About the Online Ruler
The Online Ruler is a free, browser-based tool that turns any screen into an accurate physical measuring tool. Unlike other online rulers, the Online Ruler uses multiple calibration methods and shows you exactly how accurate your measurements are.
How It Works
Every screen displays images using tiny dots called pixels. The number of pixels packed into each inch of screen — called pixels per inch (PPI) — varies between devices. A phone might have 460 PPI, while a desktop monitor might have 109 PPI. To draw an accurate 1-centimeter line, we need to know exactly how many pixels make up 1 cm on your specific screen.
Calibration Methods
Auto-Detect
We check your screen resolution and device pixel ratio against a database of known devices to estimate your PPI. This works well for phones and tablets but is less precise for desktop monitors.
Device Picker
Search our database of over 30 common devices — iPhones, iPads, Samsung Galaxy phones, MacBooks, and popular monitors — to find your exact model and get its factory PPI rating.
Screen Diagonal
Enter your screen's diagonal measurement in inches (you can usually find this on the device box or spec sheet). We compute PPI using the formula: PPI = √(width² + height²) / diagonal.
Object Match
The most reliable method. Hold a standard object against your screen — a credit card (85.6 × 54 mm), a US dollar bill (156 × 66.3 mm), A4 paper (210 × 297 mm), or US Letter paper (215.9 × 279.4 mm). Adjust an on-screen rectangle until it matches the physical object, and we calculate your exact PPI.
The Math
Converting between pixels and real-world units is straightforward once you know your PPI:
- 1 inch = PPI pixels
- 1 cm = PPI / 2.54 pixels
- 1 mm = PPI / 25.4 pixels
For example, on a 264 PPI iPad Pro, 1 cm = 264 / 2.54 ≈ 103.9 pixels.
Accuracy
No screen-based ruler can be perfectly accurate — there are inherent tolerances in manufacturing, and browser zoom can change the effective pixel density. The Online Ruler shows a consistency level based on your calibration method:
- Object-match: high consistency (best case — you're directly comparing physical to digital)
- Device matched: high consistency (uses factory specs)
- Diagonal estimate: medium consistency (depends on accurate diagonal input)
- Auto-detected: starting estimate (heuristic-based)
Open Source
The Online Ruler is built with Astro and Preact. It runs entirely in your browser — no data is sent to any server. Your calibration profile is stored locally in your browser's localStorage.