Online Ruler

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How can we help you?

Have questions, feedback, discovered a bug, or want to suggest a new calibration profile? Send us a message and our team will get back to you.

Send Us a Message

Direct Assistance

Online Ruler is maintained as a free engineering and measurement utility. We review all incoming community suggestions and bug reports promptly.

Reporting Calibration Issues?

To help us diagnose display calibration deviations quickly, please include:

  • Device make and model (e.g., MacBook Pro 16" M3, Surface Pro 9)
  • Operating system & browser version
  • Screen resolution & OS scale factor (100%, 125%, 150%, 200%)
  • Measured difference compared to a physical ruler

Open Reference

Looking for quick tips on getting physical 1:1 precision? Check out our how it works guide or review our about page.

Frequently Asked Questions

Quick answers to common questions about calibration, measurement precision, and device support.

How do I calibrate the ruler for 100% accuracy?

Click the "Calibrate display" button in the header. For the most accurate result, choose the Credit Card method and adjust the on-screen card until it precisely matches a physical card (ISO/IEC 7810 standard: 85.6mm).

Why does browser zoom affect measurements?

Browser zoom changes CSS pixel scaling. For true 1:1 physical dimensions, ensure your browser zoom level is set to 100% (Ctrl+0 or Cmd+0).

Does Online Ruler work on smartphones and tablets?

Yes! Online Ruler is fully responsive and supports touch drag guides on iOS and Android devices. For best results on mobile, calibrate using your device model preset.

How do I switch between inches and centimeters?

Use the unit selector in the ruler control bar at the top of the canvas to toggle between Millimeters (mm), Centimeters (cm), Decimal Inches (in), and Fractional Inches.

Is my calibration data uploaded anywhere?

No. All calibration ratios and display settings are stored locally in your browser via localStorage. Nothing is sent to our servers. Review our Privacy Policy.

Can I use this for precision engineering?

Online Ruler is optimized for design, crafts, drafting, and general measurements. For aerospace, machining, or medical applications, always verify with certified vernier calipers or steel rules.