Frequently Asked Questions

Why do I need to calibrate?

Every screen has a different pixel density (PPI). A 27-inch 4K monitor packs ~163 pixels per inch, while a smartphone can exceed 460 PPI. Without calibration, an online ruler assumes the old web standard of 96 PPI, which can be off by 60% or more. Calibration stores your screen's true PPI so measurements are pixel-perfect accurate.

How do I calibrate my screen?

Click the "Calibrate" button in the top right. Three methods are available:

• Auto detect: Uses your browser's screen information (fastest).
• Screen diagonal: Enter your monitor's diagonal size in inches — the tool calculates PPI from your resolution.
• Credit card: Place an ISO-standard credit card against your screen and align the ruler; the tool measures the card's known 3.37-inch length.

Will my calibration data be lost if I clear my browser cache?

Your calibration is stored in your browser's localStorage, separate from cached web files. Clearing the cache won't affect it. Only clearing localStorage specifically will remove calibration data. You'll need to recalibrate if localStorage is cleared.

Can I use the ruler on mobile devices?

Yes. Real Online Ruler works on smartphones and tablets. Calibration is just as important on mobile — phone screens often have very high pixel densities. Use the same three calibration methods; the credit card method usually works best.

How accurate is the ruler?

Once calibrated, the ruler is accurate to the precision your screen can display — typically within 1-2% of true physical measurement. The limiting factor is your display's physical pixel size and browser rendering, not the tool itself. Always verify critical measurements with a physical ruler.

Can I snap the ruler to a screen edge?

Yes. Click and drag the ruler's edge (top, bottom, left, or right) to align it with your screen's edge. The ruler stays snapped while you measure. Drag it away from the edge to float it freely again.

How do I switch between the ruler and protractor?

Use the "Ruler" and "Protractor" buttons in the navigation bar at the top. Both tools share the same calibration, so you only need to calibrate once.

What is the protractor tool?

The protractor measures angles on your screen. Calibration isn't required for angle measurement (angles are screen-space only), but it's helpful for placing the protractor accurately. Use it to check corners, slopes, or any angle visually.

Does the tool work offline?

Yes. After your first visit, the tool is cached in your browser and works completely offline — no internet required. Your calibration data stays on your device and never leaves your browser.

Is my data collected or sold?

No. Real Online Ruler does not require an account, does not collect personal data, and does not show ads. Calibration data is stored locally on your device only. We do not track you or analyze your usage.

What browsers are supported?

Real Online Ruler works on all modern browsers: Chrome, Firefox, Safari, Edge, and Opera. It requires JavaScript to be enabled. For best results, use a recent version of your browser.

I found a bug or have a feature request. Who do I contact?

Please reach out at hello@realruleronline.com. We'd love to hear your feedback, bug reports, and feature ideas.