Uncalibrated — sizes are approximate. Click Calibrate above and match a credit card to your screen for true actual-size measurements.
Uncalibrated rulers use a rough estimate of your screen's pixel density and are approximate. Calibration is stored only in your browser (localStorage). No camera, no uploads, no network calls.
Quick Answer
An online ruler displays real measurement units (centimeters and inches) directly on your screen so you can measure small physical objects against it. Because screens vary in pixel density, first calibrate: hold a credit card (85.60 mm wide) against the display and drag a slider until the on-screen card matches. The correction is saved in your browser, and every measurement afterwards reflects true actual size — all computed locally, nothing is uploaded.
How It Works
Calibrate: hold a credit card (85.60 mm edge) against the screen and drag the slider until the outlined card matches, then click Apply
Pick cm or inch mode — ticks and labels redraw at true scale
Place your object flat against the horizontal or vertical ruler edge and read its size
Toggle fullscreen for longer objects; your calibration is saved in this browser
Key Facts
- Shows centimeters with millimeter subdivisions and inches with 1/4" ticks
- Two calibration methods: automatic screen estimate or credit-card matching
- Credit-card calibration uses the ISO ID-1 standard width of 85.60 mm
- Horizontal and vertical rulers with a fullscreen measuring mode
- Calibration is stored in your browser via localStorage — set it once
- Uncalibrated mode is clearly flagged as approximate, not actual size
- 100% client-side: no camera, no uploads, no data leaves your device
- Free to use with no sign-up and no limits
Frequently Asked Questions
Is an online ruler actually accurate?
Only after calibration. Out of the box the ruler uses an estimate of your screen's pixel density, which can be off by 10–30%. Hold a standard credit card (85.60 mm wide edge) against the screen and drag the slider in the Calibrate panel until the outlined card matches the physical one — the ruler is then accurate to within about a millimeter on most displays.
Why is a credit card used for calibration?
Credit cards follow the ISO/IEC 7810 ID-1 standard, so their short edge is almost exactly 85.60 mm worldwide. When you match that edge on screen, the tool derives an exact pixels-per-millimeter ratio for your display and saves it in your browser for future visits.
Do I have to calibrate every time I visit?
No. Your calibration is saved in localStorage on your own device and re-applied automatically when you return. Use the Reset button to discard it and go back to the screen estimate.
What do the tick marks mean?
In centimeter mode, large numbered ticks appear every 10 mm (1 cm), medium ticks every 5 mm, and small ticks every millimeter. In inch mode, large numbered ticks mark each inch, medium ticks mark half inches, and small ticks mark quarter inches.
Does this tool use my camera or send anything to a server?
No. There are no camera permissions, no network calls during measurement, and no uploads of any kind. All drawing and calibration math runs entirely in your browser, which makes it safe to use on any device.
What objects can I measure with it?
Anything that fits on your screen and can be held flat against the display: coins, keys, screws, jewelry, phone widths, photo frames, or craft pieces. For best accuracy, view the screen straight on (not at an angle) and place the object against the ruler edge.