Screen Ruler: Actual-Size cm and Inch Ruler Calibrated with a Credit Card
Hold a credit card (ISO/IEC 7810 ID-1, 85.60 × 53.98 mm) up to your screen and match the frame to it. The tool uses that to work out your screen's real size and draws a ruler in cm, mm and inches. Handy for measuring a small object against your display. To work out pixel density from a screen's physical size and resolution, try the Screen Size Calculator. Your calibration is stored only in this browser and nothing is sent to a server.
🔒 Runs in your browser · Your data is never sent to a server
Calibrate your screen
Resize the frame below until it matches a credit card (or any ID-1 card, such as a driver's license) held against the screen. When the card's long edge equals the frame width (85.6 mm), you are done.
Your calibration is saved in this browser automatically.
Ruler
How to use
- Hold a credit card against the screen and use the "Card width" slider (or the fine-tune buttons) until the frame is exactly as wide as the card's long edge.
- Pick cm or inches under "Unit", and horizontal or vertical under "Orientation".
- Hold the object you want to measure against the ruler and read the tick marks.
Notes
- Changing the browser zoom changes the real size on screen. Use the ruler at the same zoom level you calibrated at, and recalibrate after you change it.
- Accuracy depends on how well you line the card up with the frame. Expect an error of about 1 mm; use a real ruler or calipers for precise work.
- On a small phone screen the card frame may not fit and will scroll sideways. This tool is meant for a computer monitor.
- The calibration is kept in this browser's localStorage. In private browsing, or where storage is blocked, it lasts only while the page is open.
Glossary
ID-1 (ISO/IEC 7810)
The international card size used for credit cards, bank cards and many driver's licenses: 85.60 × 53.98 mm. Because it is nearly universal, it works as a reference for calibrating a screen.
CSS pixel
The reference unit browsers use when drawing on screen. It is not the same as a hardware pixel and depends on zoom and display scaling, which is why real size has to be calibrated.
PPI (pixel density)
The number of pixels per inch. This tool shows the value implied by your calibrated CSS pixels per centimeter, for reference.
FAQ
Why calibrate with a credit card?
A browser cannot read your screen's real size in inches, so you need a physical reference. Credit cards, bank cards and many driver's licenses follow ISO/IEC 7810 ID-1 at 85.60 × 53.98 mm, so any of them works.
How accurate is the ruler?
It depends on how well you match the card to the frame; expect an error of about 1 mm. After calibration the cm and inch ticks are calculated from that value, so a calibration error grows over longer distances. Use a physical ruler for precise measurements.
What happens if I change the browser zoom?
Zooming changes the size of a CSS pixel, so the real-world size shifts. Use the ruler at the zoom level you calibrated at, or match the frame to your card again after changing it.
Where is my calibration stored?
Only in your browser's localStorage; it is never sent to a server. If you switch to another browser, device or external monitor, calibrate again there.
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