1. Calibrate to your screen
Hold a standard card (credit / bank / IC card — they are all 85.6 mm wide) against the screen and drag the slider until the outline matches its width exactly.
Your screen: —
2. Read the ruler — shown at actual size
Both centimetres and inches are shown. Your calibration is saved on this device only — nothing is uploaded. For accurate sizes keep the browser zoom at 100%.
A ruler that displays at actual (real-world) size right in your browser. Because screen pixel density (DPI / pixels per inch) differs from one device to another, you calibrate once: credit, bank and IC cards are all exactly 85.6 mm wide (ISO/IEC 7810 ID-1), so hold a card against the screen and drag the slider until the outline matches its width — that solves the pixels-per-millimetre of your screen. After calibration the ruler is drawn with both a centimetre scale (top edge) and an inch scale (bottom edge) at their true sizes, so you can measure length even without a physical ruler nearby. Switch between horizontal and vertical, and use it on a large computer monitor or on a phone. Your calibration is stored on your device (in the browser) only, so it is ready next time; nothing you enter or calibrate is ever sent to a server — it all stays on your device. For accurate measurements keep the browser zoom at 100%.
How to use
- Hold a standard card (credit / bank / IC card — all 85.6 mm wide) against your screen.
- Drag the slider until the outline matches the card's width to calibrate.
- The cm and inch ruler is drawn at actual size. Toggle horizontal/vertical (calibration saved on-device, never sent).
FAQ
What kind of card can I use to calibrate?
Credit cards, bank/debit cards, ID cards and IC transit cards all follow the ISO/IEC 7810 ID-1 standard: 85.6 mm wide and 53.98 mm tall. Any of those cards in your wallet will work for calibration.
Is my calibration or what I measure sent anywhere?
No. The calibration value (the pixels-per-millimetre derived from the card width) is stored only on your device in the browser, and the ruler is drawn entirely in your browser. Nothing is ever sent to a server or API, and it is ready to reuse next time.
Why do I have to calibrate first?
The same '1 pixel' covers a different real-world length on different screens, depending on size and resolution. Calibrating once with a card solves the pixels-per-millimetre of your screen so the ruler can show centimetres and inches at their true size.
Does it work on phones and tablets?
Yes. It works on a large computer monitor as well as on phones and tablets — just calibrate once per device. For accurate measurements, keep the browser zoom at 100%.