How to Use a Protractor
Last updated: September 29, 2026
A protractor is the simplest angle-measuring tool ever made: a sheet of plastic or a piece of glass marked with degrees around a straight baseline. Almost every mistake with one comes from either the centre point sliding off the vertex or reading the wrong of the two printed scales, and both are fixed in a single attempt.
This guide walks through the parts, the measurement steps and the drawing steps. Underneath it sits a full interactive protractor where you can drag handles onto an angle and read the result, so the same steps can be practised on real examples without printing anything.
Angle Properties & Complementary
Keyboard Shortcuts:
Measurement History
Saved in this browser only - never uploaded.
| Time | Mode | Degrees | Radians | Slope |
|---|---|---|---|---|
| Every angle you lock is logged here. | ||||
The parts you have to line up
Every protractor has a centre point – sometimes called the origin – that must sit exactly on the vertex of the angle, and a baseline running through it that you lay along one of the angle’s two rays. On a semicircular protractor the baseline joins the 0° and 180° marks; on a round one it passes straight through the middle, joining opposite pairs of numbers.
The degrees are printed twice for a reason. One scale starts at 0 on the left and counts to the right, the other starts at 0 on the right and counts back. Which scale you read depends only on which way the angle opens: always follow the scale that begins at 0 on the ray you aligned, and the number on the other scale will be the supplement of your angle.
Measuring an angle in five steps
Place the centre point exactly on the vertex. Rotate the protractor until its baseline lies along one ray of the angle. Look at where the second ray crosses the rim and read the scale that started at 0 on that aligned ray. Write the number down in degrees, then classify what you measured: below 90° is acute, exactly 90° is a right angle, and anything up to 180° is obtuse.
Two mistakes account for nearly every wrong answer. The first is the centre sliding off the vertex, which measures the arcs from the wrong point altogether. The second is reading the scale that starts at 180, which hands back the supplement – 180° minus your answer. A quick way to catch it: look at the corner. If it clearly opens less than a right angle and you got 100-something degrees, you read the wrong scale.
Drawing an angle, and measuring past 180°
To draw a 40° angle, rule a straight baseline, mark the centre point on it, find 40° on the scale that starts at 0 along your baseline, make a dot on the rim, then draw a line from the centre through the dot. Lay the protractor back over the result and the two should agree exactly.
Reflex angles – anything larger than 180° – do not exist on a semicircular protractor. Measure the outside of the corner instead and subtract from 360°, or switch the interactive tool below into its 360° mode, which sweeps a full circle. The same trick rescues a reading taken from the wrong side of a wall: the answer is simply 360° minus the number on the dial.