Online Protractor

Measures the angle of an object's corner with a protractor drawn on your screen, or an angle in a photo by placing three points. Use it to check angles for woodwork, drawings or math homework.

How to use

1. On the "On-screen protractor" tab, set "Shape" to "Half, 180°" or "Full, 360°".
2. Put the corner of the object on the red dot and drag the ends of arms A and B onto its two edges; the "Angle" appears in large type. Drag the plate to move it and the round handle to rotate it.
3. For an angle in a picture, open "Measure on a photo", add an image, and click the corner, a point on one side and a point on the other side.
4. Under "Unit", choose "Degrees" or "Radians".

How it works

• An angle does not depend on the size of the screen, so no calibration is needed.
• The on-screen arms move in steps of 0.5°. The half shape shows the smaller angle between the arms (0–180°); the full shape shows the angle counter-clockwise from arm A to arm B (0–360°).
• The supplementary angle is 180° minus the angle, the complementary angle is 90° minus it, and the angle on the other side is 360° minus it. Radians = degrees × π ÷ 180.
• Photos are processed on this device only and are never uploaded.

Questions and answers

Can I measure an angle with my phone?

Yes. Hold the object against the screen, put its corner on the red dot and move the two arms onto its edges. If the object rests on the screen, use the − / + buttons to move the arms.

Why is the angle in my photo different from the real one?

A photo taken at a slant makes angles look wider or narrower than they are. Use a photo taken straight on to the surface you want to measure.

How many radians is 90 degrees?

π ÷ 2, about 1.5708 rad. 180° is π (about 3.1416) rad, and 1 rad is about 57.3°.