Knowing how to calculate the distance around a circle, or its circumference, has many practical implications, such as in designing circular objects, working with crafts or figuring out how much fencing you will need to encircle your hot tub. Although learning geometry may seem difficult at first, you just need to memorize one formula and apply some basic math. (1 & 2)
What is Pi?
Mathematicians use Pi, the 16th letter of the Greek alphabet, to denote the ratio of the circumference of a circle to its diameter. Pi is an unknowable value. It is an irrational number, meaning that its digits never end or repeat. It is about 3.1416, but for some engineering calculations with low tolerances, a much more exact number is needed. If you use a scientific calculator, the value for pi has far more than 3 digits.
Learn the Formula
The mathematical formula used to calculate a circle's circumference is C = pi*d. In this formula, the C stands for circumference; pi, is approximately 3.1416, the * symbol tells you to multiply and the d stands for diameter, which is a line segment drawn from the circle's circumference, passing through its center. Therefore, to figure out the circumference of a circle, you need to know its diameter or have information that could lead you to extrapolate its diameter.
Use the Diameter
In classic geometry problems, mathematics teachers give just one piece of information -- a circle’s diameter, and ask students to calculate the circumference. You must remember that pi is roughly 3.1416, as they typically don’t remind you of this information. For example, if a teacher gives tells you that a circle's diameter is 5, you can calculate its circumference by multiplying 5 by 3.1416. The circumference of the circle in this example is, therefore, 15.71. Remember to write the units, such as inches, feet or yards, in your final answer.
Use the Radius
Sometimes you have to calculate the diameter before you can work the problem. If you are given the radius of the circle instead of its diameter, just multiple the radius by 2 because twice the radius is equal to the diameter, or 2r=d. For example, If the radius of the circle is 2, its diameter is 4 and so its circumference is 12.56, because C also equals 2r*pi. It works the same for fractions. If the radius is 16.2 then the diameter is 32.4, so the circumference is 101.79.
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