Place a ruler on any edge.

Free Online Ruler — Measure in cm, Inches, mm & Pixels

An online ruler is a free browser-based measurement tool that works directly on your screen — no physical ruler, no app to download. Once calibrated to your display's pixel density, this virtual ruler shows measurements at actual physical size, letting you measure real-world objects in centimeters, millimeters, inches, points, or pixels.

How to Use This Ruler Online

Click any edge button — Top, Bottom, Left, or Right — to place a ruler along that edge of your screen. The ruler appears instantly at actual size. You can activate multiple rulers simultaneously: place a top and left ruler for an L-shaped measurement grid, or enable all four edges to create a full-screen workspace.

Move your cursor over the workspace to see your exact position displayed on the ruler in real time. The live cursor indicator tracks your measurement continuously so you can take readings without clicking.

Five Measurement Units: cm, mm, Inches, Points & Pixels

Switch between five units at any time using the unit selector in the nav bar:

• Centimeters (cm) — ideal for everyday measurements, A4 paper, packaging, and most metric tasks
• Millimeters (mm) — high-precision metric measurement for small objects and technical work
• Inches (in) — standard for US measurements, letter paper, and imperial sizing
• Points (pt) — the unit of typographers and print designers (72 pt = 1 inch)
• Pixels (px) — shows CSS pixel dimensions, essential for web and UI/UX design

A built-in unit converter (press C) converts between mm, cm, inches, and px in real time without interrupting your workflow.

Screen Calibration for Actual-Size Accuracy

For an online ruler to show accurate physical dimensions, it must know your screen's pixel density (PPI — pixels per inch). This screen ruler offers three calibration methods:

• Auto-detect — uses browser media query APIs to estimate your screen's DPI automatically. Works well on most modern desktops and laptops.
• Screen diagonal — enter your display's diagonal size in inches or centimeters (from the manufacturer's spec sheet) for precise PPI calculation.
• Credit card — hold a standard credit card against your screen and drag the slider until the on-screen card matches exactly. Works on any device including phones and tablets.

Your calibration is stored in your browser so it's ready the next time you return — no need to recalibrate on every visit.

More Than a Basic Digital Ruler

This free online measuring tool includes features typically found in professional design software:

• Guide lines — drag from any ruler edge to drop a dashed guide line at any position. Double-click to delete. Perfect for aligning elements in designs or mockups.
• Pointer markers — click to drop x/y coordinate markers and record exact positions across the workspace.
• Reference objects — overlay actual-size outlines for a credit card (85.6 × 54 mm), A4, US Letter, A5, or iPhone 16 to verify proportions at a glance.
• Device presets — instantly calibrate to 12 popular devices including MacBook Pro, iPhone 16, iPad Air, and Samsung Galaxy S24.
• Dark mode — press D to switch between light and dark themes for comfortable use in any lighting condition.
• Zoom warning — the tool detects when your browser zoom isn't 100% and alerts you, since zoom level directly affects measurement accuracy.

Who Uses an Online Ruler?

Graphic designers and UI developers use a screen ruler to verify spacing and element dimensions in mockups. Web developers use pixel mode to check CSS layouts without leaving the browser. Students reach for it when a physical ruler isn't nearby. Tailors and hobbyists use it to measure pattern pieces or fabric on screen. Photographers check print crop dimensions before ordering. Architects verify drawing scales against real dimensions.

Any time you need a quick, accurate digital measuring tool without installing software, open this free online ruler in any desktop or laptop browser — no signup required, no cost.

Have questions about accuracy, calibration, or units? Read the FAQ →