Drag the blue and red handles to set the angle. Drag the middle to move the protractor and the round arrow to turn it. Keyboard: press Tab to pick an arm, then the arrow keys (hold Shift for 10°). Photo: put the middle on the corner, one arm on each line. The photo stays on your device.
About the online protractor
This is a protractor on your screen. It has two arms, a blue one and a red one. Drag them, and the angle between them shows in big numbers at the top.
It also works on a photo. Open a picture of a roof, a ramp, a bent pipe or a homework page, put the middle of the protractor on the corner and lay one arm on each line.
There is nothing to set up. An angle stays the same when a picture gets bigger or smaller, so the size of your screen and of the photo do not matter — no calibration is needed, unlike a ruler.
Features
- Half or full circle — a classic 180° protractor or a 360° one
- Two scales — the outer scale counts 0 to 180, the inner one 180 to 0, like a real protractor
- Measure in a photo — open, drop or paste a picture, or take one with your phone camera
- More numbers — the other side (360° minus the angle), the supplement (180° minus), the slope in % and the roof pitch in 12
- Snap — to 1° or 5°, or off for 0.1° steps
- Zoom the photo — with the mouse wheel, two fingers or the buttons
- Keyboard — move each arm with the arrow keys
- Download picture — a PNG of the photo with the protractor and the numbers on it
How to use it
- Drag the blue and the red round handles to make the angle
- Drag the middle of the protractor to move it, and the round arrow to turn it
- To measure a photo, press Open a photo (or drop it on the protractor)
- Put the middle exactly on the corner, then lay one arm on each line — zoom in for more detail
- Read the angle at the top; press Download picture to save it
How to read a protractor
A paper protractor has two rows of numbers. The outer row goes from 0 on the right to 180 on the left. The inner row goes the other way, from 180 to 0. Both rows are right — which one you read depends on where the angle starts.
Put the centre of the protractor on the corner of the angle. Turn it so one line goes through 0. Now follow the other line out to the scale and read the row that starts at 0 on your first line. If your first line goes through the 0 on the right, read the outer row. If it goes through the 0 on the left, read the inner row.
Less than 90° is acute, 90° is a right angle, 90° to 180° is obtuse, and more than 180° is reflex. The number at the top of this page is always the right one, so you can check your homework with it.
Slope and roof pitch from an angle
A slope is often given in percent, and a roof as a pitch like 6 in 12: it goes up 6 inches for every 12 inches across. The tool shows both. For a roof, lay the blue arm on a level line (the gutter) and the red arm along the roof.
| Angle | Slope | Roof pitch |
|---|---|---|
| 5° | 8.7% | 1.0 in 12 |
| 10° | 17.6% | 2.1 in 12 |
| 15° | 26.8% | 3.2 in 12 |
| 18.4° | 33.3% | 4.0 in 12 |
| 22.5° | 41.4% | 5.0 in 12 |
| 26.6° | 50.1% | 6.0 in 12 |
| 30° | 57.7% | 6.9 in 12 |
| 33.7° | 66.7% | 8.0 in 12 |
| 45° | 100% | 12.0 in 12 |
| 60° | 173.2% | 20.8 in 12 |
The maths: slope % = 100 × tan(angle), and pitch = 12 × tan(angle). A slope of 100% is 45°, not 90°.
Tips for measuring angles in photos
- Shoot straight on. Stand in front of the thing, not to one side — from the side a roof looks flatter or steeper than it is.
- Keep the corner near the middle of the photo. Phone lenses bend straight lines near the edges, most of all the wide (0.5×) camera.
- Use the long arms. They go to the edge of the picture — line them up with a long, straight part of each line.
- Zoom in on the corner to put the middle of the protractor exactly on it, then zoom out to line up the arms.
Frequently asked questions
How do I measure an angle in a picture?
Press Open a photo, or drop the picture on the protractor. Drag the middle onto the corner, then the blue handle along one line and the red handle along the other.
Do I need to set the size first, like with a ruler?
No. Angles do not change when a picture is made bigger or smaller, so the protractor needs no calibration. Only lengths need it.
What is the difference between the half and the full protractor?
The half one (180°) is the one used at school. It shows the smaller angle between the two arms. The full one (360°) counts all the way round, from the blue arm counter-clockwise to the red arm, so it can show angles bigger than 180°.
How exact is it?
With snap off, the arms move in 0.1° steps. With a photo, the photo matters most: shoot straight on and zoom in to place the arms carefully.
Is my photo uploaded?
No. The photo is opened by your browser and stays on your device. Nothing is sent anywhere.
Good to know
- Snap and the protractor type are remembered in this browser
- Full screen gives you a bigger protractor and photo
- The downloaded picture shows what you see on the screen, at double size, with the numbers in the corner
Related tools
- Online Ruler — measure lengths in real size on your screen
- Square Footage Calculator — the area of a room, a floor or a roof
- Scientific Calculator — sin, cos and tan for your own sums
- Crop Image — cut the photo down to the part you need
- GPS Coordinates — find where you are on the map
- Graphing Calculator — plot functions and find where they cross
- Bubble Level — check if a surface is level with your phone
