Online Ruler CM
A true-size on-screen ruler. Calibrate once with a credit card, then measure in cm or inches.
Step 1 — Calibrate your screen
Hold a real credit card (85.6 mm wide) against your screen, then move the slider until the blue box below exactly matches the width of your card.
Calibrate once per device — each screen has a different pixel density.
Step 2 — Measure
Using 3.78 px/mm calibration. Scroll sideways for longer lengths.
Tips for accurate measuring
- Take your time on calibration — a careful match gives near-millimeter accuracy.
- Keep the card flat against the screen, aligned with the top edge of the blue box.
- If you switch devices or zoom the page, calibrate again.
Related tools
You may also like our Tailor Measurement Chart and Screen Size Finder.
Frequently Asked Questions (FAQ)
How do I calibrate the online ruler?
Hold a real credit or debit card (85.6 mm wide) against your screen and move the calibration slider until the on-screen reference box exactly matches the card width. The tool then computes your screen pixels-per-cm and draws the ruler at true size. Calibration is saved in your browser.
How accurate is an on-screen ruler?
After careful calibration the ruler is typically accurate within about one millimeter, which is enough for quick everyday measuring. Accuracy depends on how precisely you match the reference box to your card, so take your time on the calibration step.
Does the ruler work on phones and tablets?
Yes. The ruler is mobile friendly and redraws for any screen size. Calibrate once per device, because each screen has a different pixel density. The ruler scrolls horizontally so longer measurements fit small screens.
Can I measure in inches instead of centimeters?
Yes. Use the unit toggle above the ruler to switch between centimeters (0 to 15 cm with millimeter ticks) and inches (0 to 6 inches with sixteenth-inch ticks). Both scales use the same saved calibration.
Is this online ruler free?
Yes, the online ruler is 100 percent free with no signup and no limits. Measure as often as you like.