Though a protractor generally measures the sizes of angles, its shape and features also lend the tool to measuring straight-line distances as well. Formed as a semi-circle, a protractor's rounded upper portion labeled with degrees sits atop a miniature ruler, allowing for drawing and measuring of shorter lengths. This ruler can help you gauge a circle's radius, which is the distance from the circle's center point to its circumference, by finding and working with the circle's diameter, which is the longest straight-line distance within the circle, running through the circle's center and equaling twice the radius' length.
Instructions
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1
Draw a line through the center of the circle with the straight edge of your protractor, dividing the circle into two equal halves. This line is the diameter.
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2
Measure the diameter's length with the protractor's ruler along its straight edge. For this example, let the diameter measure 3 inches.
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3
Divide the diameter's length in half to find the length of the circle's radius. With this example, 3 inches divided by 2 equals 1.5 inches --- the circle's radius is 1.5 inches.