Online ruler at actual size
This ruler shows centimetres, millimetres and inches at their real size on your screen. Browsers can't tell a website how big your screen is, so calibrate it once: hold any bank card or ID card to the screen and match the outline. It takes about ten seconds and the ruler remembers it on this device.
Runs in your browser: nothing is uploaded, it's free and there's no sign-up.
How do I measure with this online ruler?
- Calibrate once: tap Calibrate, hold a bank card, ID card or driver's licence flat against the screen, and drag the outline until it matches the card's edges.
- Lay the object on the screen or hold it against it, with one end on the zero mark.
- Drag the ruler to move it. Turn it with the round brass handle, or press R to turn it 15° at a time.
- Read the length where the object ends. Move your pointer or finger along the edge: the guide line shows the exact reading in both units.
- On a phone or tablet, tap Lock first, so an object resting on the screen can't move the ruler.
Questions people ask
Is this online ruler really actual size?
Yes, once you calibrate it. Before that it uses the browser's standard size of 96 CSS pixels per inch, which is close on some desk monitors but over a third too small on many phones, so the reading is marked as an estimate until you calibrate.
How do I calibrate it without a ruler?
Use any card the size of a bank card: a credit or debit card, a national ID card or a driver's licence. They all share one standard size, 85.60 × 53.98 mm (ISO/IEC 7810 ID-1), so any of them gives the same result.
Can I use it as a ruler on my phone?
Yes. Turn the phone sideways so a card fits across the screen, calibrate, then lock the ruler before you lay anything on it. On many popular phones the ruler can also start from the maker's published screen density, marked as an estimate until you check it with a card.
Does zooming the page change the size?
No. The calibration is kept in your screen's own pixels, and the ruler works out its scale again whenever you zoom with Ctrl or Cmd and plus or minus, or pinch on a touch screen, so 1 cm stays 1 cm.
How accurate is it?
As accurate as your calibration. Matching a card's edge to within a tenth of a millimetre typically gives a few tenths of a millimetre over 10 cm, and the chip next to the reading shows the margin worked out for your own calibration.
Is anything uploaded or saved online?
No. The ruler runs entirely in your browser. Your calibration is saved on this device only, there's no account, and measuring makes no network requests.
Does it work on a second monitor?
Yes. Each screen keeps its own calibration. When you move the window to another monitor, the ruler loads that screen's calibration or asks you to calibrate it once.
A short guide
Why does an on-screen ruler need calibrating?
Because a centimetre on a web page isn't a real centimetre. Pages are laid out in CSS pixels, and the standard fixes 96 of them to one inch on every screen. A phone packs its pixels far tighter than a desk monitor, so the same "inch" can measure 16 mm on one and 26.5 mm on the other. Matching a card tells the ruler how many pixels make a real millimetre on your screen.
What can I measure with it?
Anything small or flat enough to hold to the screen: screws, beads, a ring's inside diameter, a photo print, a seed packet, a page margin. For something longer than the screen, measure it in parts and add them up, or turn the phone sideways.
How do I get an exact reading?
Look straight at the screen rather than from an angle, so the object's edge and the tick line up. Put one end on the zero mark, keep the object flat, and read at the guide line. A thin edge, like a card's, is easier to line up than a thick, rounded one.