Online ruler
Real size on any screen — calibrated with the card in your wallet.
Not calibrated yet: the ruler assumes 96 pixels per inch, so it may be off. Calibrate it once with any bank card.
Length: 0.0 cm (0 mm)
Drag the marker to measure
Nothing leaves your device
This tool runs entirely in your browser. What you type, play or record here stays on your device: nothing is uploaded or saved on a server.
Example
Example: calibrating on a laptop and measuring an SD card
Before calibration the ruler assumes 96 CSS pixels to the inch (3.78 px a millimetre), which few screens match. On this laptop the bank-card outline matched the real card at 323 px wide.
- Calibration: 323 px ÷ 85.6 mm = 3.77 px a millimetre, so on this screen a centimetre is 37.7 px and an inch 95.8 px.
- Lay the SD card along the top edge from zero and drag the marker to its end.
- Switch between centimetres and inches: the marker snaps to whole millimetres or sixteenths of an inch.
| Object | Real size | Centimetres | Inches |
|---|---|---|---|
| SD card (long side) | 32 mm | 3.2 cm (32 mm) | 1 1/4 in (1.25 in) |
| US quarter | 24.26 mm | 2.4 cm (24 mm) | 15/16 in (0.94 in) |
| Bank card (long side) | 85.6 mm | 8.6 cm (86 mm) | 3 3/8 in (3.38 in) |
| US business card | 88.9 mm | 8.9 cm (89 mm) | 3 1/2 in (3.50 in) |
The marker reads 3.2 cm (32 mm), or 1 1/4 in. Readings snap to the nearest millimetre or sixteenth, so a quarter, 24.26 mm across, shows as 2.4 cm or 15/16 in.
How it works
- Press Calibrate and hold any bank or ID card against the screen; resize the outline until it matches the card.
- Choose centimetres or inches. The ruler now shows real size, and the calibration is remembered for this screen.
- Lay the object along the top edge from zero, or drag the marker to read the length.
Frequently asked questions
- Why does the ruler need calibrating?
- Browsers don’t know how big your screen physically is, so a “centimetre” on the page varies from one screen to another. A bank card is the same size everywhere — 85.60 × 53.98 mm (ISO/IEC 7810 ID-1) — so matching it gives the true scale.
- How accurate is it?
- After calibration, to within about half a millimetre over the length of the screen — as good as your matching of the card’s edges. Zooming the page changes the scale, so it asks for a new calibration.
- Can I use it on a phone?
- Yes. The ruler is as long as the screen is wide; turn on the vertical ruler to measure along the longer side.
- What else can serve for calibration?
- Any ID-1 card: a credit or debit card, most driving licences and identity cards, a transport card. They all share the same size.
- How do I read inches and fractions?
- The longest marks are whole inches, then halves, quarters, eighths and sixteenths. The marker writes the length both ways: 1 1/4 in (1.25 in), 15/16 in (0.94 in).
- Can I use it without a card?
- Only roughly: until it is calibrated, the ruler assumes 96 pixels to the inch, the browser’s nominal size, which is often off on phones and laptops. Any card the size of a bank card gives the true scale in a few seconds.
- Is anything uploaded?
- No. It runs in your browser; nothing is sent anywhere.
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