How to Figure Out the Perimeter of a Circle

Written by april sanders
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How to Figure Out the Perimeter of a Circle
(Creative Commons)

Calculating the perimeter and area of shapes is a skill usually mastered in early middle school. There are formulas for figuring out the perimeter and area of every closed shape. Thankfully, the formulas for circles are some of the easiest to remember, and figuring out their perimeter, or circumference, is a cinch. This article will show you how--or refresh your memory, if you've been out of school for a while!

Skill level:

Things you need

  • Ruler
  • Calculator

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  1. 1

    Understand what "circumference" means. The circumference is the rim of an unrolled circle. It is identical in concept to the perimeter of another shape, such as a square. Simply put, it is the distance around the edge of the circle.

  2. 2

    Find the diameter. The diameter of a circle is the distance across it. You can find the diameter by locating the centre of the circle, measuring from the centre to the edge, and then doubling that number. Alternately, you can simply measure across the centre of the circle.

  3. 3

    Multiply the diameter by Pi. Most math teachers will allow you to use 3.14 as Pi, but others may want you to take it out to seven digits: 3.1425926. So, to figure out the perimeter of a circle, your formula is Pi * d. This equation will give you the perimeter, or circumference, of a circle.

  4. 4

    Use an alternate formula. You can also use P = 2 * Pi * r to calculate the perimeter, if you know the radius (value of r) and do not want to worry about having to find the diameter. It's easy--and simple--to figure out the perimeter of a circle!

Tips and warnings

  • Use a calculator to multiply with decimals if you want quick and accurate answers.

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