a laptop or monitor, where the outline lies flat; on a phone it stands upright so the whole card fits.
How to read the inch scale
The inch side is divided by halving, not by tenths: halves, quarters, eighths and sixteenths, each with a shorter mark than the one before. The longest mark between two numbers is the half inch; the shortest are sixteenths. When you place the marker, the reading gives the nearest sixteenth — the way a US tape measure is read — next to the decimal value, which is what you need for the inches to centimeters converter.
When not to trust it
Treat a calibrated screen ruler as good to about a millimeter. The glass sits a little above the pixels, so looking at an angle shifts the reading, and pinch-zooming on a phone scales the ruler with everything else. For sewing, crafts, checking a screw or a ring against a size chart, that is enough. For machining, medical devices or anything where a millimeter matters, use a physical ruler or caliper — and for small parts the millimeters to inches page is the more precise route.
Frequently asked questions
Is an online ruler accurate?
Only after calibration, and only to about a millimeter. Browsers define an inch as 96 CSS pixels rather than a physical length, so an uncalibrated on-screen ruler is usually off — most visibly on phones.
What if I do not have a bank card?
Any plastic card in the ID-1 format works: bank cards, many driving licenses, and most ID and membership cards share the same 85.60 × 53.98 mm size.
Will the calibration work on my other devices?
No. It is saved in this browser only, and the ruler asks you to calibrate again if the browser zoom changes, because zoom changes the size of every pixel on the page.