Tap or drag on the ruler to measure
Set the real size with a bank card
Hold any bank card, ID card or loyalty card against the screen, on the blue card. Move the slider until both are exactly the same width.
No card? Type your screen size instead
This only works when the page uses the whole screen of this device, and it is less exact than the card.
About the online ruler
No ruler at hand? This page shows one on your screen in real size: centimetres and millimetres along the top edge, inches (down to 1/16) along the bottom.
A website cannot know how big your screen's pixels are — a phone, a laptop and a big monitor are all different. So you set it up once, with something everybody has: a bank card. All bank cards, ID cards and most loyalty cards are exactly the same size (85.60 × 53.98 mm). Match the card on the screen to the real one, press save, and the ruler is exact on that screen.
How to use it
- Hold a bank card flat against the screen, on the blue card
- Move the slider (or press − and +) until the blue card is exactly as wide as the real one
- Press It fits — save
- Put the thing you want to measure against the screen, with one end at 0 on the left edge
- Tap or drag on the ruler to put a red mark at the other end, and read the length
Tips for an exact result
- Keep the browser zoom at 100%. If you zoom in or out, the ruler asks you to set it up again — on purpose, because the size has changed.
- Match the edges carefully. Look straight at the screen, not from the side.
- One setup per screen. On a second monitor, set it up again there. Each screen is remembered.
- Use full screen for a longer ruler — on a laptop it is about 30 cm.
What people use it for
- Measuring a screw, a bolt or a nail
- The size of a ring, a coin or a button
- Checking a photo or a print size
- The width of a cable or a pencil
- Ring size: wrap a strip of paper round your finger, mark where it meets, lay it flat on the screen and measure
Frequently asked questions
Is the online ruler accurate?
After you set it up with a card, it is usually right to within half a millimetre over 10 cm. Without setup, the size is only a guess and can be wrong by 20% or more.
Why do I need a bank card?
Every screen has a different pixel size, and a web page cannot read the real size of the screen. A bank card is a size everyone has and that is the same all over the world, so it is the easiest way to tell the page how big a centimetre is on your screen.
I have no card. What can I use?
Open No card? Type your screen size instead and type the size of your screen in inches (it is often on the box, or in the computer's settings). This is less exact than a card.
Does it work on a phone?
Yes. A phone screen is narrower than a card, so the card stands up on its short side. The ruler on a phone is only about 5–6 cm long; turn the phone sideways or use full screen for a little more.
Is my card information read?
No. You only hold the card against the glass. The page never sees it, and nothing is sent anywhere.
Good to know
- The setup is saved in this browser, separately for each screen and zoom level
- Tap anywhere on the ruler to move the red mark; press Clear the mark to remove it
- Convert lengths with cm to inches and inches to cm
Related tools
- Unit Converter — length, weight, temperature and more
- cm to Inches — quick length conversion
- mm to Inches — for small sizes
- Change Image DPI — set the print size of a picture
- Screen Test — find dead pixels
