How to Calculate the Circumference of a Circle

Calculate the circumference using the diameter

1. 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. 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. 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. 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. 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.