Online Ruler — Measure Your Screen with Pixel-Perfect Accuracy

This free online ruler lets you measure anything on your screen in centimeters, millimeters, inches, or pixels. Calibrate once and your online ruler stays accurate forever.

100% Free No Install 4 Units Calibrate Once
Unit
Edges
Click two points to measure — press Esc or double-click to clear.
Shape
U Unit · M Measure · E Edges · Esc

Why This Online Ruler Beats the Rest

Smart Auto-Detect

Automatically identifies your device from a database of 100+ profiles and sets the correct PPI, so the online ruler is accurate from the very first click.

3 Calibration Methods

Choose auto-detect, manual screen diagonal input, or credit card overlay for the best accuracy on any device.

Multi-Edge Placement

Place rulers on any edge of your screen — top, bottom, left, right, or any combination — and the online ruler stays aligned on every edge.

Click-to-Measure

Click any two points on the screen to instantly measure the distance between them. Works horizontally, vertically, and diagonally.

Live Cursor Position

See real-time cursor coordinates as you move across the screen. Perfect for precise alignment and positioning tasks.

Dark Mode

Switch between light and dark themes for comfortable measuring in any lighting. Your system preference is respected automatically.

How the Online Ruler Works

1

Calibrate your screen.

Choose auto-detect, screen diagonal, or credit card calibration. It takes a few seconds, and your online ruler keeps that calibration for future visits.

2

Place your rulers.

Select which edges to display rulers on — top, bottom, left, right, or all four. Pick your unit: cm, mm, inches, or pixels.

3

Measure anything.

Click two points to measure distances, track your cursor position in real time, or use the edge rulers for a quick reference — the online ruler shows every reading instantly.

Three Ways to Calibrate Your Online Ruler

Fastest

Auto-Detect Device

We match your screen resolution, pixel ratio, and user agent against our database of 100+ device profiles to find the correct PPI for the online ruler.

Most Common

Screen Diagonal

Enter your monitor diagonal in inches (usually printed on the box or in specs). We calculate the exact PPI from your screen resolution.

Most Accurate

Credit Card

Place any standard credit card on screen and adjust until it matches. Uses the ISO standard card size of 85.6mm × 54mm — the most accurate online ruler calibration.

FAQ About the Online Ruler

How accurate is the online ruler?
After proper calibration, this online ruler achieves near pixel-perfect accuracy — typically within 0.5mm on well-calibrated screens. Accuracy depends on your calibration method: auto-detect works well for known devices, while credit card calibration gives the best results on any screen.
How do I calibrate the ruler for my screen?
Pick one of three methods: Smart Auto-Detect (recognizes your device from 100+ profiles), Screen Diagonal (enter your display diagonal in inches), or Credit Card calibration (match a standard card outline on screen). Credit card calibration is the most accurate.
What units can I measure in?
The online ruler supports four units: centimeters (cm), millimeters (mm), inches (in), and pixels (px). Switch between them at any time from the toolbar or with keyboard shortcuts.
Does it work on mobile devices?
Yes. The ruler works on tablets and phones with touch support. For the best accuracy we recommend a laptop or desktop monitor, since mobile screens are small and harder to calibrate precisely.
Do I need to install anything?
No installation is required. The online ruler runs entirely in your browser. Visit the page, calibrate once, and start measuring — your calibration is stored locally.
Can I measure diagonal distances?
Yes. Click two points anywhere on the screen and the tool shows the straight-line distance, plus the horizontal and vertical components, in your selected unit.
Is the ruler free to use?
Absolutely. The online ruler is completely free, with no ads, no account required, and no usage limits.
Can I place rulers on different edges?
Yes. Show rulers on the top, bottom, left, or right edge — or any combination. Use the edge selector in the toolbar to choose your layout.

Start Measuring with the Online Ruler

Calibrate once and your online ruler stays accurate forever. No downloads, no accounts, no limits.