Measure / Screen
Online Ruler (Actual Size)
A ruler in centimetres or inches on your screen, calibrated to actual size with a bank card, for measuring small objects without a tape.
Online Ruler (Actual Size): Screens differ in pixel density, and browsers do not reveal a screen's physical size, so no web page can know how long a centimetre is on yours. The calibration fixes that: ID-1 cards, the size of almost every bank card, ID card, and driving licence, are 85.60 mm wide, so matching one sets the scale. Centimetres are marked every millimetre and inches every sixteenth. Runs 100% locally in your browser with zero server file uploads.
- Category
- Everyday tools
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Runs entirely in your browser
Cards in the ID-1 size, which almost all bank cards, ID cards, and driving licences use, are 85.60 × 53.98 mm.
Why a card works
Bank, ID, and loyalty cards share the ID-1 format of the international standard ISO/IEC 7810: 85.60 by 53.98 millimetres, with tight tolerances. That makes them the one object of known size almost everyone has to hand, and a good reference for the screen.
The calibration belongs to one screen at one zoom level. If the ruler looks wrong after you zoom, connect another monitor, or move the window, calibrate again. The monitor test helps with other screen checks, such as dead pixels.
Converting what you measure
To switch a measurement between systems, use centimetres to inches or inches to centimetres. For drawing to scale on paper, print graph paper, which is generated at exact millimetre or inch spacing.
How to use it
- Press Calibrate with a card and hold a bank card, ID card, or driving licence against the screen.
- Drag the slider until the outline matches the card exactly, then save.
- Hold the object against the screen at the 0 mark and read its length.
Privacy & limitations
The calibration and your choice of units are stored only in this browser.
Related tools
Frequently asked questions
Why does the ruler need calibrating?
A web page works in CSS pixels, which the browser maps to the screen's real pixels differently on every device. Without calibration the page assumes 96 pixels to the inch, which is only roughly right on some desktop screens.
What can I use instead of a bank card?
Any ID-1 card: an ID card, driving licence, library card, or most membership and gift cards are the same 85.60 × 53.98 mm. A card with a raised logo or worn corners is fine; match the edges.
How accurate is it?
After careful calibration, to about half a millimetre over a short length, limited by how precisely you can line up the card and the size of the screen's pixels. Calibrate again after changing the browser zoom or moving the window to another screen.
Can I measure something longer than the screen?
Measure in sections and add them up, or use a tape. To switch between units, use the centimetres to inches converter.
Free tool · runs in your browser · no account required