How to Calculate the Circumference of a Circle

Whether you’re designing a project, doing craft work, or you need to know how much fencing you should buy to go around that hot tub in the backyard, knowing how to find circumference of a circle can come in handy. In this article, you’ll learn just how and the promise is that it’s easy as pi!

Edit Steps

Calculate the circumference using the diameter

  1. Plug in the numbers. The formula for finding the circumference of a circle is this:

    • C=πd. But what does that mean? There are three elements in this equation to figure out. These are:

      • C, which is the circumference. This is the number you’re looking for;
      • d, which is the mathematical shorthand for diameter, and is the distance between the widest part of a circle, measured across the center; and finally
      • π, or pi, which is pronounced “pie”. Pi is an infinitely long number, but in this case, 3.14 will be close enough for fencing out the neighbors.
    • Summarizing all that in words––the circumference of a circle is equal to the diameter of that circle, times pi.
  2. Try it out. Take a real world example of how you’re going to use this formula to solve your neighbor problem.
    • You have an 8-foot hot tub, as measured from one side to the other. You want to build a fence 6 feet out from it, all around. The diameter (d) of the tub is 8. Add 12 to that (remember—–6 feet all the way around), for a fence diameter of 20 feet.
    • You know that π=3.14
    • Grab your calculator, and enter 20 feet x 3.14=62.8 feet for the circumference. That’s how much fencing you’ll need to buy to get a little privacy.

Calculate the circumference using the radius

  1. Know your radius. The radius (r) is the point from dead center in the middle of a circle, to the edge. It is exactly half the diameter, which will make this next formula easy:

    • C= 2πr. You know already that C is the circumference, and that π = 3.14.
  2. Try it out. For this example, you’re cutting out a decorative strip of paper to wrap around the edge of a pie you’ve just made.
    • Measure from the center of the pie to the edge of the crust. (You don’t want any crumbling). For this example, you find that the distance is 5 inches.
    • Armed with this knowledge, you can now calculate:
    • C=2 x 3.14 x 5, or 2 x 15.7, which just happens to equal 31.4.

Bonus points – calculating the area of a circle

  1. Speaking of π, you may have heard the old joke, “Pi are squared? What? Pie are not square, pie are round!” That refers to the area of a circle, which is found by multiplying the the radius by itself, then multiplying that by 3.14. The formula is A=πr2.

    • Therefore, in the second example above, the area of the pie is 78.5 inedible square inches.

Edit Video

Edit Tips

  • Consider buying an advanced calculator that already has π as one of the buttons. This will mean less typing for you and a more accurate answer because the π button produces an approximation to π that is much more accurate than 3.14.

Edit Warnings

  • Remember to always double-check your work because one mistake will set off all your data.
  • Take your time. It will come to you.
  • If you are stuck, ask a friend, family member or teacher. They will often help!

Edit Related wikiHows

  • How to Calculate the Volume of a Sphere
  • How to Calculate Pi by Throwing Frozen Hot Dogs
  • How to Calculate the Radius
  • How to Discover Pi for Yourself Using Circles
  • How to Calculate the Area of a Circle

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