About TrueRuler

Last updated: October 17, 2026

TrueRuler is a free online ruler that turns any screen — phone, tablet, laptop or monitor — into a real measuring tool. It draws centimetre, millimetre and inch scales at true physical size by detecting your device's pixel density, or through a quick one-time calibration.

What you can do with it

• Measure small objects directly on your screen in cm, mm and inches
• Calibrate automatically from a device database, with your screen's diagonal size, or with a standard bank card (85.60 mm)
• Drag-to-measure box with live readings, copy and arrow-key nudging
• Built-in 180° protractor, printable paper ruler and unit converter
• Works in 19 languages, with dark mode and offline support

Our approach

An on-screen ruler is only useful if it is honest about its limits. TrueRuler tells you when your device is recognised, warns you when browser zoom is off, and explains exactly how the calibration math works. No account, no installation, no tracking of what you measure — your calibration never leaves your device.

Contact

Questions, feedback or business enquiries: admin@trueruleronline.com.