Why an online ruler needs calibrating
Web pages are drawn in "CSS pixels", and how many of those fit into a real inch depends on your screen size, resolution and zoom level. A 13-inch laptop and a 27-inch monitor can both report the same pixel width. No website can know your physical screen size, so any online ruler that claims to be actual size without calibration is guessing. With the 20-second card calibration above, this one matches your screen to within a fraction of a millimetre.
- Open Calibrate and hold a credit, debit or ID card flat against the screen.
- Drag the slider (or use − / +) until the blue card exactly matches your card's width.
- Press Save calibration. The setting stays on this device and browser until you reset it.
Keep browser zoom at 100% (Ctrl/Cmd + 0). Changing zoom changes the size of everything on the page. If you zoom, calibrate again.
Measuring with the screen ruler
- Tap or drag anywhere on the ruler to move marker B. Drag near marker A to move the starting point, which helps when the object does not start at zero.
- On phones, switch to Vertical to use the full height of the screen.
- With a keyboard, the arrow keys move B by 1/16" or 1 mm. Hold Shift to move A.
Measuring something longer than your screen
Use the running total, which works like an online measuring tape built from parts. Measure the first part from A to B and press Add A–B to total. Then move the object along so the point you reached sits at A, measure the next part, and add it again. The total shows the full length in inches and centimetres. For long or curved things, the printable tape measure is more practical.
Good for screws, coins, jewellery, SD cards, pills, buttons and anything else small enough to hold against the screen. Switch to cm / mm for a millimetre ruler online.
Frequently asked questions
Is the online ruler accurate on a phone?
Yes, after calibration. Phone screens are dense, so marks are sharp, and Vertical mode gives you 12–15 cm of length on most phones.
Which coins can I calibrate with?
US quarter and penny, 1 euro, UK £1, India ₹10, Canada $1 and Australia $1. A bank card is the most accurate option because it is the widest standard object.
Does it work offline or store my data?
The ruler runs entirely in your browser. The only thing saved is your calibration value, stored in your browser on this device.
Last reviewed 2026-10-07.