Measurement
Online Protractor
Measure any angle by dragging three points, on a blank canvas or straight over your own photograph. Reads in degrees and radians, snaps to 5° for geometry work, and exports a PNG with the measurement marked.
Drag the points to measure, or drag the protractor to move it
Drop or paste an image. Hold Space (or middle mouse) to pan. Use Ctrl/Cmd + wheel to zoom. Drag the three points to measure. Toggle Move all to reposition the whole protractor.
Four steps
How to measure an angle online
- 1
Load an image (optional)
Open a photo or diagram to overlay the protractor on real geometry.
- 2
Place the vertex
Drag point B onto the corner where the two lines meet.
- 3
Set the arms
Drag A and C out along each line, the angle reads live as you move them.
- 4
Read or export
Switch between degrees and radians, then export a PNG with the measurement marked.
Where a digital protractor beats a plastic one
A physical protractor requires the angle to be in front of you, flat, and small enough to fit the tool. A digital one works from a photograph, which opens up everything a plastic semicircle cannot reach:
- Roof pitch and stair rake measured from a photo taken side-on.
- Physiotherapy and sports: joint range of motion from a video still.
- Design and typography: the slant of an italic, the angle of a logo mark.
- Engineering drawings: checking an angle on a scan or PDF export.
- Astronomy and navigation: angular separation in an image.
Angle types, briefly
| Type | Range | Radians |
|---|---|---|
| Acute | 0° – 90° | 0 – π/2 |
| Right | 90° | π/2 ≈ 1.5708 |
| Obtuse | 90° – 180° | π/2 – π |
| Straight | 180° | π ≈ 3.1416 |
| Reflex | 180° – 360° | π – 2π |
| Full turn | 360° | 2π ≈ 6.2832 |
Getting a precise reading
- Put the arm points far apart. Placing A and C well away from the vertex reduces the effect of a pixel or two of placement error.
- Photograph square-on. A photo taken at an angle introduces perspective distortion that no measurement tool can undo.
- Zoom the image first if the feature is small, placement precision is the limiting factor, not the maths.
- Turn snap off for real-world angles. Real roofs are not 35.000°.
Degrees, radians and gradians
Degrees divide a circle into 360, a convention inherited from Babylonian astronomy and convenient because 360 divides evenly so many ways. Radians measure the angle by arc length relative to radius, which is why calculus and physics use them, derivatives of trigonometric functions only come out cleanly in radians. If you are writing code, check which your language expects; most use radians.
Related measuring tools
For lengths on screen at true size, use the calibrated online ruler. For pixel dimensions in a mockup, the pixel ruler. For drawing angles by hand, isometric and polar graph paper both build the angles into the grid.
Answers