Online Protractor
Measure and draw angles directly on your screen. Use the interactive protractor scale or upload a photo to measure angles from any image.
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Click 3 points (Arm 1, Vertex, Arm 2) to measure any angle inside the photo. 100% browser-based.
About the Online Protractor Tool
The Online Protractor is a free, 100% browser-based mathematical utility designed for students, educators, engineers, artists, and woodworkers who need to measure, construct, or analyze angles quickly on-screen. Traditional physical protractors require laying plastic tools against physical paper, but our digital protractor operates directly on your web browser screen with zero download or installation required. If you also need to match the color of a line, shape, or fill, pick the exact color with our Color Picker & Converter and copy the HEX, RGB, or HSL value.
Whether you are verifying geometry homework, checking architectural drafting blueprints, analyzing joint angles in orthopedics, or extracting angles from digital photos and screenshots, this tool provides dual operational modes: an interactive vector protractor and an image-based angle finder. For documenting your measurements in a report or README, convert your notes to HTML with our Markdown to HTML Converter.
How to Use the Interactive Protractor (Standard Mode)
- Select Scale Mode: Choose between the 180° semi-circle scale for standard geometric angles or the 360° full-circle scale for reflex angles and complete rotations.
- Adjust the Pointer Arm: Click and drag the blue handle knob on the protractor arm to rotate it around the origin vertex.
- Precision Numeric Entry: Type an exact degree value into the angle input box to sync the protractor arm instantly to any angle.
- Read Angle Properties: View the real-time angle value alongside its complementary angle (90° - x), supplementary angle (180° - x), and standard classification.
- Fine-Tune with Keyboard: Click the protractor workspace and use the Left and Right Arrow keys to adjust by 1° increments, or hold Shift with Arrow keys for 10° steps.
- Flip & Copy: Click Flip to invert the angle direction or Copy to save the measurement summary to your clipboard.
How to Measure an Angle from an Image or Photo
Measuring angles on physical objects, diagrams, or digital screenshots is effortless with our built-in Image Mode:
- Switch to the Image Mode (Photo Measurement) tab at the top of the tool container.
- Upload any image file (PNG, JPG, WEBP, GIF, SVG) or drag and drop a screenshot onto the dropzone. Large photos from mobile cameras are automatically scaled to fit your display cleanly — then use our Color Picker & Converter to pick colors from the same image.
- Click three points directly on the image: 1st point at the tip of the first arm, 2nd point at the vertex (corner point), and 3rd point at the tip of the second arm.
- The tool automatically connects the points with vector rays, shades the angle sector, and displays the exact interior angle value.
- Drag any of the 3 points to fine-tune their position. Touch input is fully supported for mobile devices and tablets.
How to Read a Physical Protractor
Reading a physical or digital protractor relies on aligning three reference elements accurately:
First, locate the origin center point (the crosshair vertex) and align it exactly over the corner point where the two line segments meet. Next, align the 0° baseline of the protractor flat along one of the angle's rays. Finally, trace the second ray outward to the point where it intersects the degree scale along the curved outer arc.
Standard protractors feature dual scales: an inner scale (0° to 180° counterclockwise) and an outer scale (0° to 180° clockwise). If your baseline ray points to the right, read the inner scale starting from zero on the right side. If your baseline ray points to the left, read the outer scale starting from zero on the left side.
Overview of Angle Types and Classifications
Angles are categorized based on their degree measurement relative to a right angle (90°) and a straight line (180°):
| Angle Type | Degree Range | Description & Properties |
|---|---|---|
| Zero Angle | 0° | Both rays overlap completely with no rotation between them. |
| Acute Angle | 0° < x < 90° | Sharp angle smaller than a right angle (e.g., 30°, 45°, 60°). |
| Right Angle | 90° | Perpendicular lines forming a perfect square corner. |
| Obtuse Angle | 90° < x < 180° | Wide angle greater than a right angle but less than a flat line. |
| Straight Angle | 180° | Forms a completely flat continuous horizontal line. |
| Reflex Angle | 180° < x < 360° | Bends backward, larger than a straight line but less than a full circle. |
| Full Angle | 360° | A complete 360-degree rotation back to the starting reference line. |
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