Online Ruler (Actual Size, cm and Inches)

A screen cannot know its own physical size, so an on-screen ruler is only accurate after calibration. This ruler converts your screen's size or pixel density into CSS pixels per millimetre, draws a centimetre or inch ruler at that scale, and adds a bank-card outline so you can check and fine-tune it against a card you already have — every standard bank or ID card is exactly 85.60 mm wide. It is a handy approximation, not a measuring instrument. For paper, it also makes a printable PDF ruler that is true to size when printed at 100%.

How to use this tool

1. Choose how to calibrate: the screen diagonal from the device's specifications, its pixel density (PPI), or a value you saved earlier.
2. Hold a bank or ID card against the card outline and adjust the percentage until the card exactly covers its width.
3. Choose centimetres or inches and a length; a ruler too long for your screen is shortened to fit. Then measure.
4. Tick the PDF option for a printable ruler; print it at 100% (“Actual size”) and check the 5 cm square.

Formula

CSS px per inch = √(width² + height²) ÷ diagonal; px per mm = px per inch ÷ 25.4

width and height are the screen size in CSS pixels as the browser reports it. With a pixel density, CSS px per inch = PPI ÷ devicePixelRatio. ID-1 cards (bank, credit and most ID cards) are 85.60 × 53.98 mm under ISO/IEC 7810, which makes them a handy reference. In a PDF, 1 inch is exactly 72 points.

Example

A 15.6-inch laptop that reports 1536 × 864 CSS pixels has a diagonal of √(1536² + 864²) ≈ 1762 px, so 1762 ÷ 15.6 ≈ 113 px per inch (4.45 px per mm). A 10 cm ruler is then about 445 px long, and the card outline about 381 × 240 px.