The most precise online ruler tool. Calibrate once, measure forever.
Supports centimeters, millimeters, inches, and pixels.
A complete measurement toolkit built for precision and ease of use.
Automatically identifies your device from a database of 100+ profiles and sets the correct PPI for instant, accurate measurements.
Choose auto-detect, manual screen diagonal input, or credit card overlay for the most accurate calibration on any device.
Place rulers on any edge of your screen — top, bottom, left, right, or any combination — for maximum measurement flexibility.
Click any two points on the screen to instantly measure the distance between them. Works in any direction — horizontal, vertical, or diagonal.
See real-time cursor coordinates as you move across the screen. Perfect for precise alignment and positioning tasks.
Switch between light and dark themes for comfortable measuring in any lighting condition. Respects your system preference automatically.
Three simple steps to pixel-perfect measurements.
Choose from auto-detect, screen diagonal, or credit card calibration. This takes just a few seconds and is saved for future visits.
Select which edges to display rulers on — top, bottom, left, right, or all four. Choose your preferred unit: cm, mm, inches, or pixels.
Click two points to measure distances, track your cursor position in real-time, or use the edge rulers for quick reference. All measurements appear instantly.
Choose the calibration method that works best for your setup.
We automatically match your screen resolution, pixel ratio, and user agent against our database of 100+ device profiles to find the correct PPI.
FastestEnter your monitor's diagonal size in inches (usually printed on the box or in specs). We calculate the exact PPI from your screen resolution.
Most CommonPlace any standard-size credit card on your screen and adjust the overlay until it matches perfectly. Uses the ISO standard card size (85.6mm × 54mm).
Most AccurateEverything you need to know about using Online Ruler.
After proper calibration, our ruler achieves near pixel-perfect accuracy — typically within 0.5mm for well-calibrated screens. The accuracy depends on your calibration method: auto-detect works well for known devices, while credit card calibration provides the best accuracy for any screen.
We offer three calibration methods: Auto-Detect (automatically recognizes your device from our database of 100+ devices), Screen Diagonal (enter your screen's diagonal size in inches), and Credit Card Calibration (place a standard credit card on screen and adjust until it matches). Credit card calibration is the most accurate.
The ruler supports four measurement units: centimeters (cm), millimeters (mm), inches (in), and pixels (px). You can switch between units at any time using the toolbar or keyboard shortcuts.
Yes! The ruler works on tablets and mobile devices with touchscreen support. However, for the best experience and accuracy, we recommend using a laptop or desktop monitor, as mobile screens are smaller and harder to calibrate precisely.
No installation required. Online Ruler runs entirely in your web browser. Just visit the site, calibrate once, and start measuring. Your calibration is saved locally so you don't need to recalibrate on each visit.
Yes! Our interactive measurement tool lets you click two points anywhere on the screen to measure the distance between them — horizontally, vertically, or diagonally. The distance is displayed in your selected unit.
Absolutely. Online Ruler is completely free with no ads, no account required, and no limitations. We believe measurement tools should be accessible to everyone.
Yes! You can place rulers on the top, bottom, left, or right edges of your screen — or any combination of edges. Use the edge selector in the toolbar to choose your preferred layout.