Online protractor
Drag the two coloured handles around the dial, or focus a handle and use the arrow keys. The angle between the arms updates live, along with the supplementary and complementary angles. It measures what you set up, so use it to check an angle you read from a diagram or to draw a specific one.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Drag arm A and arm B to the angle you want, or step them with the arrow keys.
- Read the angle between the arms for a normal 0–180° value, or the sweep for the full 0–360° counter-clockwise reading from A to B.
- Turn off Snap to whole degrees for tenth-of-a-degree control.
How it works
Each handle’s direction is found with atan2 from the centre point. The sweep is (B − A) wrapped into 0–360°. The angle between the arms is the sweep if it is 180° or less, otherwise 360° minus the sweep. The supplement is 180° minus that angle, and the complement is 90° minus it when the angle is 90° or less.
Worked example
Arm A at 0° and arm B at 270°: the sweep is 270°, the angle between the arms is 360 − 270 = 90°, its supplement is 180 − 90 = 90°, and its complement is 90 − 90 = 0°.
Tips
- A right angle is exactly 90°, so with arm A at 0° put arm B at 90° or 270°.
- To mark a 45° angle, set A to 0° and B to 45°, or use the arrow keys from the default position.
Frequently asked questions
Do I need to calibrate the screen?
No. Angles do not depend on screen size or resolution, so the readings are correct without calibration. If you have saved a calibration in the Screen Ruler tool, the protractor is drawn at about 10 cm wide on your screen; otherwise it scales to fit.
Can I use it to measure a photo or drawing?
You can hold the screen over a printed angle or place the same shape on screen, but this tool does not load images. It is a standalone protractor where you set the arms yourself, which suits checking homework, drawing a target angle, or comparing angles.
What are supplementary and complementary angles?
Two angles are supplementary if they add up to 180° and complementary if they add up to 90°. Only an angle of 90° or less has a complement.
Why are there two different angle readings?
The first is the sweep from arm A to arm B going counter-clockwise, from 0 to 360°. The second is the ordinary geometric angle between the two lines, always 0–180°, which is the smaller of the sweep and 360° minus the sweep.