# Radius

Definition 1 of 4

Pro Tip 1/3

Circle Basics

A radius is essential in geometry for calculating area and circumference of a circle. Find the area of a circle by using the radius in the formula Area = πr^2.
Pro Tip 2/3

Measure Twice

Always double-check the length of a radius, as even a small error changes the circle size significantly. She carefully measured the radius again before cutting the round cake board.
Pro Tip 3/3

Not a Diameter

Do not confuse radius with diameter; the radius is half the length of the diameter. The diameter is twice the radius, so if the radius is 4 cm, the diameter will be 8 cm.