Online protractor
Updated — measuring an angle in a photo now takes drag-and-drop, extends both arms across the picture as guide lines, and has its own step-by-step.
A free online protractor that doubles as an image protractor: drag the arm to measure any angle, type the degrees to draw one, or open a photo — or drop it onto the protractor — and measure the angle in the picture. It runs in your browser with no download, no upload and no ads. A screen protractor can't touch the real world, though: for angles on a wall, a workpiece, or a roof, the iPhone app reads them straight from the phone's sensors.
Drag the orange handle, type a number, or use the arrow keys.
How to use this online protractor
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Set the angle
Drag the orange handle around the dial, click anywhere on the arc, use the arrow keys, or type an exact number of degrees into the Set angle box.
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Read the value
The angle between the baseline arm and the orange arm is shown live above the protractor, rounded to the nearest degree.
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Switch to 360° for reflex angles
Press 360° to turn the half-circle into a full-circle protractor for angles past a straight line.
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Measure an angle in a photo
Choose a photo, or drag and drop it onto the protractor. Drag the centre dot onto the vertex, drag the grey baseline arm along one edge and the orange arm along the other — the readout is the angle between them. Use + / − to resize the protractor.
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Practise reading angles
Press Practice: the readout hides and a target appears. Set the arm where you think the target is, press Check, and see how close you were.
How to measure an angle in a photo or picture
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Add the picture
Press Measure in a photo and choose an image, or drag and drop it onto the protractor. A photo, a screenshot or a scanned worksheet all work; on a phone, the picker can also take a new photo.
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Put the centre on the corner
Drag the protractor by its centre dot, or by any part of the dial, until the centre sits on the vertex — the point where the two edges meet. + and − make it bigger or smaller.
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Line up both arms
Drag the grey arm along one edge and the orange arm along the other. A thin guide line carries each arm across the whole picture, so you can match a long edge exactly.
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Read the angle
The readout is the angle from the grey arm to the orange arm, counter-clockwise, to the nearest degree. If it shows more than 180° and you wanted the angle on the other side, subtract it from 360°.
Everything runs locally in your browser: a picture you choose or drop is opened on your device and never uploaded; close the tab and it is gone. The protractor reads the angle as the picture shows it, so take the photo square-on to the angle — shot from the side, perspective changes it. More on that in how to measure an angle in a photo.
The placement rules are the same as on paper — origin on the vertex, baseline on one ray, read the scale that starts at 0° on that ray. If that is new to you, start with How to use a protractor, the step-by-step guide with diagrams. Need a physical one? Print a real-size printable protractor (180° and 360°).
What you can do with it
- Measure an angle on screen. Line the baseline up with one edge of anything on your display and swing the arm to the other — a diagram, a PDF, a screenshot.
- Draw an exact angle. Type 37 and the arm sits at 37°. Hold paper against a tablet to trace it, or use it as an angle maker to show what each angle looks like.
- Measure an angle in a photo or image. Drop in a picture of a roof, a corner, a bike frame, a golf swing — or a screenshot — put the vertex on the corner and the two arms on the edges, and read the angle. It works as an image protractor for any picture your browser can open.
- Full-circle protractor. 360° mode handles reflex angles and full turns.
- Practice reading angles. Guess the target, check, improve — useful before a geometry test.
- Touch, mouse, or keyboard. Arrow keys nudge by 1°, Shift + arrow by 10°.
When a screen protractor is enough — and when it isn't
An online protractor is perfect for measuring an angle in an image, teaching how angles work, or a quick sanity check. What it can't do is measure a real object: you can't lay your monitor against a table leg or a roof. That's the gap the Protractor app fills — rest your iPhone on the surface and it reads the angle from the same sensors behind Apple's level, in degrees, percent slope, or an X-in-12 ratio. For the underlying method, see how to measure an angle and how to use a protractor.
An online protractor for students and teachers
A virtual protractor is a clean way to learn how angles work. There's no parallax error from reading a printed scale at an angle, the value updates as you drag, and it's the same on every device — useful for classrooms, homework, and quick practice. Project it on a whiteboard to demonstrate acute versus obtuse angles, let students check an answer, or run the practice mode as a two-minute warm-up. The names of each range are in types of angles; drawing on paper is in how to draw an angle with a protractor.
For many more free virtual manipulatives, from fractions to geometry, Virtual Manipulatives on the Web by Dr. Patricia Deubel at CT4ME is a long-running, well-organized directory for teachers.
Common angles at a glance
| Angle | Type | Where you meet it |
|---|---|---|
| 30° | Acute | Roof pitch of 7-in-12, set square |
| 45° | Acute (half a right angle) | Miter for a square corner, 12-in-12 roof |
| 60° | Acute | Equilateral triangle, hexagon corner |
| 90° | Right angle | Square corners, plumb and level |
| 120° | Obtuse | Hexagon interior, splayed chair legs |
| 135° | Obtuse | Octagon interior, bay-window corner |
| 180° | Straight angle | A flat line — the protractor baseline |
| 270° | Reflex | Three-quarter turn (360° mode) |
When you're ready to measure angles out in the world, see angle measuring tools and the angle finder page — which also has a sensor-based angle finder that runs in your phone's browser.
For angles you meet as a slope rather than a drawing, two other tools on this site do the conversion for you: the roof pitch calculator turns a rise-over-run pitch into degrees (and back), and the digital level reads the tilt of a real surface straight from your phone's sensors.
Measure any angle,
right from your pocket.
Protractor turns your phone into a precision angle finder, level and inclinometer. Measure anything, anywhere.
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