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Online Protractor - Measure Angles on Images
Free browser-based protractor for students, teachers, designers, and engineers
Upload a photo, diagram, or PDF screenshot, place rays, and export results for homework, visual design checks, or documented review. The measuring workflow works without an account.
Core Features
Built for real measurement tasks, from quick checks to documented exports
Ray-Based Measurement System
Click to place rays from the protractor center. Values between adjacent rays are calculated automatically and displayed in real-time. Switch between degrees (0-360°) and radians. Adjust decimal precision from 0 to 4 places. Toggle between inner and outer display. Drag ray endpoints to adjust—you'll see a live preview as you move.
Image Upload and Overlay
Upload PNG, JPG, or other image formats. This online protractor works on photos, diagrams, or technical drawings. Images appear at the canvas center, automatically scaled to fit 60% of your viewport. Drag images to reposition them. Use corner handles to resize, and the rotation handle (top-center) to rotate. Adjust opacity from 0% to 100% to balance visibility between your image and measurement overlays. Lock images to prevent accidental changes.
Multiple Protractors
Free includes one protractor. Plus and Pro let you add and independently move, rotate, scale, duplicate and lock multiple protractors for comparing different areas in one workspace. Each protractor supports semicircle (180°) and full-circle (360°) styles.
Customizable Display
Change protractor tick mark colors to create contrast against your background images. Adjust protractor scale from 0.1x to 5x for precise alignment. Rotate protractors from 0° to 360° to match any orientation. Set ray colors individually for better visibility. The protractor uses a glass-like appearance that adapts to light and dark themes automatically.
Angle Snapping and Grid Overlay
Plus and Pro add angle snapping, an adjustable grid and ruler overlays. Snapping can align rays to configured increments, while grid spacing and opacity help with repeatable visual placement. These overlays support alignment but do not calibrate an image or physical distance.
Export and Save Your Work
Every plan can manually export/import the full workspace as JSON and export a 1x PNG without a watermark. Plus and Pro add CSV, JPEG, overlay-only and 2x/4x image export. Pro also adds a professional PDF report with a measurement disclaimer.
What Is an Online Protractor?
An online protractor is a browser-based tool for reading angles on your screen. This tool uses a ray-based measurement system: you place rays from the protractor center, and the angle between them is calculated automatically. Results can be displayed in degrees or radians with up to four decimal places, but those digits describe display resolution—not guaranteed real-world accuracy. It works best for images, diagrams, screenshots, and on-screen documents; verify physical materials or important measurements with the object itself and an appropriate calibrated tool.
- Measure angles on uploaded images, photos, or technical drawings
- Display calculated results with 0–4 decimal places and documented accuracy limits
- Export a manual JSON backup and reload it later
- Choose optional Plus or Pro tools for advanced exports and local recovery
- Work from any device with a modern web browser—desktop, tablet, or phone
How to Use This Online Protractor
Step-by-step guide to measuring angles
Step 1: Add a Protractor to the Canvas
A Free workspace starts with one protractor. Select it, then drag it or enter X and Y coordinates in the panel. Plus and Pro users can use the '+' button to add or duplicate additional protractors for other areas. Press 'V' to activate Select before moving or resizing objects.
Step 2: Switch to Measure Angle Mode
Click the protractor to select it—you'll see a blue dashed outline. Then click the 'Measure Angle' tool in the left toolbar (triangle icon), or press 'M' on your keyboard. If you haven't selected a protractor first, you'll see a message asking you to select one. The protractor must be selected before you can draw rays.
Step 3: Place Rays to Find Angles
Move your mouse around the protractor center. You'll see a preview line showing where the ray will go. Click to place the first ray. Move your mouse again and click to place a second ray. The measurement between the two rays appears automatically as a label on the arc. Continue clicking to add more rays. Each new ray creates a measurement with the previous ray. Drag the endpoint of any ray to adjust—the label updates in real-time.
Step 4: Fine-Tune and Export
Adjust position, rotation, scale, color, decimal display and angle unit. Every plan can download JSON and a 1x PNG. Plus and Pro unlock snapping, grid/ruler overlays, CSV, JPEG and higher-resolution export; Pro also adds PDF reports.
Comprehensive Angle Knowledge
Everything you need to know about angles, types, and measurements
What is an Angle?
An angle is formed when two rays meet at a common endpoint called the vertex. It measures the amount of turn between the two rays.
Why 360 Degrees?
The 360-degree convention grew from ancient Mesopotamian base-60 mathematics and later astronomical practice. Historians discuss several contributing reasons, so the exact original choice should not be reduced to a single explanation.
Read full historyNaming & Notation
- •Typically named using three letters (e.g., ∠ABC), where the middle letter is the vertex.
- •Greek letters like θ (theta), α (alpha), β (beta) are often used to represent the angle value.
- •Positive angles are measured counter-clockwise; negative angles are measured clockwise.
The 5 Fundamental Types
Explore all typesAcute Angle
Sharp, smaller than a right angle.
Right Angle
A perfect 'L' shape. The most common angle in construction.
Obtuse Angle
Wide, larger than a right angle but smaller than a straight line.
Straight Angle
A straight line. It represents half of a full turn.
Reflex Angle
Bent backwards, larger than a straight angle.
Angle Relationships
Complementary
Two angles that add up to 90°.
Supplementary
Two angles that add up to 180°.
Vertical (Opposite)
Angles opposite each other when two lines cross. They are always equal.
Adjacent
Angles that share a common side and vertex but don't overlap.
Angles in Daily Life
In ball sports, launch angle works together with speed, release height, spin, drag, and the target; there is no single best angle for every shot.
Honeycomb cells approximate hexagonal patterns with repeated angles that can be studied using geometry; natural cells are not assumed to be perfectly regular.
Roof pitches are measured in angles to ensure proper water drainage.
Astronomers use angles to measure distances between stars and the size of celestial bodies.
Measurement & Units
Learn about unitsHow to Use a Protractor
- 1
Align the center point of the protractor with the angle's vertex.
- 2
Rotate the protractor so the 0° line (baseline) lies on one side of the angle.
- 3
Read the number where the other side crosses the scale.
- 4
Note: Use the inner scale if measuring from the right, outer scale if from the left.
Why Use This Online Protractor?
Browser-Based Online Protractor
This online protractor runs in your browser without an extension, and Free measurement needs no account. Manual JSON backups work on every plan. Plus and Pro can automatically recover the last workspace from IndexedDB in the same browser; Pro named projects are also local and do not sync across devices.
Transparent Calculation and Limits
Angles are calculated from the two ray directions with Math.atan2 and can be displayed with 0–4 decimal places. Extra digits do not correct an off-center vertex, thick source lines, blur, scaling, or perspective distortion. Angle snapping offers repeatable increments of 1°, 5°, 10°, 15°, 30°, 45°, or 90°, while grid and ruler overlays support alignment. Recheck important results with a second method.
Designed for Real Work
Free supports one image and one protractor with ray measurement, keyboard shortcuts, undo/redo, JSON backup and 1x PNG. Plus adds multiple objects, overlays and advanced exports. Pro adds local project management and PDF reports.
Free and Private
The core measuring tool is free and exports do not add watermarks. Most measurement work happens in your browser, so images and workspace data stay local during normal use. Umami analytics loads on each page, and GA4 loads when configured; the Privacy Policy explains the data involved.
Who Uses This Tool?
Students and Teachers
Students use this online protractor to check angles in geometry homework, verify their protractor readings, and explore relationships in triangles and polygons. Teachers create visual examples for lessons by uploading diagrams and measuring in real-time during class. The tool helps explain concepts like complementary angles (adding to 90°), supplementary angles (adding to 180°), and angle bisectors. Export measurements as images to include in homework submissions or presentations.
Designers and Architects
Designers can check angles in mockups and layouts. Architects can use plans or elevations for preliminary visual review, while verifying critical dimensions against source drawings and calibrated methods. Plus and Pro provide 2x/4x exports for documentation.
Engineers and Technicians
Engineers and technicians can use screenshots for preliminary visual checks and discussion. Plus and Pro can export CSV; Pro can document ray measurements in PDF. None of these replace calibrated CAD, metrology or quality-control equipment.
DIY Enthusiasts and Hobbyists
Woodworkers can make preliminary visual checks before verifying cuts on the material. Metalworkers, model builders, home improvement enthusiasts and crafters can compare visible angles in reference images. Results from photos remain estimates affected by image quality and perspective, so verify any cut, fit or safety-critical decision with an appropriate physical tool.