Online Protractor
Drag the two arms of a full-circle protractor to measure any angle, or upload a photo and lay the protractor over it. Your image is opened locally in your browser and is never uploaded.
Drag the round handles to move the arms. Drag the center dot to move the protractor.
- Inner angle
- 60.0°
- Supplement
- 120.0°
- Complement
- 30.0°
- Reflex angle
- 300.0°
- Radians
- 1.0472 rad
- Angle type
- Acute
Click point A on the first line, then the vertex B, then point C on the second line.
How to use the Online Protractor
- Drag the arm handles (the round dots at the ends of the green and orange arms) to line them up with the two sides of the angle.
- Read the angle in the panel: the counter-clockwise angle from the green arm to the orange arm, plus the inner angle, supplement and reflex angle.
- Measure a photo: press Upload image, then drag the center dot to move the protractor onto the corner and use the size slider to scale it.
- Use 3-point mode for photos: click a point on the first line, then the vertex, then a point on the second line; drag any point to fine-tune.
- Press Copy angle to copy the result.
Types of angles
| Angle type | Size | Everyday example |
|---|---|---|
| Acute | greater than 0° and less than 90° | The tip of a pizza slice, a typical roof pitch |
| Right | exactly 90° | The corner of a book or a door frame |
| Obtuse | greater than 90° and less than 180° | A reclined chair back, an open laptop |
| Straight | exactly 180° | A flat line, a fully opened book |
| Reflex | greater than 180° and less than 360° | The outside of a corner |
| Full turn | 360° | One complete rotation |
Related angles: two angles are complementary when they add up to 90° (e.g. 35° and 55°) and supplementary when they add up to 180° (e.g. 120° and 60°). An angle and its reflex angle always add up to 360°.
How the 3-point angle is calculated
In 3-point mode you click a point A on the first line, the vertex B and a point C on the second line. The tool measures the direction of each line from the vertex with the two-argument arctangent, atan2(Δy, Δx), and takes the difference. The equivalent vector formula is:
∠ABC = arccos( (BA · BC) / (|BA| × |BC|) )
Worked example: A = (4, 0), B = (0, 0) and C = (−3, 3). Then BA = (4, 0) and BC = (−3, 3), the dot product is −12, |BA| = 4 and |BC| = 4.243, so cos ∠ABC = −12 ÷ 16.97 = −0.7071 and ∠ABC = 135°. Its supplement is 45° and its reflex angle is 225°.
Converting units: radians = degrees × π ÷ 180, so 90° = π/2 ≈ 1.5708 rad and 1 rad ≈ 57.296°.
Tips for measuring angles in photos
- Shoot straight on. Perspective distorts angles. Hold the camera parallel to the surface you are measuring, as square to it as you can, or use a scan or screenshot.
- Pick far-apart points. In 3-point mode, place A and C as far from the vertex as possible; a small placement error then changes the angle much less.
- Turn snapping off for tenths of a degree, or keep it on when you only need whole degrees and want steady readings.
- Use it for slopes and posture. Measure roof pitches, ramp slopes, stair angles, bike fit, joint angles in exercise photos or angles in geometry homework. For a live slope reading with your phone's sensors, try the bubble level.
Frequently asked questions
How do I measure an angle with this online protractor?
Place the vertex of the angle on the center of the protractor (drag the center dot to move it), then drag the green arm along one side and the orange arm along the other side. The angle between them is shown in degrees.
Can I measure an angle in a picture?
Yes. Press Upload image to open a photo or screenshot, then either move and resize the protractor over it or switch to 3-point mode and click three points: one on each line and the vertex in the middle. The image stays on your device.
What is the difference between the angle, the supplement and the reflex angle?
The inner angle is the smaller angle between the arms, from 0 to 180°. Its supplement is 180° minus that angle, which is what the angle on the other side of a straight line measures. The reflex angle is 360° minus the inner angle, the 'outside' of the angle. The complement (90° minus the angle) only exists for angles up to 90°.
Which direction does the protractor measure?
Like a math protractor, 0° points to the right and angles increase counter-clockwise, so 90° points up. The main reading is the counter-clockwise turn from the green arm to the orange arm, which is why it can go up to 360°.
How accurate is it?
With snapping off and the arms dragged out to full length, readings are accurate to about a tenth of a degree. In practice, accuracy depends on how precisely you place the arms, so zoom into a photo or use a larger protractor size for fine work. Typing exact arm angles is also supported.
Can I measure a real object held against the screen?
Yes, for flat objects. Hold the object against the screen with its corner on the center dot and line up the arms with its edges. Unlike a ruler, angle measurements do not need any screen calibration.