Screen Ruler Online

Free browser-based measuring tools — ruler, protractor, and more. Accurate measurements on any device, no download required.

Open Ruler

Free Online Tools

Screen Ruler

Measure real-world objects in cm, mm, and inches directly on your screen. Calibrate with a credit card for millimeter-level accuracy.

  • cm / mm / inch
  • Device auto-detection
  • Credit card calibration

Protractor

Measure angles on your screen with a digital protractor. Drag two rays to measure any angle from 0° to 360°.

  • 0° – 360° range
  • Snap to common angles
  • Touch & mouse support

Random Spinner

Spin the wheel to make random decisions. Add your own options, customize colors, and let the wheel decide.

  • Custom options
  • Spin animation
  • History tracking

Device Specs Database

Verified screen specs for 69+ phones, tablets, laptops, and monitors. Filter by brand, OS, year, and screen size to compare devices side-by-side.

  • 69+ verified devices
  • Filter by brand / year / OS
  • Cross-linked with ruler

Dead Pixel Test

Cycle full-screen solid colors to spot dead, stuck, and hot pixels on any monitor, laptop, phone, or tablet. Includes a Stuck Pixel Fixer mode.

  • 9 solid-color screens
  • Stuck pixel fixer mode
  • Full-screen + keyboard shortcuts

Aspect Ratio Calculator

Calculate aspect ratio from pixel dimensions or find a missing width/height for a known ratio. Includes 8 common presets — 16:9, 4:3, 1:1, 9:16, 21:9, 3:2, 2:3, and 2.39:1 cinema.

  • 8 common ratio presets
  • Width × height ↔ ratio
  • Shareable URL + copy result

Pixel Converter

Convert pixels to millimeters, centimeters, and inches at any DPI. Five presets cover web (96), Mac legacy (72), and print (150/300/600).

  • 4-unit live conversion
  • 5 DPI presets + custom
  • Shareable URL + copy

Printable Ruler PDF

Generate an actual-size ruler PDF in cm, mm, inch, or dual units on A4, US Letter, or Legal paper. Built-in credit-card calibration corrects any printer scale error.

  • 4 unit modes + 3 paper sizes
  • Credit-card calibration loop
  • Lazy-loaded PDF (no main bundle bloat)

Screen Size Calculator

Convert diagonal + aspect ratio to physical width × height. Compare two to four screens at true scale. Built-in THX, SMPTE, and 4K viewing-distance recommendations.

  • 4 modes (diagonal / compare / viewing distance / preset)
  • 8 aspect ratios + 15+ device presets
  • Multi-screen visual compare at true scale

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Why Screen Ruler Online?

Works on Any Screen

Desktop, laptop, tablet, or phone — our tools work on any device with a web browser.

No Download Required

Open your browser and start measuring. No apps, no plugins, no installation needed.

Auto Device Detection

We detect your device model and screen PPI automatically for accurate measurements.

20 Languages

Available in English, Chinese, Japanese, Korean, Spanish, French, German, and 13 more languages.

How It Works

1

Open the Tool

Visit screenruler.online and choose your tool — ruler, protractor, or spinner.

2

Calibrate (Optional)

For the ruler, place a credit card on screen and adjust the slider until sizes match.

3

Measure

Place your object against the screen and read the measurement in your preferred unit.

Frequently Asked Questions

How accurate is an online screen ruler?▾
After calibration with a credit card or known reference object, our online ruler achieves accuracy within 0.5mm on most devices. The precision depends on your screen resolution and calibration quality. Higher PPI displays like Retina screens generally provide better accuracy.
Does the ruler work on mobile phones and tablets?▾
Yes. The ruler works on any device with a web browser, including iPhones, Android phones, iPads, and Android tablets. Touch controls are fully supported for mobile measurement. Simply open the website, calibrate with a credit card, and start measuring.
Do I need to download an app to use this ruler?▾
No download or installation is required. This is a web-based ruler that runs directly in your browser. You can also add it to your home screen on mobile devices for quick access, making it work like a native app.
Why do I need to calibrate the ruler?▾
Every screen has a different pixel density (PPI). A 14-inch laptop and a 27-inch monitor may have the same resolution but very different physical pixel sizes. Calibration tells the ruler your screen's exact scale so it can convert pixels to real-world measurements like centimeters and inches.
What objects can I use for calibration?▾
The most reliable calibration object is a standard credit card or debit card (85.6mm x 54mm, an ISO standard). You can also use coins like a US quarter (24.26mm diameter) or an A4 sheet of paper (297mm x 210mm). Any object with a known size works.
Does the calibration persist between visits?▾
Yes. Once you calibrate, your settings are saved in your browser's local storage and remain valid for 30 days. If you return to the ruler on the same device and browser, it will remember your calibration automatically.
Which tool should I use — ruler, protractor, or spinner?▾
Pick the ruler for measuring length (cm, mm, inches), the protractor for measuring angles from 0° to 360°, and the random spinner when you need a fair random pick from a list of options. All three are free, run entirely in your browser, and require no sign-up.
How accurate is the online protractor?▾
The protractor displays angles with 1° precision by default, and with 0.1° precision when you disable snapping. Accuracy depends on how carefully you align the two rays with the lines you want to measure — for pixel-perfect work, enlarge the reference diagram or photo before dragging the rays.
Is the random spinner actually random?▾
Yes. The spinner uses a uniform random distribution, so every option you add has exactly the same probability of winning. The outcome is decided before the animation starts — the visible spin is purely for show. Your list of options stays in your browser and is never sent to a server.