Online ruler
30-second setup: match a card, then measure
Accurate for this screen
Hold the card flat against the screen, one corner on the outline's top-left corner, and press + or − until the outline matches the card's edges exactly.
For a laptop or monitor whose size you know. Enter the diagonal as sold — 13.3, 15.6, 24, 27. A card is more accurate, because the advertised size is rounded.
Kept on this device only
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Online ruler — actual size, in cm, mm and inches
Match the outline to a bank card once, and the ruler below is true to size on this screen. Hold the object against the glass and drag the two markers to its ends.
The short answer
To measure with an online ruler, first calibrate it: hold a bank card against the screen and resize the outline until it matches. Every bank card is 85.6 mm wide, so the page can work out the real size of your screen. Then hold the object against the ruler and drag the two markers to its ends — the length shows in centimetres, millimetres and inches. Without calibration an on-screen ruler is usually wrong, because browsers do not know how big the screen is.
Four steps to a true length
Step 1: Calibrate with a card
Hold any bank or ID card flat against the screen and press + or − until the outline matches its edges. A coin works too.
Step 2: Hold the object to the screen
Rest it against the ruler with one end at the first marker. Turn the ruler upright for tall things.
Step 3: Drag the markers
Move the two markers to the ends of the object. Arrow keys move a marker by a fifth of a millimetre.
Step 4: Read the length
The distance between the markers shows in cm, mm and inches, with the margin of error for your screen.
Everyday things of a known size
These sizes are fixed by a standard, which is what makes them usable for calibrating a screen — or for checking any ruler.
| Object | Size | In inches |
|---|---|---|
| Bank, credit or ID card (ISO ID-1) | 85.6 × 53.98 mm | 3.37 × 2.13 in |
| A4 sheet | 210 × 297 mm | 8.27 × 11.69 in |
| US Letter sheet | 215.9 × 279.4 mm | 8.50 × 11.00 in |
| ₹10 coin | 27 mm | 1.06 in |
| US quarter | 24.26 mm | 0.96 in |
| €1 coin | 23.25 mm | 0.92 in |
| €2 coin | 25.75 mm | 1.01 in |
Small things that move a reading
Look straight on
Glass has thickness, so from an angle the object and the ruler line shift against each other. Put your eye directly above the end you are reading.
Leave browser zoom alone
Zooming the page makes the ruler bigger without making a millimetre bigger. The page notices and asks you to calibrate again — it takes a few seconds.
A card beats a coin
The longer the reference, the smaller the error. A card is 85.6 mm; a coin is about 25 mm, so the same one-pixel slip costs three times as much.
For long things, mark and move
A screen is a short ruler. Measure up to the far marker, make a light pencil mark on the object, slide it along and add the lengths.
One number, kept on your device
The calibration is a single figure — pixels per millimetre for this screen — stored in your own browser. Nothing you measure is sent anywhere, and there is no account. The margin of error is shown beside every result, because a tool that hides it is guessing on your behalf.
- No upload, no account, no email
- Calibrate once for every measuring tool here
- Margin of error shown for your own screen
- Warns you when browser zoom would make it wrong
- No ads beside the ruler or the sizing circles
Questions people ask
Anything we've missed?
Is this ruler really actual size?
After calibration, yes — to within about a pixel, which the page states in millimetres for your screen: roughly 0.2 mm on a phone and 0.3 mm on a laptop. Before calibration it is not, and no on-screen ruler is, whatever it claims.
How do I measure without a ruler?
Use something of a known size. A bank card is 85.6 mm by 53.98 mm everywhere in the world, an A4 sheet is 210 by 297 mm, and a ₹10 coin is 27 mm across. This page uses the card to set its scale, then works as a ruler.
Can I use it on a phone?
Yes. A phone is a short ruler — about 6 to 7 cm across when upright — but the screen is sharp, so it is the more accurate one. Turn the phone sideways or use the vertical ruler for a little more length.
Why does the ruler change when I zoom?
Browser zoom scales every pixel on the page, the ruler included, while a real millimetre stays the same. The page detects the change and asks for a fresh calibration rather than showing a wrong ruler.
Why do I have to calibrate first?
Because a browser does not know how big your screen is. Web pages assume 96 pixels to the inch, and almost no real screen matches that, so an uncalibrated on-screen ruler can be out by 30% or more. Matching an outline to a bank card once tells the page how many pixels make a millimetre on your screen.
Do I have to calibrate every time?
No. The calibration is kept on this device, in this browser, and every measuring tool on Spolvero uses it. You are asked again only if the browser zoom changes, because zoom changes the size of everything on the page.
Is anything uploaded?
No. The calibration is one number stored in your own browser. Nothing you measure is sent anywhere, and the tools work with the connection switched off once the page has loaded.
Guides worth reading
- Measuring
ATM and credit card size in cm, mm and inches, and how to use a card as a ruler
Every ATM, debit and credit card is 8.56 × 5.398 cm (85.6 × 53.98 mm, or 3.370 × 2.125 inches). How to measure with one and set a screen ruler.
5 min read - Measuring
How to measure something without a ruler, using what is in your wallet
Bank cards, A4 paper and coins are made to exact sizes. How to use them as a ruler, how to turn your screen into an accurate one, and how far to trust each.
5 min read
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See everything on the Measure page.