What you’ll learn
Calculate circle area.
Key idea
Circle area squares the radius and has square-length units; circumference is a different measure.
Square radius, not diameter; area units are squared, and inverse geometric length must be positive.
Worked example
Enter the exact value or ordered values in the format requested below.
A circle's diameter is 18cm. Enter exact area in square cm as a multiple of pi, without a unit suffix.
- Radius=9cm; if diameter 18cm is supplied, halve it first.
- Area=pi×radius²=81pi cm²; the positive radius is the number whose square is 81.
Answer: 81pi
Quick Check
Evaluate a worked attempt at calculate circle area, identify the first invalid step, and provide the correction.
Circle radius=3cm. Step1 identifies radius=3cm. Step2 squares diameter to get area=36pi cm². Locate the first invalid step and correct area.