Fourth Grade Geometry: How to Find the Circumference

Finding the Circumference in Fourth Grade

The circumference is the distance around a circle. For example, if you wanted to know the distance around a circular swimming pool, you could find it by measuring the pool's circumference.

First, you'll need to know either the circle's diameter or its radius to figure out its circumference. The diameter is the distance across the circle at its widest point. For instance, if you cut a round pizza in half, the length of the cut is the circle's diameter.

The radius is the distance from the exact center of a circle to the outside edge. It is equal to exactly half of the circle's diameter. To find a circle's circumference, you also need to know pi, which is always equal to 3.14.

The Equation for Circumference

Using the Diameter

There are two different equations you can use to find a circle's circumference. If you know the circle's diameter, you can use this equation:

C =(pi)(d)

The 'C' in this equation represents the circumference of the circle, and the 'd' represents the diameter. Since 'pi' and 'd' are right next to each other, this means that you multiply them together to get the circumference.

For example, if you know the circle's diameter is 15 centimeters, you multiply 15 by pi, or 3.14, to find the circumference. The number 15 times 3.14 equals 47.1 (15 x 3.14 = 47.1), so the circumference of the circle is 47.1 centimeters.

Using the Radius

Some geometry problems ask you to find the circumference using the radius instead of the diameter. Since the radius is half of the diameter, the equation to find the circumference using the radius is written like this:

C = 2(pi)(r)

Just like before, the 'C' stands for circumference and 'pi' equals 3.14. The 'r' represents the circle's radius. To find the circumference, just multiply two by 3.14 by the length of the radius. If the radius is five inches, multiply two by 3.14 by five inches to find the circumference (C = 2 x 3.14 x 5). A circle with a radius of five inches has a circumference of 31.4 inches.

Practice Finding Circumference

Example 1

Problem: Ellen is cutting out a construction paper circle to decorate her class bulletin board. The circle has a radius of seven inches, and Ellen wants to know its circumference.

Solution: To find it, Ellen uses the equation C = 2(pi)(r). She multiplies two, 3.14 and seven inches to find the circumference of 43.96 inches.

Example 2

Problem: Jamie has just made a pie with a diameter of 12 inches, and he wants to figure out its circumference.

Solution: Jamie uses the formula C = (pi)(d) to calculate it. He multiplies pi, which is 3.14, by the diameter of 12 inches to get a circumference of 37.68 inches.