What "actual size" means on a screen
An actual size ruler shows lengths as they are in the real world. One centimeter on the screen should match one centimeter on a physical ruler. A web page cannot do this alone, because it does not know the physical size of your pixels.
This page asks you to teach it. You match an object of known size, a credit card, to an outline. From that match the tool works out how many device pixels fit in a millimeter on your screen. It saves that number for this screen and draws every ruler from it.
Why 96 PPI is wrong on most screens
PPI means pixels per inch. Web standards define a CSS pixel as 1/96 of an inch, so a simple ruler draws 96 pixels for each inch. That value is a convention. The panel in your device has its own density.
A laptop with a 13.3 inch, 2560 by 1600 panel, set to look like 1440 by 900, shows about 128 CSS pixels per inch. A ruler that draws 96 of them per inch ends up about a quarter too short. Phones are further away from 96. Change the display scaling and the number changes again.
So a fixed pixel count per inch cannot be right everywhere. The only fix is to measure on your own screen.
The credit card method
Credit and debit cards follow ISO/IEC 7810 ID-1: 85.60 by 53.98 mm. That is 3 3/8 by 2 1/8 inches. The size is the same all over the world.
- Press Calibrate at the top right of the page, then open the Card tab.
- Hold the card flat against the screen. Line up one edge with the outline.
- Drag the grip on the right edge until the other edge matches. Zoom in with your eyes: look straight at the screen, not at an angle.
- Use the 0.1 mm buttons, or the arrow keys on the grip, for fine changes.
- Press "My card matches".
On a narrow phone the card is shown on its short side, 53.98 mm, because the long side will not fit. The result is the same.
Check your scale
After the card step, the tool shows a coin outline at the size it now believes is right. A US quarter is 24.26 mm wide (US Mint). Lay a quarter on it. If it fits, press Yes. If not, press No and calibrate again.
This check uses a second object, so it can catch a mistake in the first match. Other methods work too: pick your device from the list, type your screen size, or use a coin or a sheet of paper. The device and screen size methods are approximate. They trust the size printed on the box.
| Method | Time | How exact | You need |
|---|---|---|---|
| Credit card | About 20 seconds | You match a real object. The most exact of the four. | A credit or debit card |
| Coin or paper | About 20 seconds | You match a real object. Paper needs a wide screen. | A coin, or a sheet of Letter or A4 |
| My device | Under 10 seconds | Approximate. It trusts the size listed for the device. | Your device name |
| Screen size | About 10 seconds | Approximate. It trusts the diagonal printed on the box. | The size in inches or cm |
Per screen, saved in your browser
The calibration is saved for this screen in your browser. A laptop and an external monitor each keep their own record. If you drag the window to another monitor, the tool notices and offers to calibrate it, or to reuse a saved one. Nothing is sent to a server.
Display scaling matters. If you change it, calibrate again. After calibration, go to the full online ruler to measure.
Pixels, CSS pixels and device pixels
Three words come up often, so here is what each one means.
- Device pixel. One physical dot on the panel. It has a real size, which depends on the hardware.
- CSS pixel. The unit a web page uses. Browsers map it to device pixels. On a laptop with a high-density panel, one CSS pixel may use four device pixels.
- Device pixel ratio. The number of device pixels in one CSS pixel along one side. Zooming the page changes it.
The tool stores device pixels per millimeter, which does not change when you zoom. That is why the scale holds in most browsers when you press Ctrl and plus. The card tells the tool how long a millimeter is in device pixels, and everything else follows from that.
How exact is the result?
We have not measured the accuracy on a set of devices yet, so we do not quote a figure. What we can say is how the error arises. When you match a card, you line up an edge by eye, so your match is only as good as your eyesight and the width of one pixel. The device and screen size methods trust a number printed by the maker, so their error is larger.
To keep the error small, hold the card flat and close to the glass, look straight down, and use the 0.1 mm buttons to fine-tune. Then run the check step. If a second object fits too, your scale is good for most small jobs.
When to calibrate again
Calibrate again if you change the display scaling or the resolution in the system settings, or if you plug in a new monitor. The tool notices a new screen and asks you. A calibration is also worth redoing if you used a quick method first and later need a better result.
You do not need to calibrate again after a restart, or on another day. The saved value stays in your browser until you reset it or clear the site data.