Online Ruler — Measure Real Objects with ±0.5% Accuracy
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Measure Anything on Your Screen.

Auto-detect your device or calibrate manually. Accurate to cm, mm, or inches.

Detecting device...
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Guide

Three Ways to Calibrate.

1

Auto-Detect Device.

The ruler automatically matches your screen from 130 device profiles — iPhones, Galaxy, Pixels, MacBooks, monitors, and more.

Instant±1%
2

Screen Diagonal.

Enter your screen's diagonal size (e.g., 15.6" for a laptop). The ruler calculates exact PPI from your resolution.

Manual±2%
3

Credit Card Calibration.

Place any bank card on the on-screen guide and adjust the slider until it matches. Cards are a standard 85.60 × 53.98 mm worldwide.

Most Accurate±0.5%
Features

Everything You Need for Precise Measurement.

Draggable Rulers - reposition to any edge for maximum workspace
130 Device Models - Apple, Samsung, Google, Xiaomi, OnePlus, Windows, monitors
cm, mm & Inches - switch units instantly without losing calibration
Dark Mode - comfortable viewing with dark-themed rulers
Fullscreen Mode - distraction-free measuring across the entire display
Persistent Settings - calibration and preferences saved locally
About

How This Online Ruler Works.

This tool renders a ruler at actual physical scale on your screen using your display's pixel density (PPI). It draws precise tick marks in centimeters, millimeters, and inches — no downloads, no installs, just open and measure.

The ruler automatically detects your device from a database of 130+ models including iPhones, Samsung Galaxies, Google Pixels, MacBooks, and monitors. Three calibration methods let you fine-tune accuracy: auto-detect, credit card matching, or screen diagonal entry.

Accuracy

How Calibration Achieves ±0.5% Precision.

This online ruler's accuracy depends on how precisely the tool knows your screen's PPI (pixels per inch). Three methods, three confidence levels:

Credit Card ( ±0.5% ): ISO/IEC 7810 ID-1 cards are a worldwide standard at 85.60 × 53.98 mm. Placing your card on the on-screen guide and adjusting the slider until it matches yields the highest precision because the reference object itself has zero variance.

Auto-Detect ( ±1% ): The tool matches your device against manufacturer-published screen specs (diagonal size + native resolution) to compute PPI. Factory tolerances between units of the same model introduce the small variance. Covers 130+ devices.

Screen Diagonal ( ±2% ): You enter your advertised screen size; the tool calculates PPI from your display's resolution. Advertised diagonals are rounded (e.g., 15.6" may be 15.59") which accounts for the wider tolerance. A reliable fallback when no card is available.

Compatibility

Works on Every Modern Device.

Automatically Detected

  • Apple: iPhone, iPad, MacBook, iMac, Studio Display
  • Samsung: Galaxy S, Note, Fold, Flip series
  • Google: Pixel phones and tablets
  • Xiaomi: Mi, Redmi, Poco series
  • OnePlus: All models
  • Windows: Surface devices, popular laptops
  • Monitors: Common desktop monitor sizes

Browser Support & Tips

  • Chrome (desktop & mobile)
  • Safari (iOS & macOS)
  • Firefox (all platforms)
  • Edge (Windows)
  • Opera
  • Samsung Internet

Tips for Best Results

  • Keep browser zoom at 100%
  • Re-calibrate when switching monitors
  • Use Credit Card method for highest precision
FAQ

Frequently Asked Questions.

Yes. The auto-detect feature identifies your phone model — iPhone, Samsung Galaxy, Google Pixel, and many others — and calibrates the ruler to your screen's exact PPI. The ruler also works on tablets including iPad and other Android tablets. Place the object against the edge of your screen to measure.

Accuracy depends on the calibration method. Credit card calibration is the most accurate at ±0.5% because credit cards have a worldwide standard size (85.60 × 53.98 mm). Auto-detect is typically ±1%, and screen diagonal calibration is ±2%.

For highest accuracy, use the credit card method — place any bank card on the on-screen guide and adjust the slider until it matches. For convenience, auto-detect works great and requires no manual steps. Screen diagonal is a good fallback when other methods aren't available.

Use the unit buttons at the top of the ruler to toggle between centimeters, inches, and pixels. Your calibration stays the same when switching units — the ruler recalculates tick marks automatically.

Camera-based measurements are unreliable due to lens distortion and perspective errors. This tool uses a screen-based approach instead, which is inherently more accurate and repeatable.

Auto-detect covers over 130 devices including Apple (iPhone, iPad, MacBook, iMac), Samsung (Galaxy S, Note, Fold, Flip), Google Pixel, Xiaomi, OnePlus, Microsoft Surface, and common monitor sizes.

The largest marks with numbers are whole units. In centimeter mode, smaller lines between numbered marks represent millimeters. In inches mode, subdivisions represent fractional inches. You can also hover over the ruler to see exact position in a tooltip.

Yes, completely free. No account required, no data collected, no ads that interfere with measuring. Your calibration settings are saved locally in your browser.

The ruler works on Chrome, Safari, Firefox, Edge, Opera, and Samsung Internet across desktop and mobile platforms.

Every screen has a different pixel density (PPI). Calibration tells the ruler your screen's actual PPI so it can render tick marks at real-world scale. Without calibration, the ruler won't show accurate measurements.