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Online Protractor - Measure Angles with Precision
Free browser-based protractor for students, teachers, designers, and engineers
Upload images, place protractors, draw rays to find angles, and export your data. Works in your browser—no downloads or sign-ups needed.
Core Features
Everything you need from an online protractor
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
Add as many protractors as you need to your workspace. Each protractor operates independently—move, rotate, and scale them separately. Click the '+' button to add a new protractor, or duplicate an existing one to copy its color and settings. Choose between semicircle (180°) and full circle (360°) styles. Lock protractors to prevent accidental movement while you work.
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
Enable angle snapping to lock rays to whole degrees or custom increments (1°, 5°, 10°, 15°, 30°, 45°, or 90°). When you drag a ray near a snap point, you'll see a visual hint. Toggle grid overlay with adjustable spacing (10px to 100px) and opacity. Add ruler overlays to measure distances. Snapping helps you create consistent measurements and align rays to specific values.
Export and Save Your Work
Export measurements as CSV files with columns for protractor ID, ray ID, angle, length, and color. Save your entire workspace as JSON to reload later—includes all protractors, rays, images, and settings. Export the canvas as PNG or JPEG at 1x, 2x, or 4x resolution for high-quality prints. Choose transparent, white, or black backgrounds. Optionally export only the measurement overlay without background images.
What Is an Online Protractor?
An online protractor is a browser-based tool that measures angles on your screen. Unlike physical protractors limited to 1° precision, digital protractors calculate angles mathematically. This tool uses a ray-based measurement system: you place rays from the protractor center, and the angle between them is calculated automatically. The result is displayed in degrees or radians with up to 4 decimal places of precision.
- Measure angles on uploaded images, photos, or technical drawings
- Achieve precision up to 0.0001° (4 decimal places)
- Save your workspace and reload it later to continue work
- Export measurements as CSV or images for reports and documentation
- 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
Look for the Protractor panel on the right side of the screen. Click the '+' button to add a protractor. It appears at the canvas center. Drag the protractor to move it, or enter exact X and Y coordinates in the panel. You can add multiple protractors if you need to measure angles in different areas. Press 'V' to activate the Select tool if you need to move or resize the protractor.
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
Use the Protractor panel to adjust position (X, Y), rotation (0-360°), and scale (0.1x to 5x). Change the protractor color if you need better contrast. In the Display Options panel, toggle angle snapping to lock rays to whole degrees. Adjust decimal precision (0-4 places) and switch between degrees and radians. When finished, click the download icon in the left toolbar to export as PNG, JPEG, CSV, or JSON.
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 system dates back to ancient Babylonians (c. 2000 BC), who used a base-60 numeral system. They likely chose 360 because it's close to the number of days in a year (365) and is highly divisible (by 2, 3, 4, 5, 6, 8, 9, 10, 12, etc.), making calculations easier.
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 basketball, the optimal release angle for a jump shot is around 45-48 degrees.
Honeycombs are built with perfect 120° angles (hexagons) to maximize space and strength.
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 entirely in your web browser using HTML5 Canvas technology. No downloads, no browser extensions, no account creation. Just visit the website and start measuring. Works on Chrome, Firefox, Safari, and Edge. Your work is saved in your browser's local storage, so you can close the tab and return later.
Built for Accuracy
Angles are calculated using the arctangent function (Math.atan2), providing mathematical precision. Decimal precision goes up to 4 places (0.0001°). Angle snapping locks rays to exact increments—choose from 1°, 5°, 10°, 15°, 30°, 45°, or 90° steps. Grid and ruler overlays help you align protractors precisely. The ray-based system eliminates the guesswork of aligning physical protractor edges.
Designed for Real Work
Upload images up to several megabytes in size. Add multiple protractors to measure different areas of the same image. Lock protractors and images to prevent accidental changes. Use keyboard shortcuts (V, M, Space, Ctrl+Z, Delete) to work faster. Undo and redo support up to 50 steps. Export measurements as CSV for spreadsheet analysis or JSON to save your entire workspace.
Free and Private
This tool is completely free with no usage limits, no watermarks on exports, and no premium features locked behind paywalls. All processing happens in your browser—your images and measurements never leave your device. We don't collect, store, or track any of your data. No cookies, no analytics, no third-party scripts.
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 check angles in mockups, UI layouts, and graphic designs to ensure visual balance. Architects upload floor plans or elevation drawings to verify roof pitches, stairways, and structural elements. Check perspective in 3D renderings. Measure between walls, windows, and doors. Export high-resolution images (2x or 4x) for client presentations or construction documentation.
Engineers and Technicians
Mechanical engineers analyze CAD drawings to verify component tolerances. Electrical engineers check circuit board layouts. Quality control technicians upload photos of manufactured parts to verify against specifications. Export measurements as CSV files to include in inspection reports. Use multiple protractors to check several values on the same technical drawing simultaneously.
DIY Enthusiasts and Hobbyists
Woodworkers plan miter cuts and verify furniture joints. Metalworkers check welding projects. Model builders measure reference photos to replicate details accurately. Home improvement enthusiasts check roof, stair, and deck angles from photos. Crafters measure for origami, paper crafts, and sewing patterns. This online protractor helps you get precise measurements without buying a physical protractor.