Rulers pinned to the edges of the viewport at true millimetre / inch scale. Physically fixed even when you zoom your browser.
Use our free on-screen ruler to measure real-world items directly against your computer screen, tablet, or mobile phone in inches, centimeters, or millimeters. Calibrate using a standard credit card for 100% true physical accuracy.
A standard ISO 7810 ID-1 credit card is exactly 85.60 mm wide (3.370 inches). By matching the on-screen card border to a physical card in your hand, our algorithm calculates your screen exact PPI (Pixels Per Inch) and scales the digital ruler ticks to match real physical dimensions.
Click the Calibrate button in the toolbar and hold a standard physical credit card or driver license up to your monitor screen. Adjust the slider until the on-screen card guide matches your physical card exactly.
Standard web rulers assume all screens are 96 DPI. Modern 4K monitors, Retina displays, and laptops have pixel densities ranging from 120 DPI to 300+ DPI. Manual calibration guarantees 1:1 millimeter precision.
Yes! The ruler dynamically listens to browser viewport scale and device pixel ratio events to recalculate scale factors so your measurements remain physically true.
Yes! You can use it on iPhones, iPads, Android tablets, or desktop monitors. Just calibrate once using any standard ID or credit card.
No. The screen ruler renders 100% in CSS and SVG locally in your browser thread. No camera, screen recording, or server requests are involved.