Online Ruler FAQ

Q: Is the online ruler accurate?

After calibrating with a real object, the on-screen ruler shows the true size for that specific screen and zoom level. Before calibration it is only an approximation because screens have different pixel densities. Use a precise, easy-to-align object and keep browser zoom at 100% for the most reliable result.

Q: Why does the ruler need calibration?

A CSS pixel in the browser is not a fixed physical length. The same pixel count maps to different real distances on different displays, and OS scaling changes it further. Calibration measures your screen's actual pixels-per-millimeter so the ruler matches reality.

Q: Why is a bank card recommended for calibration?

A bank card is a convenient reference: its size is fixed at 85.6 mm wide by ISO/IEC 7810, it is easy to find, and it lies flat against the screen for a clean alignment.

Q: Do I need to recalibrate on a different computer or monitor?

Yes. Calibration is tied to the current display and zoom. Switch monitors, change OS display scaling, or alter browser zoom, and recalibrate so the ruler stays true to size.

Q: Does the site upload my calibration data?

No. Calibration happens in your browser and the resulting value is stored only in your browser. Nothing you measure or calibrate is sent to a server.

Q: How do I print a true-size ruler?

Use the printable ruler, set print scale to 100 percent, and print on A4 or Letter paper; the printed ruler then matches real centimeters and inches.

Last updated: 2026-10-15

Quick answers about accuracy, calibration and privacy. Still unsure? Reach out on the contact page.

Is the online ruler accurate?

After you calibrate with a real object, the on-screen ruler shows the true size for that specific screen and zoom level. Before calibration it is only an approximation, because screens have different pixel densities. For the most reliable result, use an object with a precise, easy-to-align size (a bank card works well) and keep browser zoom at 100%.

Why does the ruler need calibration?

Because a CSS pixel in the browser is not a fixed physical length. The same pixel count maps to different real distances on different displays, and OS scaling (125% or 150% on Windows, display scaling on macOS) changes it further. Calibration measures your screen’s actual pixels-per-millimeter so the ruler matches reality.

Why is a bank card recommended for calibration?

A bank card is a convenient reference: its size is fixed (85.6 mm wide by ISO/IEC 7810), it is easy to find, and it lies flat against the screen for a clean alignment. Any object with a known size works, but a card is usually the simplest.

Do I need to recalibrate on a different computer or monitor?

Yes. Calibration is tied to the current display and zoom. When you switch monitors, change OS display scaling, or alter browser zoom, recalibrate so the ruler stays true to size.

Does the site upload my calibration data?

No. Calibration happens in your browser, and the resulting value is stored only in your browser (localStorage). Nothing you measure or calibrate is sent to a server. Clearing your browser data removes it.

How do I print a true-size ruler?

Use the printable ruler, open the print dialog, set scale to 100% (disable “fit to page”), and print on A4 or Letter paper. The printed ruler then matches real centimeters and inches.

What units can I use?

The ruler and the converter support centimeters, millimeters and inches. See cm, mm and inches explained for how they relate.

Can I use the online ruler on a phone or tablet?

Yes. Open the site in a mobile browser, lay a bank card on the screen to calibrate, and the ruler matches real size. Touch and drag work for marking and measuring.

Does screen resolution affect accuracy?

Accuracy depends on the screen’s physical pixel density, your OS display scaling, and browser zoom — not the resolution number itself. Calibrating with a real object cancels these out for that device and zoom level.

Does the ruler work offline?

Once the page has loaded, the ruler runs entirely in your browser. It needs no internet connection to measure, and no measurement or calibration data is uploaded to any server.