The short answer
Why use Protractor?
A plastic protractor needs the drawing on paper and a steady hand to line up the baseline. On a photo of the page, you place the points where the lines are and read the number.
The corner point is the one labelled End, which starts 75% across and 65% down. Drag it onto the vertex, then drag Start and Third along each arm of the angle. On a phone, tap near a point to move the closest one there, or grab it and drag. The reading updates as you move, to two decimal places.
Photograph the worksheet flat and from straight above. If the phone is tilted, the page is foreshortened and a 90° corner can read as 85° or 95°. Short arms are another source of error: if a line is only a few dozen pixels long, a one pixel slip moves the result by a degree or more, so place points far along each line.
Measure Angle is the same tool with a page aimed at roofs, ramps and posture. If you want to draw an arrow or a label on the worksheet afterwards, open the downloaded PNG in Add Arrow or Add Text to Image. The downloaded PNG keeps the full resolution of your photo, so the marked angle stays sharp when printed.
Where Protractor fits
Checking homework
Confirm the angles a child measured on a printed geometry sheet.
Drawing angles
Check that a hand-drawn 60° angle really is 60° before inking it.
Lesson slides
Show a class how an angle is read by marking it on a projected photo.
Start to finish
How to use Protractor
- 01Tap Choose image and pick a straight-down photo or scan of the worksheet.
- 02Put the End point on the angle's corner, then drag Start and Third along the two lines.
- 03Read the degrees, then tap Apply changes and Download result if you want the worksheet marked.
What to expect from Protractor
| Check | What it means here |
|---|---|
| Supported input | JPEG, PNG, WebP, GIF, BMP, SVG and browser-supported AVIF images |
| Main controls | Draggable points, exact X and Y fields, and an optional pixels-per-unit scale |
| Output | The measurement on screen and a marked-up copy of the image |
| Working limit | 50 MB and 50 megapixels per image |
Protractor tips
- Lay the worksheet flat and hold the phone parallel to it before taking the photo.
- Place Start and Third near the far ends of each drawn line.
- A right angle that reads 88° to 92° usually means a slight camera tilt.
- Answers show two decimals; round to the nearest degree for homework.
Protractor FAQ
Something missing? Ask us.
Which point goes on the corner?
Can it read angles over 180 degrees?
Is this different from Measure Angle?
Does Protractor upload my file?
Is Protractor free to use?
Why did Protractor fail or look wrong?
Related tools
More in Measurement.