Online protractor: measure angles on screen
Measure any angle on your screen: put the centre on the corner, drag the two arms along its sides and read the angle at once. An angle doesn't depend on how big your screen is, so the reading is exact without calibrating; calibrate only if you want the protractor itself the size of a real one.
Runs in your browser: nothing is uploaded, it's free and there's no sign-up.
How do I measure an angle on screen?
- Drag the protractor so its centre sits on the corner (the vertex) of the angle.
- Drag the first arm along one side of the angle.
- Drag the second arm along the other side.
- Read the angle in degrees above the protractor. To set an arm precisely, type the angle, or use the arrow keys: 1° per press, 0.1° with Shift.
- Tap Lock so the protractor stays put while something rests on the screen.
Questions people ask
Do I need to calibrate the protractor?
No. An angle is the same at any scale, so the reading is right on any screen straight away. Calibration only changes the size of the protractor itself, which helps when you hold a real object against it.
How do I measure the angle of a real object?
Lay the screen flat, ideally a phone or tablet on a table, and lock the protractor. Set the object on the screen with its corner on the centre, then line the arms up with its edges.
How precise is the reading?
The reading shows tenths of a degree. You can type an exact angle or nudge an arm with the arrow keys, 1° at a time or 0.1° with Shift. Longer sides on screen are easier to line up precisely.
Can it measure angles over 180°?
Yes. Switch from the half-circle protractor (0° to 180°) to the full circle (0° to 360°) to read reflex angles.
Does it work on a phone?
Yes. Drag the centre and the arms with a finger, turn the phone sideways for longer arms, and lock the protractor before you lay anything on the screen.
Is it free, and is anything uploaded?
It's free with no sign-up, and it runs entirely in your browser: nothing you measure leaves your device.
A short guide
How do you read a protractor?
Put the centre on the vertex, line up zero with one side and read where the other side crosses the scale. A paper protractor has two scales running in opposite directions, which is why it's easy to read the wrong one; this one shows the number directly, and the arms show which angle is being measured.
Acute, right, obtuse or reflex?
An angle under 90° is acute, exactly 90° is a right angle, between 90° and 180° is obtuse, 180° is a straight angle, and over 180° is reflex. The two sides of a corner make two angles that add up to 360°; the full-circle protractor shows the larger one too.