Frequently Asked Questions

Screen Ruler Online is a free browser-based measuring tool that turns any screen into an accurate ruler. It supports measurements in centimeters, millimeters, and inches, and works on phones, tablets, and desktops without any download.

Yes, completely free. No sign-up, no hidden fees, no premium tier. All tools (ruler, protractor, spinner) are available to everyone.

No. Screen Ruler Online works entirely in your web browser. Just open screenruler.online and start measuring. No app, no plugin, no extension needed.

Any device with a modern web browser — iPhones, Android phones, iPads, tablets, laptops, and desktop computers. We auto-detect 50+ popular devices for optimal accuracy.

No. All measurements and calibration data stay in your browser (localStorage). We do not collect, store, or transmit any measurement data. We use Google Analytics for anonymous page view statistics only.

After calibration with a reference object (like a credit card), the ruler achieves millimeter-level accuracy on most devices. Without calibration, accuracy depends on how well we detect your screen's PPI (pixels per inch).

Place a credit card (or other reference object) on your screen, then adjust the calibration slider until the on-screen outline matches the card exactly. This tells us your screen's exact pixel density.

We support credit/debit cards (85.6 × 53.98 mm), US Quarter, 1 Euro coin, £1 coin, Indian ₹10 coin, Indonesian 1000 Rupiah coin, Turkish 1 Lira coin, and A4 paper. Credit card is recommended for best results.

Browsers report screen resolution in CSS pixels, not physical pixels. Without knowing your screen's actual DPI/PPI, we can only estimate. Calibration fixes this by establishing the true pixels-per-millimeter ratio.

The ruler is most accurate on flat screens. Curved displays (like some Samsung phones) may introduce slight measurement errors at the edges. For best results, measure near the center of the screen.

Currently, the ruler measures up to your screen's physical length. For longer objects, measure in sections and add the values together, or use the ruler as a reference to mark and measure with a physical tool.

The protractor shows two colored rays extending from a center point. Drag either ray handle to rotate it. The angle between the two rays is displayed in the center, with 0.1° precision.

Yes. Enable the "Snap" feature and choose an interval (1°, 5°, 15°, or 45°). The rays will snap to the nearest multiple of your chosen angle when you drag them.

The protractor always shows the smaller angle between the two rays (0° to 180°). For reflex angles (>180°), subtract the displayed angle from 360°.

Yes. The spinner uses JavaScript's Math.random() to determine the result. Each option has an equal probability of being selected. The animation is visual only — the result is predetermined when you click Spin.

There is no hard limit. You can add as many options as you need. For readability, we recommend 2-20 options. Labels longer than 12 characters are truncated on the wheel.

Not in the current version. All options have equal weight. Weighted spinning is planned for a future update.

No. Spin history is stored in memory and resets when you close the tab. Your custom options are also not persisted between sessions (this is planned for a future update).

Screen Ruler Online is available in 20 languages: English, Chinese (Simplified & Traditional), Japanese, Korean, Spanish, French, German, Hindi, Indonesian, Vietnamese, Turkish, Thai, Malay, Croatian, Czech, Romanian, Serbian, Slovenian, and Filipino.

Partially. Once loaded, the ruler, protractor, and spinner tools work without an internet connection. However, you need an initial connection to load the page. Full offline/PWA support is planned.

All modern browsers — Chrome, Firefox, Safari, Edge, and their mobile versions. We require JavaScript enabled and a screen that supports the Canvas API (virtually all devices since 2015).