Online ruler (actual size)

Built by Nitin Raja · Last updated October 24, 2026

How to use an online ruler at actual size

Most "actual size" rulers on the web are not, unless you set them up. A web page draws in CSS pixels, which are defined as 96 to the inch, but real screens ignore that: a phone may fit about 160 CSS pixels into an inch and a big monitor fewer than 96. Browser zoom and the operating system's display scaling stretch things again. So the same ruler image can be 30% too small on one screen and 20% too large on another, and the browser has no way to know which.

The fix is to show the page something of a known size. This ruler uses a bank card, because every bank, credit, debit and ID card in the world is made to the same standard, ISO/IEC 7810 ID-1: 85.60 by 53.98 mm. You hold the card against the screen and stretch an outline until it matches. From then on the page knows exactly how many pixels make a millimetre, and draws the ruler to that scale.

Calibrating with a card, step by step

1. Pick any standard card: bank, credit, debit, ID, driving licence, library or gift card.
2. Hold it flat against the screen, with its top-left corner in the outline's top-left corner. On a phone, hold it upright.
3. Drag the orange handle until the outline's right edge lines up with the card's right edge. Use the + and - buttons for the last pixel or two.
4. Save. The calibration stays in this browser, and the ring sizer uses it too.

After saving, the page shows the screen size that your calibration implies, measured across the diagonal. If it says 13.3 in and you have a 13.3 inch laptop, you got it right. If it is far off, calibrate again.

Reading the centimetre and millimetre scale

The top edge (or the left edge, when vertical) is metric. The longest lines are whole centimetres, numbered; the medium lines are half centimetres (5 mm); the short lines are single millimetres. A reading of 4 numbered lines plus 7 short ones is 4.7 cm, or 47 mm.

Reading inches to 1/16

The bottom edge (or the right edge, when vertical) is in inches, with each inch split into 16. The line halfway between two numbers is 1/2 inch; the next shorter lines are quarters, then eighths, and the shortest are sixteenths. To read a length, take the last whole inch and count the sixteenths after it, then simplify: 2 inches and 10/16 is 2 5/8 in. The readout above the ruler gives both the fraction and the decimal, so you do not have to count.

Tips for accurate measuring

• Lay the object flat on the screen with one end exactly on the 0 line. On many phones the screen's rounded corners and frame sit close to 0, so leave a small gap and subtract it, or start from the 1 cm line.
• Look straight down. The glass is a millimetre or so above the image, so looking from an angle shifts the line you see.
• Turn the ruler vertical on a phone. Phones are taller than they are wide, so the vertical ruler gives you more length.
• Keep the browser zoom at 100% once you have calibrated, or calibrate again after changing it.

What it is good for

Small things: screws and bolts, jewellery, buttons, a strip of paper wrapped round your finger for a ring size, the size of a photo or a card, or checking a measurement in a listing. For anything longer than the screen, or where a millimetre really matters, use a physical ruler or tape. To size a ring, the ring sizer turns your measurement into US, UK, EU and Japanese sizes.

Why we built this online ruler

Before building CheckMyHeight, I tested a lot of height and sizing websites to see what people actually need from them and what just gets in the way. "Actual size" rulers are a clear example. A web page draws in CSS pixels, defined as 96 to the inch, but real screens do not follow that, and browser zoom and display scaling change it again. A ruler that skips calibration can be 20 or 30% off and gives no sign that it is wrong. We wanted a ruler that is set to your own screen in under a minute with a card from your wallet, and that tells you whether the calibration looks right.

What this online ruler measures

• Calibration: you stretch an outline to match a bank or ID card (ISO/IEC 7810 ID-1, 85.60 x 53.98 mm), which sets the number of pixels per millimetre. As a check, the page shows the screen diagonal that this implies, so you can compare it with the real size of your display.
• The ruler: millimetres and centimetres on one edge, inches split into sixteenths on the other, drawn at that scale.
• The reading: tap or drag the marker to the end of the object; the readout gives centimetres, millimetres, the inch fraction to the nearest 1/16 and decimal inches.
• Zoom and screens: if the browser zoom changes on the same screen, the scale is corrected; on a different screen, the page asks you to calibrate again.

Last checked on 24 October 2026. It does not measure anything longer than your screen, anything through the camera, or angles. An outline 1 mm off the card makes a 10 cm reading about 1 to 2 mm off.

Why this online ruler is different

• Calibrated to your screen with a card, instead of assuming 96 pixels per inch.
• A built-in check: the implied screen size tells you when the calibration is off.
• A vertical mode for phones, which have more length down the screen than across it; a phone held upright starts in it.
• Both scales at once, with the inch fraction worked out for you.
• Nothing to print or install, no account. The calibration stays in your browser and is shared with the ring sizer.