Online Ruler Tools
About

Online Protractor

Drag the arms to measure an angle, or type one in to draw it. Degrees and radians, with optional 15° snapping.

Protractor

Angle 60.0° 1.047 rad reflex 300.0°

Drag either arm to set the angle. The nearest one follows your pointer. With the protractor focused, ← → move the active arm by 0.5°, or by 15° with Shift held. Use the Arm A / Arm B buttons to choose which arm the keys move.

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Measuring an angle on your screen

An online protractor behaves much like the plastic one from school, with one advantage: nothing slips. Drag either arm and it follows your pointer, snapping to the nearest half degree while the reading updates. To measure something real, hold it flat against the display with its corner on the vertex mark at the centre of the baseline, then bring each arm onto one of the two edges.

There is a second mode people often actually want, which is drawing a specific angle rather than measuring an unknown one. Type the value into the exact-angle box and the arms move to it, so you can hold paper against the screen and trace a true 45° or 30°. Switch on 15° snapping and dragging lands cleanly on the angles that come up most: 15, 30, 45, 60, 90 and so on.

Why no calibration is needed here

The ruler and the actual size viewer both need to know your display's pixel density before you can trust them, because both report physical lengths. A protractor does not, and the reason is worth stating: an angle is a ratio rather than a length. Scale a figure up or down and every angle inside it stays exactly the same. So this tool is right on any screen, at any resolution, with no setup.

What does limit precision is your eye. The readout resolves to a tenth of a degree, finer than you can read off a moulded plastic scale, so in practice the accuracy you get depends on how carefully you align the arms to the edges in front of you. If you want more room to do that, the fullscreen button roughly doubles the radius, which means a degree spans twice the arc length.

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Degrees, radians and naming angles

Degrees divide a full turn into 360 parts. Radians measure the same turn as 2π. Radians are the natural unit in trigonometry and in most programming languages' maths libraries, which is why both appear. The conversion is degrees × π/180, so a right angle is π/2, or roughly 1.571 rad.

The vocabulary is worth having straight, since it is usually what a homework question is really testing. Below 90° an angle is acute. At exactly 90° it is a right angle. Between 90° and 180° it is obtuse. At 180° it is a straight angle. Beyond 180° it is reflex. Because a semicircular protractor only reads to 180°, whether plastic or on screen, a reflex angle is found by measuring its smaller partner and subtracting from 360. That figure sits beside the main reading, so the arithmetic is done for you.

Frequently Asked Questions

How do I use an online protractor?
Drag either arm until both line up with the angle you are measuring. The nearer arm follows your pointer and the reading updates as you move. For something physical, hold it against the screen with its corner on the vertex mark, then bring one arm onto each edge.
Can I draw an exact angle?
Yes. Type the value into the Set exactly box and the arms move to it, which is what you want when you need to reproduce 45°, 30° or 120° rather than measure something unknown. Turning on Snap to 15° also makes dragging land cleanly on the common angles.
Does this protractor need calibration?
No. An angle is independent of scale, so a reading taken on screen is correct whatever your display's pixel density. Calibration only matters for tools that report physical lengths, like the online ruler.
How do I measure an angle greater than 180°?
Measure the smaller angle and read the reflex value shown beside it. The two always add to 360°. This is the same method you would use with a physical semicircular protractor, which also stops at 180°.
What is the angle in radians?
The readout gives radians next to degrees. To convert by hand, multiply degrees by π/180. So 90° is π/2, about 1.571 rad; 60° is π/3, about 1.047 rad; and 180° is π, about 3.142 rad.
What are acute, obtuse and reflex angles?
Below 90° is acute. Exactly 90° is a right angle. Between 90° and 180° is obtuse. Exactly 180° is a straight angle. Above 180° is reflex. The reading above tells you which one you have as you drag.
Can I use it on a phone or tablet?
Yes, drag the arms with a finger. A tablet suits this particularly well, since you can lay a physical object flat on the glass and align the arms to its edges.
Is it accurate enough for schoolwork?
For a printed or drawn angle held against the screen, yes. It reads to a tenth of a degree, finer than you can read a moulded plastic scale. The limit is how precisely you can align the arms by eye, not the tool.