Online Ruler
Measure with an actual-size online ruler in cm, mm and inches — calibrate with a bank card or your screen size for real accuracy. Free online ruler.
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Free, no sign-up, works in the browser — send it to a friend or add it to your site.
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Part of the toolkit
How to use Online Ruler
- 1Press Calibrate and hold a bank card against the screen, then drag the slider until the box matches it — or enter your screen size
- 2Choose centimetres or inches
- 3Place the object against the screen at the zero mark and read its length; move over the ruler for an exact readout
Frequently asked questions
Is this online ruler actual size?▾
After calibration, yes. Browsers assume 96 pixels per inch, but real screens range from about 90 to over 450, so an uncalibrated ruler can be off. Calibrating with a bank card takes seconds and makes it accurate to about a millimetre.
How do I calibrate with a credit card?▾
All bank, credit and ID cards are the same standard size — 85.60 × 53.98 mm (ISO/IEC 7810 ID-1). Hold one against the screen and drag the slider until the outline matches its width exactly, then save.
How can I measure something without a ruler?▾
Use this screen ruler for small objects, or common items with known sizes: a bank card is 8.56 cm wide, a US dollar bill 15.6 cm (6.14 in), an A4 sheet 21 × 29.7 cm and a US letter sheet 8.5 × 11 in.
How many centimetres are in an inch?▾
Exactly 2.54 cm (25.4 mm) — the inch has been defined that way since 1959. So 1 cm is about 0.394 in and 12 in (1 ft) is 30.48 cm.
Why does the ruler look wrong after I zoom?▾
Browser zoom scales the page, including the ruler. Keep zoom at 100% (Ctrl/Cmd + 0) when measuring, or recalibrate at the zoom you use.
Can I use it on my phone?▾
Yes. Calibrate once with a card and the ruler works across the width of your phone screen — about 6–7 cm in portrait, more if you rotate to landscape.
Embed Online Ruler on your website▾
Paste this snippet into your page. The embedded tool runs in your visitor's browser and shows a small “Powered by clkr.work” link.