What this test can tell you
The ruler draws centimeter, millimeter and 1/16-inch scales from a calibration you set with a bank card or your screen diagonal, and measures the distance you drag.
Accuracy depends on your calibration, browser zoom and display scaling. Websites cannot read physical screen size, so uncalibrated sizes are only approximate.
How to use it
- Press “Calibrate” and hold a bank or ID card against the screen. Change the size until the outline matches the card exactly, then save.
- Or enter your screen diagonal in inches and press “Apply”, then save.
- Place the object at the left edge of the ruler, the zero line.
- Tap or click the ruler where the object ends, or drag along it, and read the length in cm and inches.
- On a phone, open fullscreen for a longer ruler along the long edge of the screen.
Why an online ruler needs calibration
Websites cannot read the physical size of your screen. Browsers use CSS pixels, defined as 96 per inch, but real screens have very different pixel densities and display scaling. Without calibration, the ruler can be noticeably too small or too large. A bank card is a good reference because ID-1 cards are standardised at 85.60 × 53.98 mm.
Tips for accurate measurements
Keep browser zoom at 100% and do not change display scaling after calibrating. Look straight at the screen to avoid parallax, and place objects gently to protect the display. If you move the window to another monitor, calibrate again for that screen.
Not working? Try these fixes
The ruler does not match a real ruler
Calibrate again with a bank card and check that browser zoom is 100%. Changing zoom or display scaling after calibration changes the ruler size.
The screen-size method is slightly off
Some devices report a rounded screen size to websites, and rounded corners or notches are part of the stated diagonal. The bank-card method is usually more precise.
The ruler is too short on my phone
Use the fullscreen button. Held upright, the phone shows a vertical ruler along its long edge; for longer objects, measure in several steps.
Frequently asked questions
Is this ruler actual size?
Careful calibration makes the scale approximate actual size on this screen. Accuracy depends on the reference, alignment, zoom and display scaling. No physical accuracy tolerance has been verified; check against a real ruler when precision matters.
How many centimeters are in an inch?
One inch is exactly 2.54 cm, and one centimeter is about 0.394 inches. The ruler shows both scales at once.
Is my calibration stored online?
No. It is saved in your browser’s local storage on this device and is never uploaded.