A real size ruler on your screen in centimetres and inches, calibrated in seconds with any bank card
How to use
Hold any bank, credit or ID card flat against your screen and drag the slider until the outline is exactly as wide as the card.
Press It matches my card. The calibration is saved for this screen, so you only do it once.
Place the object against the screen with one edge on the left marker.
Drag the second marker to the other edge and read the length in centimetres, inches and tape measure fractions.
Why use this tool
Actual size, not a guess: Websites cannot know how big your pixels are, so an uncalibrated ruler is wrong on almost every screen. A bank card is the same size in every country, which makes it a perfect reference.
Centimetres and inches together: Millimetre marks on the top edge and sixteenth of an inch marks on the bottom edge, like a real ruler.
Measuring markers: Drag two handles to measure anything between them, with the result in cm, mm, decimal inches and the nearest fraction.
No card? Use your screen size: On a laptop or monitor, type the diagonal from the model sticker and the ruler sets itself.
Remembered per screen: Each screen and scaling setting keeps its own calibration, so a laptop and an external monitor both stay accurate.
Frequently asked questions
Is this ruler really actual size?
Yes, once calibrated. Before calibration it uses the browser default of 96 pixels per inch, which is rarely the true size of your screen. Matching a real card fixes it to within a fraction of a millimetre.
Why a bank card?
Bank cards, credit cards, driving licences and most ID cards follow the ISO/IEC 7810 ID-1 standard: exactly 85.6 mm by 53.98 mm everywhere in the world.
Does browser zoom change the measurement?
Yes. Zooming makes everything on the page bigger or smaller, so keep zoom at 100%. The ruler notices a zoom change and asks you to calibrate again.
Does it work on a phone?
Yes. Calibrate with a card the same way. A phone screen is short, so it measures up to about 6 to 7 cm, which is fine for screws, coins, jewellery and small parts.
Does the ruler send anything anywhere?
No. The calibration is a single number saved in your own browser.