Free browser tool
Online Protractor
Instant digital angle measurement on screen and photos with zero upload. Drag the rays, place the vertex, and read exact degrees or radians live.
Keyboard: Tab handles, arrows nudge 1px center / 1° ray (Shift 10px / 5°), R reset, G grid, L lock baseline, Z undo, Shift+Z redo, +/- zoom. Endpoint magnifier shows while dragging. Drag center, baseline ray, second ray. Snap preview before commit. Image stays local — never uploaded. Press Esc to exit fullscreen.
Camera stays local — capture is processed in your browser
Tip: precision depends on pointer accuracy and image alignment. Accuracy guide
Place Vertex
Drag the glowing central ring to the vertex where the two angle lines intersect. Use grid lines and crosshair for exact alignment.
Align Rays
Drag Handle A along the baseline (0°), then drag Handle B to align with the second ray. Lock baseline if fixed reference is needed.
Read & Copy
Read the interior degrees, reflex complement, or radians in real time. Click Copy to place exact measurement values on your clipboard.
How it works
Pointer geometry in plain language
An angle is the rotation between two rays from the same vertex. The tool does pure vector math — arccos(dot/|A||B|) — on the points you place. Four moves control it.
- 1Set center
Drag the glowing ring to the vertex — the single point where the two rays meet. Use crosshair and grid for sub-pixel alignment; magnifier appears while dragging.
- 2Align baseline
Drag handle A so the first ray sits exactly on one edge at 0°. Lock baseline if you need it fixed while testing the second ray.
- 3Drag second ray
Drag handle B to the second edge. Lengthen rays away from the vertex for stability — short rays amplify placement error.
- 4Read angle
Live chip shows interior (≤180°), reflex (360°−inner) and radians. Toggle Measure reflex or 180°/360° mode, snap, and Copy in your chosen unit.
Tip: Keyboard: Tab to focus center/A/B, Arrows nudge 1° (Shift 5°), R reset, L lock, S snap. Also try full how-to with live tool.
Angle types — quick reference
The pill next to your reading classifies the angle live. Five types cover every case the tool can show.
0° < acute < 90° · eg 30°, 45°, 60°
Exactly 90° — highlighted green in tool
90° < obtuse < 180° · eg 120°, 135°
A straight line — flat edge in 180° mode
Larger side · eg 210°, 270°, 300°
Reflex + interior = 360°. Use 360° protractor or the reflex toggle. More in angle finder.
Accuracy & limitations
Decimal precision is not the same as measurement accuracy
We publish limits, not inflated ±0.01° badges. The vector math is exact — real error comes from how you place the points and how the photo was taken. Showing 45.12° does not mean ±0.01° real.
Error grows near the vertex. Zoom the view, use crosshair and grid, drag handles far along the true edge centerline. Magnifier appears while dragging.
Low-resolution or compressed images blur edges. Use the highest-resolution crop, avoid recompression, fit image to view. Thick lines hide the true centerline.
A tilted photo skews angles — a rectangle becomes a trapezoid, a right angle can look acute. Keep the lens parallel and centered; otherwise note it as an apparent 2D angle.
Wide-angle lenses bend straight lines near edges. Center the subject, avoid extreme wide-angle, and keep the protractor away from image margins where distortion is strongest.
Resized, stretched or exported screenshots change on-screen pixels without changing the underlying angle — placement still drifts. Use the original aspect, or fit image without non-uniform stretch.
Browser zoom (Ctrl +/−) and OS display scaling do not change the angle calculation, but they change touch target size and placement. Reset to 100% for repeatability.
Precision 0–2 decimals controls display only. Showing 89.84° does not mean ±0.01°. Use decimals to track trends, but describe real accuracy as “within a few degrees given photo and placement.”
- Good: “~90° (photo, uncalibrated)” · Avoid: “90.00° ±0.01°” without calibration.
- Toggle degrees/radians; supplement/complement helpers live on angle relationships.
Trust & privacy
Built as a fast, honest reference — not a credential. Here’s what’s true today:
No price, no paywall, no credits. All protractor modes and image features are free in your browser.
No sign-up, no sign-in and no database. Close the tab and nothing about your measurement is retained on a server.
Images are read locally via FileReader and blob: URL. Check DevTools → Network: you’ll see no upload. Camera, when supported, is permission-gated and HTTPS-only.
We show no fake security badges, no invented expert endorsements, no counter widgets and no “±0.0° certified” claims. For methodology and image workflow, see About and Privacy.
Frequently asked questions
Concise answers only — claims are limited to what the tool actually does.
What is an online protractor?⌄
An online protractor is a free browser tool that measures angles directly on your screen. You place its center on the vertex where two rays meet, align the rays along the edges, and read the angle live without installing an app.
How do I measure an angle online?⌄
Drag the glowing center to the vertex, align the baseline ray (handle A) to 0° on one edge, drag the second ray (handle B) to the other edge, and read the live interior and reflex values. Use Snap 5° while learning and Off or 1° for fine adjustments.
Can I measure an angle from a photo?⌄
Yes. Click Use image, drag and drop, or paste with Ctrl+V a PNG, JPG/JPEG or WebP up to 12 MB. The file is read locally with FileReader and a blob: URL and never leaves your device. Pan, zoom and rotate the photo, then move the protractor center independently.
Is the online protractor accurate?⌄
The angle math is exact for the points you place. Real accuracy depends on pointer placement, image resolution, perspective, camera lens distortion, transformed or scaled screenshots, browser zoom and line thickness. For safety-critical work, verify with a calibrated physical protractor.
Does it work on phones and tablets?⌄
Yes. The canvas is touch-first with large 28 px hit targets, scrollable toolbar and full-width view optimized for 320–430 px screens. It works with touch, mouse and keyboard and renders useful HTML before JavaScript loads.
Can I measure reflex angles over 180°?⌄
Yes. The tool shows the interior angle up to 180° and the reflex complement 360° minus interior together. Toggle Measure reflex angle to make the larger side primary, or switch to 360° mode for a full-circle view such as 270° or 300°.
What is the difference between degrees and radians?⌄
Degrees and radians are two units for the same angle. 180° equals π radians (about 3.1416), so 90° equals π/2 (about 1.571). Toggle Degrees and Radians in the toolbar — Copy and sharing use the active unit, and precision 0–2 decimals only changes display.
Does the tool upload my image?⌄
No. Images are processed entirely in your browser. They are loaded via FileReader and URL.createObjectURL as a blob: URL, never uploaded to a server. Replacing or clearing the image revokes the URL with URL.revokeObjectURL, and nothing is stored in localStorage or IndexedDB.
Why can a photo angle differ from the real-world angle?⌄
The tool measures the 2D projection you see on screen, not the physical 3D object. A tilted photo turns a rectangle into a trapezoid, a right angle can look acute, and wide-angle lens distortion bends straight lines near the edges. Keep the lens parallel and centered for the closest match.
Can I use it without downloading an app?⌄
Yes. It runs entirely in your browser — no install, no sign-up and no plugins. Open the site on desktop or mobile and measure immediately. You can add it to your home screen for quick access, but it works as a normal web page.