Pick the formula that matches the measurement
Circumference is the distance around a circle. If you know the radius r, use C = 2πr. If you know the diameter d, use C = πd. The diameter is twice the radius.
Work through an example
For a radius of 4 cm, C = 2 × π × 4 = 8π cm, approximately 25.13 cm. Keep π in the calculation until the final rounding step.
Check your answer
The circumference is slightly more than three times the diameter. If your result is smaller than the diameter, check whether you divided when you meant to multiply.