Protractor

A protractor is a flat, semi-circular or full-circular instrument marked with degree graduations, used to measure and draw angles. The standard protractor reads from 0° to 180°; a full-circle protractor reads to 360°.

How it works

To measure an angle, you align the protractor’s baseline with one side of the angle and read where the other side crosses the degree scale. To draw an angle, you mark the desired degree on the protractor’s scale and draw a line from the vertex through that mark.

Types

• Semicircular protractor: The classic school-room tool — a half-disc with degree markings from 0° to 180°. Inexpensive and adequate for most purposes.
• Full-circle protractor: A complete 360° disc, useful for measuring reflex angles and in navigation.
• Bevel protractor: A precision instrument with a vernier scale that reads to 5 minutes of arc (1/12 of a degree). Used in machining and metalwork.
• Digital protractor: Uses an electronic sensor and LCD to display the angle directly. Some models attach magnetically to metal surfaces for hands-free measurement.

Why angles matter

Accurate angle measurement is essential in woodworking (mitre joints), metalwork (bevel cuts), navigation (bearing), and construction (roof pitch, stair angle). A protractor is one of the most fundamental measuring tools, alongside the ruler and the square.