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
True Size
Calibrate Your Screen
Browsers assume 96 pixels per inch, but real displays vary. Calibrate once and the edge protractor below renders at real-world size — hold a physical object against the screen and measure directly.
Method 1 · Auto-Detect Device
One-click estimate from your reported resolution and pixel ratio. Fast, but honest: browsers cannot read physical panel size, so treat this as a starting point and verify with a credit card for exact work.
- Resolution
- —
- Pixel Ratio
- —
- Estimate
- Not detected yet
Method 2 · Screen Diagonal
Enter your screen’s advertised diagonal (inches). We combine it with your detected pixel resolution: PPI = √(w² + h²) / diagonal.
Method 3 · Credit Card Calibration
Most accurate. Hold a standard credit card (85.60 × 53.98 mm) against the outline below and drag the slider until the widths match exactly. Each step re-computes PPI = outline width / 3.370 in.
- Hold a real card to the screen.
- Align left edges.
- Drag until right edges line up.
Stored only in this browser (localStorage). No upload, no account.
Browser zoom must stay at 100% while calibrating — zoom changes CSS pixel size.
Physical Mode
Edge Protractor & Image Stage
Dock a true-size protractor to any edge, hold a real object against the screen, and place your photo anywhere in the stage. Drag the vertex and the ray — the readout follows live.
A quiet stage, ready to measure
Drop a photo here, or drag the vertex & ray to measure against the true-size edge scale.
True size needs calibration — calibrate above, keep browser zoom at 100%, and re-check with a ruler for exact work. Angle math is exact for the points you place; physical accuracy depends on calibration and holding the object flat to the screen.
Want the full walkthrough? Open the Edge Protractor page →
How to use the Online Protractor in three steps
Place the center on the vertex, align the baseline at 0°, then set the second ray and read the result.
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.
- 1 Set 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.
- 2 Align 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.
- 3 Drag second ray
Drag handle B to the second edge. Lengthen rays away from the vertex for stability — short rays amplify placement error.
- 4 Read 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.
Angle measurement guide
Free online protractor tool for measuring angles
This free online protractor is a digital online protractor that helps you measure an angle without installing an app or carrying a physical tool. It works as an online angle measuring tool on a drawing, a screen capture, or a photo: position the center at the vertex, align one ray to the first edge, and rotate the other ray to the second edge. The live readout makes this an online protractor to measure angles quickly while keeping the measurement controls visible. Because it runs entirely in the browser, there is nothing to install and no account to create — open the page and start measuring.
How to measure an angle accurately
For the most reliable result, zoom in until both edges are clear and place the center exactly where they meet. Set handle A on the baseline at 0°, then place handle B along the second edge and read the result. The degree markings and snap controls make it easy to work with an online protractor in degrees; switch snapping off or to 1° when you need a finer placement. For anyone looking for an accurate online protractor, the calculation is exact for the two rays you set, while real-world precision still depends on image quality, line thickness, perspective, and careful pointer placement. A sharp, high-resolution view with thin, well-lit edges is far easier to align than a blurry or heavily compressed one — if an edge looks thick or fuzzy, zoom in, aim for its centerline, and double-check pointer placement before copying the value.
Degrees, 360° views, and useful measurement features
Use the 180° display for most classroom geometry, or choose the online protractor 360 degree view to inspect reflex angles and full turns. Degree markings run around the dial in both directions so you can read the inside and outside scales just like a classroom protractor, while the digital readout removes any doubt about which scale you are on. The readout supports online protractor degrees as well as radians, so students can check conversions and engineers can compare a design specification in either unit. Open online protractor full screen mode when you want a larger working area, and enable the online protractor ruler overlay to help line up a reference image. On compatible devices, the online protractor with camera option lets you capture a subject and measure the 2D angle in the resulting image.
Built for learning, design, engineering, and DIY
Students can use this virtual protractor online to practise identifying acute, right, obtuse, straight, and reflex angles on homework diagrams and textbook screenshots. Teachers can project it live to demonstrate degree markings and baseline alignment, then share links to the angle guides for independent practice. Designers can check a logo, interface mock-up, or illustration for consistent slopes and corners, while engineers can compare measured angles on plans and CAD screenshots against specifications before moving to a calibrated instrument. For craft, woodworking, tiling, and home-repair tasks — checking a miter-cut sketch, a shelf bracket, or a photographed corner — it is a convenient real online protractor for a quick sanity check, though safety-critical or tolerance-sensitive measurements should always be verified with an appropriate physical tool.
Actual size and printable reference
The online protractor actual size mode is designed for on-screen reference: calibrate your display, keep browser zoom at 100%, and verify the scale with a ruler before relying on physical size. If you need a printable online protractor for a worksheet or bench reference, print from the browser only after checking the printed scale against a real ruler. That small verification step matters because display settings, printer margins, and paper scaling can change a measurement even when the angle itself remains correct.
For everyday work, start with the live tool and use the visual degree markings as a quick check before copying the result. It is useful for a homework diagram, a presentation graphic, a CAD screenshot, a miter-cut sketch, or a photographed corner. Camera and image measurements describe the visible two-dimensional angle, so keep the lens parallel to the surface and use a calibrated physical instrument whenever a build, safety decision, or manufacturing tolerance depends on the result.
Ready to start? Use the tool above as an online protractor for measuring angles, or follow the step-by-step protractor guide for photos, calibration, and 360° measurements.
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.