How to use the online protractor
- The black arm (arm 1) is the base line, and the scale starts at 0° along it. Drag the round handle at its end to point it along one side of the angle.
- Drag the red arm (arm 2) to the other side. The angle between them appears in the wedge, on the scale and in the Angle box, with its reflex angle and the value in radians.
- Drag the dark dot to move the vertex — the corner of the angle. The arms move with it.
- Choose 180° for a classic half-circle protractor: it flips to whichever side arm 2 is on, so it always shows the angle between the arms (0–180°). Choose 360° to read a full turn counterclockwise from arm 1, up to 359°.
- Keep Snap to whole degrees on for clean readings such as 45° or 120°; switch it off to read tenths of a degree.
Every handle also works from the keyboard: press Tab to select one, then the arrow keys turn an arm by 1° (Up and Right counterclockwise), or 10° with Shift held. On the vertex, the arrows move it.
Measure an angle in a photo
Press Add a photo and choose a picture — on a phone you can usually take one with the camera right there. The photo fills the work area under the protractor. Drag the vertex onto the corner you want to measure, then lay arm 1 along one edge and arm 2 along the other. This works for the pitch of a roof, the angle of a ramp or a staircase, a cut in a piece of wood, a joint angle in a sports photo, or a geometry exercise on a worksheet.
The photo is opened by your browser and drawn on this page only; it is never uploaded or stored. For the best result, photograph the object straight on: a picture taken at a slant changes the angles you see (a square seen from the side looks like a diamond).
Why no calibration is needed
A ruler on screen has to be calibrated, because screens show a centimeter at different sizes. An angle doesn’t depend on size at all: enlarge or shrink a drawing evenly and every angle stays the same. The protractor and the photo are always scaled by the same factor horizontally and vertically, so the reading is right on a phone, a laptop or a projector, at any zoom.
How the angle is calculated
Each arm’s direction comes from the position of its handle relative to the vertex: direction = atan2(rise, run), measured counterclockwise from the right. The angle from arm 1 to arm 2 is the difference of the two directions, brought into the range 0–360°. The smaller side of it (0–180°) is the angle between the arms; the reflex angle is the rest of the full turn:
- Reflex angle = 360° − angle
- Radians = degrees × π ÷ 180
Example
Arm 1 points to the right (direction 0°). Arm 2 ends 3 units to the right of the vertex and 4 units up — the sides of a 3-4-5 triangle. Its direction is atan2(4, 3) = 53.13°, so the angle is 53.13° (53° with snapping on). The reflex angle is 360° − 53.13° = 306.87°, and in radians 53.13 × π ÷ 180 = 0.9273.
Types of angles
| Angle | Name | Radians |
|---|---|---|
| Between 0° and 90° | Acute | 0 to π/2 (1.5708) |
| Exactly 90° | Right angle | π/2 = 1.5708 |
| Between 90° and 180° | Obtuse | 1.5708 to 3.1416 |
| Exactly 180° | Straight angle | π = 3.1416 |
| Between 180° and 360° | Reflex | 3.1416 to 6.2832 |
| 360° | Full turn | 2π = 6.2832 |
Tips for accurate readings
- Put the vertex exactly on the corner first, then the arms: a vertex a little off moves the reading more than an arm a little off.
- Drag a handle far out along a long edge — the farther it is from the vertex, the more precisely the direction is set.
- Use the keyboard for the last degree: select a handle and tap the arrow keys.
- For a slope given as rise over run (such as a 1:12 ramp), the slope calculator converts it to an angle without measuring; for trigonometry with angles, use the scientific calculator.
Frequently asked questions
How do I use a protractor online?
Drag the black arm along one side of the angle and the red arm along the other, with the dark vertex dot on the corner. The angle between the arms appears in degrees and radians, with its reflex angle. To measure something real, add a photo of it first.
Can I measure an angle in a picture?
Yes. Press Add a photo, choose or take a picture, place the vertex on the corner and line up the two arms with the edges. The photo is opened in your browser only and is never uploaded.
Is the online protractor accurate on any screen?
Yes. Angles don’t change when a picture is scaled evenly, so no calibration is needed: the reading is the same on a phone, a tablet or a monitor. Accuracy depends on how exactly you place the vertex and the arms; with snapping off it shows tenths of a degree.
How do I measure a reflex angle?
Every reading shows the reflex angle too: it is 360° minus the angle between the arms. On the 360° scale you can also read the full turn counterclockwise from arm 1 directly.
How do I convert degrees to radians?
Multiply by π and divide by 180. For example, 90° is π/2 = 1.5708 radians and 60° is π/3 = 1.0472 radians. The protractor shows the radian value of every angle you measure.
Last updated: October 19, 2026