Online Ruler — Actual Size
A real-size ruler on your screen, in centimeters and inches. Calibrate once with any bank card for true 1:1 accuracy.
Centimeters and inches at actual size
⚠ Not calibrated — 30-second setup for true 1:1 size (click to expand)
Estimate (96 px/inch reference)Click or use arrow keys + Enter to mark two points and measure.
Measure the screen, not the casing
This ruler measures objects against the display you are using. If your goal is to identify a laptop, monitor, TV or phone screen size, first follow the screen measurement guide to place the tape correctly, exclude the bezel and convert centimeters to inches.
Frequently asked questions
›How accurate is this online ruler?
After calibration, it is accurate to within about 1% — limited only by how precisely you match the card outline. Without calibration, browsers can't know your display's physical size, so the ruler falls back to a rough estimate.
›Why do I need to calibrate with a bank card?
Web browsers report sizes in pixels but don't reliably expose the physical dimensions of a display. A bank card has an internationally standardized width (85.60 mm, ISO/IEC 7810 ID-1), making it a measuring reference almost everyone has at hand.
›Is the calibration saved?
Yes, in your browser's local storage on this device. It also unlocks true-size mode in our screen size comparison tool. Nothing is sent to any server.
Related tools
- Screen Size ComparisonCompare phone, tablet, laptop and monitor screens as proportional overlays — or against your own screen. True-size mode shows devices at real physical size.
- PPI CalculatorCalculate PPI from resolution and screen size. Instant results with the formula worked through your numbers, plus physical dimensions and dot pitch.
- What Is My Screen Size?Instantly see your screen resolution, viewport size, device pixel ratio and more. Live values that update as you resize, with a copyable debug report.
- Phone Screen SizesFilterable phone screen size chart with display inches, resolution, PPI, aspect ratio and viewport for the iPhone, Galaxy S and Pixel models tracked here.