How to use the online ruler
- Grab a standard card. Any credit, debit or ID card works. They are all 85.6 mm wide.
- Match the box to the card. Hold the card against the glass and move the slider until the colored box lines up with both edges, then tap Save calibration.
- Pick units and direction. Choose centimeters or inches, and horizontal or vertical. Vertical is handy for tall items.
- Move the markers. Drag the two blue markers to the ends of the object. The distance between them updates live.
Why an online ruler needs calibration
Web pages measure in CSS pixels, and a CSS pixel is not a fixed physical size. Phones, tablets and monitors all pack a different number of pixels per inch, and browsers scale them again. That is why a ruler that is correct on one screen is wrong on another. Calibrating against an object of known size, such as a bank card, fixes it for the device you are holding.
How accurate is it?
With a careful calibration you can usually get within about one millimeter on a phone, which is good enough for jewelry, screws, ring sizes, small parts and general household measurements. For anything critical, confirm with a physical ruler or calipers. Calibration is stored in your browser only, so you do not have to repeat it next time unless you change browser zoom.
Tips
- Keep browser zoom at 100%. Changing zoom after calibrating makes the ruler wrong, so calibrate again.
- Remove thick cases or screen protectors that lift the card above the glass.
- Use the diagonal method only as a fallback. It assumes square pixels and a screen edge-to-edge.
Looking for a pixel density figure for your device? Use the PPI calculator.
Frequently asked questions
Calibrate it with an object of known size. Hold a bank card against the screen, match the on-screen box to its width, and save. The ruler then uses real millimeters for your screen.
Web pages use CSS pixels, which do not map to a fixed physical length across devices. Without calibration, any online ruler is only approximate.
A standard ID-1 bank card is 85.6 mm wide and 53.98 mm tall, or about 3.37 by 2.13 inches.
Yes. It is designed for touch. You can measure objects up to the size of your screen, and the vertical mode gives you more length on a tall phone.
No. The pixel size on your screen stays the same, so your calibration works in both orientations.
It is saved in your browser's local storage on your device only. Clearing site data removes it.