Online ruler
Measure real objects against your screen in inches, centimeters, and millimeters. Calibrate once with any bank card and the scale is saved for next time.
Calibrate with a bank card
Hold any credit or debit card against the outline and drag the slider until they match — every bank card in the world is exactly 85.6 mm wide. Your setting is saved on this device.
Current scale: 96.0 pixels per inch · 3.78 px per mm. Keep browser zoom at 100% — zoom changes the scale.
Online ruler FAQ
- How accurate is an online ruler?
- Uncalibrated, not very — browsers assume 96 pixels per inch, which almost no modern screen actually has. After matching the outline to a real bank card it's typically accurate to well under a millimeter, because every ID-1 card on earth is exactly 85.6 mm wide. Keep browser zoom at 100%, since zooming rescales everything.
- Why does the ruler need calibrating?
- A web page can read your resolution but not the physical size of your screen — the same 1920 pixels might span a 13-inch laptop or a 27-inch monitor. Calibration pins down how many pixels equal a real inch on your particular display. We save the setting locally so you only do it once per device.
- Can I measure something longer than my screen?
- Only up to the ruler's on-screen length. On a phone, rotate to landscape for extra reach; on a desktop, press F11 for fullscreen to squeeze out the window borders. For anything longer, measure in stages or admit defeat and find a tape measure.