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.
 
 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.
She carefully measured the radius again before cutting the round cake board.
   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.
The diameter is twice the radius, so if the radius is 4 cm, the diameter will be 8 cm.