How to Figure the Perimeter of a Circle Knowing the Diameter

A drafting compass is used to draw a perfect circle and measure its diameter.

The distance around the rim of a shape is called its perimeter. For a round shape like a circle, the perimeter is typically called a circumference. One important fact about a circle's circumference is that regardless of how big or how small a circle is, there is a mathematical relationship between its circumference and its diameter (the distance across the a circle). Dividing a circle's circumference by it its diameter will always result in a number called "pi." Pi is equal to 3.14. By multiplying pi times a circle's diameter you will find a circle's circumference (perimeter).

Things You'll Need

• Drafting compass
• Ruler (metric or standard)
• Calculator

Instructions

1. Draw a circle using a drafting compass.

2. Extend the point and pen legs of the compass across the center of the circle.

3. Measure the distance between the point and pen legs of the compass using a metric or standard scale ruler. This is the circle's diameter.

4. Multiply the circle's diameter by 3.14 (pi). The product is equal to the circle's circumference (perimeter). The units of the product will be the units used on the ruler, inches or centimeters.

Tips & Warnings

• Wrap a string around several round containers and mark where they meet. Measure the lengths of marks on the string and divide them by the diameter of the containers. You will find that the same number (3.14) always results.

References

• "Mathematics Plus"; Exploring Circumference; Harcourt Brace Javanovich, Inc.: 1992
• Math Goodies: Circumference of a Circle; 2011