Online Digital RulerActual-size ruler: cm, mm and inches

How the Digital Ruler works

Actual size on screen

The ruler draws centimeters, millimeters and inches at actual size, using your screen's pixel density (PPI, pixels per inch).

Zero is at the left edge of the screen (or the top when the ruler is rotated). Place your object there, then tap or drag along the ruler to its end to see the exact reading.

Why calibrate?

Browsers don't report the physical size of the screen. The ruler tries to identify your device automatically; if it can't, it uses a default estimate (96 PPI), which may not be exact.

Calibrating takes under a minute and only needs to be done once. It is saved on this device and browser.

Ways to calibrate

Auto-detect: identifies your device from the screen's resolution and pixel density.

Device: pick your model from hundreds of phones, tablets, laptops and monitors.

Diagonal: enter your screen size in inches (e.g. 6.1 or 15.6).

Card: the most precise method. Hold a credit or debit card (85.60 × 53.98 mm) against the left edge of the screen and adjust the rectangle until it matches.

Tools

Fullscreen: hides the rest of the page so you can use the whole screen.

Rotate: turns the ruler vertical, great for phones held upright.

Popup: opens the ruler in a small window you can drag over anything on screen (desktop).

Tips for accurate measuring

Keep browser zoom at 100%.

If you change system scaling or switch monitors, calibrate again.

Look at the ruler straight on, not at an angle, to avoid parallax error.

Privacy Policy and Terms of Use

Last updated: October 19, 2026

1. Summary

The Digital Ruler runs in your browser and needs no sign-up. We use Google Tag Manager and Google Analytics to understand how the site is used; these tools use cookies and collect browsing data, as explained below.

2. Data saved on your device

Your calibration (PPI and chosen model) and theme (light, dark or auto) are saved in your browser's local storage (localStorage) only to remember your preferences. They are never sent to us.

You can delete them at any time with "Reset to default" in calibration, or by clearing site data in your browser settings.

3. Device detection

For auto-calibration, the page reads your screen resolution, pixel density and browser identifier (user agent) inside your browser. These are matched against a built-in device list and never leave your device.

4. Cookies and analytics (Google Analytics)

We use Google Tag Manager to load Google Analytics, Google's statistics service. It sets cookies in your browser and sends Google information such as pages visited, time on site, device and browser type, language, how you arrived, approximate location and a random identifier. We only use this data in aggregate to improve the site; we do not sell it.

Google handles this data under its own privacy policy: https://policies.google.com/privacy

You can block or delete cookies in your browser settings, use a tracker blocker, or install the Google Analytics opt-out add-on: https://tools.google.com/dlpage/gaoptout

We do not show ads.

5. Your rights (LGPD)

The legal basis for analytics is our legitimate interest in understanding and improving the site. You can exercise your rights under Brazil's General Data Protection Law (Law No. 13,709/2018), such as access, correction, deletion and objection, by contacting us.

6. Children

The site can be used by people of any age. We do not knowingly collect data that identifies children.

7. Terms of use

The Digital Ruler is free and provided "as is", without warranties of any kind. Measurement accuracy depends on your screen, browser zoom and calibration.

Do not rely on it where a certified measurement is required (for example medical, legal or engineering uses). Use it at your own risk.

8. Changes

We may change this policy, the terms or the site at any time. The last-updated date is shown at the top of this page.

9. Contact

Questions about privacy or these terms: contato@reguadigital.com