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Online Ruler — Actual Size on Your Screen, in Inches, cm & mm

A real-size digital ruler for your phone or computer. Auto-detect your device or calibrate with any bank card for ±0.5% accuracy. Free, no download.

Detecting device...
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Tools

Free measuring tools

Everything runs in your browser at real size — no app, no sign-up.

Project calculators

Guide

Three ways to calibrate

1

Auto-detect your device

The ruler recognizes about 130 phones, tablets, laptops and monitors, including iPhones, Galaxy phones, Pixels and MacBooks.

Instant±1%
2

Enter your screen size

Enter your screen's diagonal size (for example, 15.6" for a laptop), and the ruler works out the scale from your screen resolution.

Manual±2%
3

Calibrate with a bank card

Hold any bank card against the outline on screen and move the slider until they match. Bank cards are the same size everywhere: 85.60 × 53.98 mm.

Most Accurate±0.5%
Features

Everything you need for precise measurement

Draggable Rulers - reposition to any edge for maximum workspace
About 130 device models: Apple, Samsung, Google, Xiaomi, OnePlus, Windows, monitors
cm, mm & Inches - centimeter mode marks every millimeter; 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 on-screen ruler draws a real-size ruler on your display using its pixel density (PPI), with precise tick marks in centimeters, millimeters and inches. Whether you call it a digital ruler, an electronic ruler or a screen measurement tool, it works the same way: no downloads, no installs, just open it and measure.

The ruler automatically detects your device from a database of about 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 accurate is the ruler?

How accurate the ruler is depends on how well it knows your screen's pixel density, meaning how many pixels fit in an inch. There are three ways to set it, from most to least accurate:

Bank card (±0.5%): Bank and credit cards are made to one worldwide size, 85.60 × 53.98 mm (ISO/IEC 7810 ID-1). Because every card is exactly the same, matching one on screen gives the most accurate result.

Auto-detect (±1%): The ruler looks up your device's published screen size and resolution. Screens of the same model can differ very slightly, so this is a little less exact. It covers about 130 devices.

Screen size (±2%): You type in your screen size, and the ruler works out the rest from your resolution. Advertised sizes are rounded (a "15.6-inch" screen may really be 15.59 inches), so this is the least exact method. It's a good fallback when you don't have a card.

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 and 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
Use Cases

Who uses an online ruler?

Students and teachers - measuring printouts, worksheets, diagrams and maps
Online shoppers - checking the size of rings, jewelry, screws or small parts against product specs
Designers - checking real printed size of logos, cards and labels
Crafters and tailors - measuring fabric swatches, buttons, beads and paper templates
DIY and repairs - sizing screws, bolts, batteries and cables when no tape measure is nearby
Anyone on the go - a phone is always in your pocket; a ruler usually isn't
Reference Sizes

Real-life objects to check your measurements

Use these everyday objects to confirm the ruler is calibrated correctly.

Bank or credit card: 85.60 × 53.98 mm (3.37 × 2.13 in)

A4 paper: 210 × 297 mm (8.27 × 11.69 in)

US Letter paper: 8.5 × 11 in (215.9 × 279.4 mm)

US quarter: 24.26 mm (0.955 in) across

1 euro coin: 23.25 mm (0.915 in) across

AA battery: about 50.5 mm long and 14.5 mm wide

Rulers

More online rulers

Measuring guides

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.

Yes, once it's calibrated. A screen ruler is only as accurate as the screen size it's based on, and browsers don't report that, which is why uncalibrated online rulers are often off. Calibrated with a bank card (85.60 × 53.98 mm worldwide), this ruler is within ±0.5%: about 0.5 mm over 10 cm. Auto-detect is typically within ±1%, and screen-diagonal entry within ±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: cm, in, or px. Centimeter mode marks every millimeter, so it doubles as a millimeter ruler. 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 about 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, and 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.

Choose cm. Each small tick is 1 mm and each long numbered tick is 1 cm (10 mm). Count the small ticks after the last number: 4 cm plus 7 small ticks is 47 mm.

Yes, on a screen at least 12 inches wide. Switch to inches and use the top or bottom ruler; most 15.6-inch laptops and desktop monitors fit a full 12 inches. On a phone or small laptop, measure in sections: note where the ruler ends, slide the object along, and add the lengths.

Hold an object with a known size against it. A bank card should read 85.6 mm on its long side, a US quarter 24.26 mm across, and a €1 coin 23.25 mm. If the reading is off, recalibrate with the credit card method.

One inch is exactly 2.54 cm (25.4 mm), and one centimeter is about 0.39 inches. A US quarter is just under 1 inch across. After calibrating, switch between cm and in to compare them on your own screen.