About

Updated August 28, 2026•Online Protractor

Online Protractor is a free, privacy-friendly browser tool for measuring angles. Images are processed locally — nothing is uploaded to a server. No account, no tracking, no heavy libraries.

Built with Astro + Tailwind CSS v4, the tool is designed mobile-first, works with touch, mouse, and keyboard, and degrades gracefully before JavaScript loads.

Methodology & transparency

Angles are computed from the two vectors you place: arccos(dot / |A||B|) converted to degrees, with reflex shown as 360° − inner. The math is precise for the points you place; real-world accuracy depends on image perspective, lens distortion, screen scale, and finger placement — see the accuracy guide rather than a claimed ±0.1° figure.

Browser compatibility

• Chrome / Edge / Firefox / Safari — latest 2 versions, desktop & mobile.
• Image mode via FileReader & Blob URL; camera via getUserMedia where available (HTTPS + permission-gated).
• No server, no database — works offline after first load where the browser caches the page.

Privacy

Images you upload or capture never leave your browser. We do not store them in localStorage; Object URLs are revoked on clear. See privacy policy.

Changelog

• 2026-08-28 — Initial public release; tool, image overlay, 360° mode, converters, and guide pages.
• 2026-08-31 — SEO architecture hardening (canonical / hreflang / sitemap) per Prompt 07A.

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