The short answer
Why use Measure Angle?
Holding a plastic protractor against a screen is fiddly and rarely lines up with the pixels. Placing three points on the image itself gives a repeatable number you can check again later.
Three points appear: Start at 25% across and 35% down, End at 75% and 65%, and Third at 50% and 80%. End is the vertex, the corner where the two arms meet. Drag it onto the corner first, then put Start and Third somewhere along each arm. The further out you place them, the less a one pixel slip changes the result.
The tool measures the angle as it appears in the flat picture. A roof or a ramp photographed from the side reads true; one photographed at a slant does not, because perspective squeezes lines that point away from the camera. For posture or joint angles, film square on to the body at the height of the joint.
Protractor runs exactly the same calculation, and its page is written for worksheets and homework. If the picture itself is tilted and you want to level it rather than read a number, Straighten rotates it by the angle you set, from -45° to 45°. Neither tool needs Pixels per unit, since an angle has no length.
Where Measure Angle fits
Roof pitch
Read the slope of a gable from a side photo before ordering a ladder or panels.
Knee and elbow range
Track how far a joint bends during physio by measuring the same pose each week.
Ramp gradients
Check whether a wheelchair ramp looks steeper than the angle you planned for.
Start to finish
How to use Measure Angle
- 01Tap Choose image and pick a side-on photo of the roof, ramp, joint or corner you want to check.
- 02Drag the End point onto the vertex, then place Start and Third far out along each arm of the angle.
- 03Read Angle in degrees in the status line; tap Apply changes, then Download result to keep the marked PNG.
What to expect from Measure Angle
| 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 |
Measure Angle tips
- End is always the vertex; the angle is measured between End-to-Start and End-to-Third.
- Readings never exceed 180°. Subtract from 360 for the reflex side.
- Points 300 pixels from the vertex are far steadier than points 30 pixels away.
- Shoot square on to the angle; a slanted camera squeezes lines that run away from you.
Measure Angle FAQ
Something missing? Ask us.
How is the angle computed?
How do I get the outside (reflex) angle?
Does Pixels per unit change the angle?
Does Measure Angle upload my file?
Is Measure Angle free to use?
Why did Measure Angle fail or look wrong?
Related tools
More in Measurement.