Protractor
Drag to measure, on screen
45.0°
Angle 45.0 degrees, plus or minus 0.4
- Supplement
- 135.0°
- Complement
- 45.0°
- Reflex
- 315.0°
- Arm length
- 403 / 403
Tab to a point, then use the arrow keys. Hold Shift to move further, Alt for finer steps. Paste a screenshot to measure anything on your screen.
About this tool
A precise on-screen protractor. Drag a vertex and two arms for an exact reading, or use the classic rotating protractor face with both scales. Paste a screenshot to measure anything on your screen.
How to use a protractor
- Put the protractor’s centre mark exactly on the vertex, where the two lines meet.
- Turn the protractor so its zero line lies along one arm of the angle.
- Follow the other arm out to the edge and read the scale that starts from zero on the first arm.
- Check that the answer makes sense: an angle narrower than a square corner must read less than 90°. If it does not, you are reading the other scale.
The on-screen protractor above does the same job without the guesswork: drag the vertex onto the corner and each arm onto a line, and it reads the angle for you.
Getting an exact reading
Measuring something on your screen
Paste a screenshot with Ctrl+V (⌘+V on a Mac) or use Capture screen, and the image sits behind the protractor. That lets you measure an angle in a drawing, a map, a chart or a CAD export without printing it. Drag the vertex onto the corner and each arm along a line, and the reading updates as you move them.
With a keyboard, press Tab to reach a point and nudge it with the arrow keys: hold Shift to move further, or Alt for finer steps. Snapping holds the arms to steps from half a degree up to 90° when you are drawing an angle rather than measuring one.
How precise is it?
The angle is calculated from the positions of the three points, so the arithmetic is exact. What limits the result is how precisely the points sit on the lines, and the page shows that as a margin of error. A point one pixel off matters less the further it is from the vertex, so stretch each arm as far along its line as it will go, and zoom in to place the corner.
Reading the related angles
Next to the main reading you get the reflex angle, the other way round the vertex (360° minus the reading), and, for angles under 180° and 90°, the supplement and complement. A measurement you save stays on this device and can be picked up in calculators such as the angle converter and the miter calculator, so you can turn a measured corner into saw settings without retyping it.
Frequently asked questions
What is a protractor?
A protractor is a tool for measuring and drawing angles in degrees. The usual school protractor is a half circle marked from 0° to 180°; a full-circle protractor goes all the way round to 360°, so it can measure reflex angles directly.
A digital protractor shows the angle as a number, so there is no scale to misread. The on-screen protractor on this page works that way: drag its arms onto the lines and it reads the angle for you.
Which scale do you read on a protractor, the inner or the outer?
Read the scale that starts at 0 on the arm you lined up with the baseline. On most protractors the inner scale starts at 0 on the right and the outer scale starts at 0 on the left, so the two run in opposite directions and either arm can be the starting side.
Then check the answer against the shape: an angle narrower than a square corner must read less than 90°. If a sharp angle reads 130°, you have read the other scale and found its supplement.
How do you measure the angles of a triangle with a protractor?
Measure each corner in turn: put the centre mark on the corner, line the baseline up with one side, and read where the other side crosses the scale. If the sides are too short to reach the scale, extend them with a ruler first.
The three angles of any triangle add up to 180°, so use that as a check. If you already know three of its sides and angles, the triangle calculator works out the rest exactly.
What is a bevel protractor?
A bevel protractor is a precision instrument for measuring and setting angles on metal and wooden parts. It has a flat stock, a blade that swivels against a graduated dial, and, on a vernier bevel protractor, a vernier scale for readings finer than a degree.
The least count of a standard vernier bevel protractor is 5 minutes of arc (5′), one twelfth of a degree. To turn an angle into saw settings for frames, trim or crown molding, use the miter angle calculator.
Guides
About these measurements
Readings are estimates taken from rendered pixels and consumer device sensors. Do not rely on them for structural engineering, surveying, manufacturing inspection, navigation, or medical decisions. Check anything safety-critical with a calibrated instrument.
All processing happens in your browser. No image, camera frame or sensor reading is ever uploaded.
Other ways to measure
Photo measurement
Measure angles in an image
Drop, paste or upload a photo and measure any angle in it. Magnifier for precise placement, perspective correction for off-angle shots, unlimited measurements, and export to PNG, CSV or JSON.
Camera measurement
Sight an angle through your camera
Aim your camera at a roofline, a tree or a slope and read the angle of elevation. Add the distance to work out the height, and hold the reading to note it down.
Printable protractor
Print a protractor at true size
Set up a half or full-circle protractor, size it for A4 or Letter paper, add a marked angle to use as a cutting template, and print it at exactly the right scale, with a check bar to confirm.