Online protractor
- In radians
- 0.7854
- Complement
- 45°
- Supplement
- 135°
- Kind of angle
- Acute
For measuring something on screen, hold it against the display and line the base up with one edge of the angle.
Runs in your browser · nothing is uploaded
A protractor on screen: set any angle from 0 to 180 degrees and read it in degrees and radians, with its complement, supplement and what kind of angle it is.
How to use the online protractor
The complement and supplement are the two relationships that come up constantly and get mixed up just as often. Complementary angles add to 90 degrees: the two non-right angles in any right triangle are complementary. Supplementary angles add to 180 — angles on a straight line, and the co-interior angles between parallel lines. A useful way to keep them apart is alphabetical: complementary comes first and pairs with the smaller number.
Radians are given because most programming and all calculus use them. A radian is the angle where the arc equals the radius, which makes a full turn 2π rather than an arbitrary 360. The 360 is Babylonian and survives because it divides evenly by so many numbers; 2, 3, 4, 5, 6, 8, 9, 10, 12 and more, which is genuinely convenient for a protractor and useless for mathematics. Every trigonometric function in JavaScript, Python and C takes radians, which is why Math.sin(90) returns −0.89 rather than 1.
For measuring a real object against the screen the same caveat applies as to an on-screen ruler: angles are more forgiving than lengths, because an angle does not depend on scale: a 30-degree angle is 30 degrees at any size. So a screen protractor is actually more trustworthy than a screen ruler, as long as you are looking straight at the display rather than from an angle yourself.
What people use it for
- Checking a geometry answer that has to be shown as working
- Turning a degree figure into radians for code that expects them
- Finding the supplement of an angle read off a drawing
- Setting a mitre or bevel angle before a cut
- Deciding whether an angle counts as acute, right or obtuse
Questions
Complementary angles add to 90, supplementary to 180. Alphabetically, complementary comes first and pairs with the smaller number.
Trigonometric functions take radians, not degrees. Math.sin(90) is −0.89; you want Math.sin(Math.PI / 2).
It is Babylonian and survives because it divides evenly by so many numbers. Radians are the mathematically natural unit.
More so than a screen ruler, because an angle does not depend on scale. Look straight at the display rather than from an angle.
No. A reflex angle is measured as its supplement and subtracted from 360.