Circumference is the distance around the outside of a circle or sphere. If you walked all the way around a circular track, returning to where you started, you would have traveled its circumference.

Think of it as the perimeter of a circle: just as a square's perimeter is the total of its four sides added together, a circle's circumference is the measurement of its single, continuous curved edge.

You can find a circle's circumference using a simple formula: multiply the diameter (the distance straight across through the center) by pi, which equals about 3.14. So a pizza with a 12-inch diameter has a circumference of about 37.7 inches. That's why bigger pizzas have so much more crust to eat!

The Earth's circumference at the equator is about 24,901 miles. Ancient Greek mathematician Eratosthenes calculated Earth's circumference over 2,000 years ago using only shadows and geometry, getting remarkably close to the correct answer. When someone says they want to sail around the world, they're planning to travel Earth's circumference, and when you measure around your head to find your hat size, you're measuring your head's circumference.