What you’ll learn
Calculate circumference.
Key idea
Circumference is pi times diameter, and diameter is twice radius.
Circumference equals pi times diameter, not radius; distinguish center-to-edge from whole-center-crossing lengths and retain exact pi.
Worked example
Enter the exact value or ordered values in the format requested below.
A circle's diameter is 18cm. Enter exact circumference in cm as a multiple of pi, without a unit suffix.
- Radius is 9cm and diameter is twice that, 18cm.
- Circumference=pi×diameter=18pi cm; divide its pi coefficient by2 to recover radius.
Answer: 18pi
Quick Check
Evaluate a worked attempt at calculate circumference, identify the first invalid step, and provide the correction.
A circle radius is 3cm. Step1 obtains diameter=6cm. Step2 computes circumference=pi×radius=3pi cm. Locate the first invalid step and correct exact circumference.