Help Center

What is Ruler Online?

Ruler Online is a free, accurate digital ruler that turns your screen into a real-time measuring tool. Calibrate with credit card, diagonal size, or auto-detect, then measure in centimeters (cm) or inches.

How to Use the Online Ruler?

  1. Calibrate your screen: Click Calibrate and choose Credit Card (most accurate), Diagonal (enter screen size in inches), or Auto Detect.
  2. Set measurement units: Switch easily between cm and inch.
  3. Start measuring: Place your object on the screen ruler and read the values directly.

Accuracy Tips

To ensure the most accurate measurements:

  • Do not zoom in or out in your browser. Zooming will affect scale accuracy.
  • Calibrate for best results. Credit card calibration (~0.5% accuracy) is recommended; diagonal input is a good alternative if you know your screen size.

These simple steps help guarantee your measurements are precise and trustworthy.

How to Find Your Screen Size

Not sure about your screen size in inches? Here are a few ways to check:

  • Check your device specs: Search online for your device model (e.g., “MacBook Pro 13 inch”)—the inch size is usually in the name.
  • Look at the packaging or manual: The screen size is often printed on the box or in the user guide.
  • Use system settings:
    • Windows: Go to Settings → System → Display, then find your display resolution or model.
    • macOS: Go to Apple Menu → About This Mac → Displays tab to see your display info.
  • Use a search engine: Search your laptop or monitor's model number followed by "screen size".

The screen size refers to the diagonal length of the screen, measured from one corner to the opposite corner.

Screen size measurement diagram

Is Calibration Required?

Yes—calibration sets your screen's PPI for accurate measurements. We offer three methods: <strong>Credit Card</strong> (~0.5% accuracy, ISO standard size), <strong>Diagonal</strong> (enter screen size in inches, ~2–3%), and <strong>Auto Detect</strong> (quick estimate from resolution, ~5–10%). Recalibrate anytime via the Calibrate button.

Frequently Asked Questions

  • Can I use it on any device? Yes, it works on all computers, laptops, and monitors.
  • How accurate is it? Very accurate when calibrated—credit card method achieves ~0.5% accuracy.
  • Is my data safe? 100% safe—everything runs in your browser without sending any data.
  • Do I need to install anything? No installation needed. Open the site and start measuring.

Contact Us

Have questions? Email us at [email protected]