How to Find the Circumference of a Circle
Learn how to calculate the circumference (perimeter) of a circle using the radius or diameter. Master the formula C = 2πr with our interactive calculator and practice problems.
Visual Guide:
A circular garden has a radius of 8 feet. What is the circumference?
Using C = πr instead of C = 2πr when given the radius
If radius = 5, then C = 2π(5) = 10π, not C = π(5) = 5π
Writing 31.4 instead of 10π when an exact answer is required
Exact: 10π units, Approximate: 31.42 units
Writing just "31.42" instead of "31.42 cm" or "31.42 units"
π = circumference ÷ diameter
This ratio is the same for every circle, no matter how big or small!
Ancient Greek mathematician Archimedes (287-212 BC) was one of the first to calculate π accurately by inscribing and circumscribing polygons around circles.
π is an irrational number, meaning its decimal representation never ends and never repeats. Mathematicians have calculated π to over 100 trillion digits!
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