A protractor is a handheld instrument used to measure or mark angles. The most familiar form is a transparent semicircular disc graduated in degrees from 0° to 180°, but full-circle (360°) and specialized forms exist. Protractors provide a visual scale that helps users compare, transfer and construct angles in fields ranging from classroom geometry to metalworking.
Design and components
Typical features include a straight baseline or edge, a center point or hole that aligns with the angle vertex, and degree markings around the curved edge. Most school protractors are made of clear plastic so lines on paper remain visible; precision workshop protractors and bevel protractors are often metal and include vernier or digital readouts for finer resolution. For more on the basic function see measuring angles.
Types of protractors
- Semicircular protractors (0°–180°): common in education and drafting.
- Full-circle or circular protractors (0°–360°): used when continuous rotation must be measured.
- Bevel and combination protractors: adjustable arms used in woodworking and metalwork.
- Digital and electronic protractors: provide numerical readouts and higher precision for workshop tasks.
Degree graduations are usually printed or engraved; some models include both clockwise and counterclockwise scales to make reading easier in different orientations. For a general explanation of angle notation see angle measurement.
How to use a protractor
To measure an angle: place the center hole over the vertex, align the baseline with one side of the angle, and read the number on the curved scale where the other side crosses. To draw an angle: mark the vertex, align the baseline and mark the required degree on the curved edge, then connect the mark to the vertex. Accuracy depends on proper alignment, the fineness of the scale, and material stability.
Protractors are ubiquitous in education for teaching geometry, in engineering and drafting for laying out parts, and in trades such as carpentry and metal fabrication for checking cuts and joints. Alternatives for angle work include combination squares, digital angle finders and smartphone apps; see also degree markings and scales for more detail.
Historically, instruments for measuring angles predate the modern protractor; precise, standardized protractors became more common with the growth of technical drawing and surveying. Today both simple classroom models and precision electronic devices coexist, reflecting the protractor's continuing utility as a basic tool for working with angles.
