Online Ruler
An accurate on-screen ruler in cm, mm and inches — calibrate once and measure anything.
Calibrate once with a real credit card (or your screen size), then measure anything against the ruler below.
Calibration
Hold a real bank card against the outline and drag the slider until the sizes match — every card is exactly 85.6 mm wide.
For long objects, measure in steps or rotate the device — the ruler spans the width of your screen.
About Online Ruler
The online ruler turns your screen into an accurate measuring tool in centimeters, millimeters, and inches. Because every display has a different pixel density, you calibrate it once — either by holding a real bank card against the on-screen outline, or by entering your screen’s diagonal size in inches.
The card method works because every payment card in the world follows the ISO/IEC 7810 ID-1 standard: exactly 85.6 mm wide. Drag the slider until the outline matches your card and the ruler knows precisely how many pixels one centimeter occupies on your specific screen.
Once calibrated, drag the marker along the ruler to read off a measurement — handy for measuring small objects, checking a printed dimension, or sizing a piece of jewelry when no physical ruler is around. It is free, works on phones and desktops, and remembers nothing beyond your browser.
How to use Online Ruler
- Hold a real bank or ID card against the on-screen outline.
- Drag the calibration slider until the outline matches your card exactly — or enter your screen diagonal in inches and press Apply.
- Choose your units: cm/mm or inches.
- Place the object against the screen and drag the marker along the ruler to measure it.
- For objects longer than your screen, measure in steps or rotate the device to landscape.