Measuring Angles with a Protractor

Geometry · Axiom Academy

LESSON Measuring Angles with a Protractor Learn how to use a protractor to measure and draw angles accurately. Before measuring anything, get familiar with the three features you'll use on every angle. The animation below traces each one in turn. The small hole or mark at the center of the flat edge. Align this with the vertex of the angle. The straight edge at the bottom. Align this with one side of the angle. Two scales: one reads left-to-right (outer), one right-to-left (inner). Both go from 0° to 180°. Measuring an angle is the same three moves every time. Get the setup right and the answer reads straight off the scale. The vertex of the angle should be exactly at the center point of the protractor. This is the most important step! Line up the straight edge (0° line) with one side of the angle. Make sure it passes through the center point. Use the scale that starts at 0° where your baseline is. Read the number where the second ray passes through the protractor. A protractor has two number scales running opposite ways so it works no matter which side your angle opens from. The trick is simple: read from whichever scale starts at 0° along your baseline. Use the outer scale (the top numbers), which runs 0° on the right around to 180°. Use the inner scale (the bottom numbers), which runs 0° on the left around to 180°.

This is the written version of the interactive lesson above. See the full Geometry course.