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Circle Circumference Calculator
Circumference and area of a circle, from radius or diameter.
Circumference
How it works
From radius: circumference = 2 × π × r, area = π × r² From diameter: circumference = π × d, radius = d ÷ 2
Worked example: a radius of 7 gives a circumference of 2 × π × 7 = 43.98 units and an area of π × 49 = 153.94 square units. The diameter is 14, and π × 14 = 43.98 — the same circumference, reached the other way.
Frequently asked questions
What value of π does the calculator use?
The full JavaScript value of π, about 3.14159265. Using 3.14 instead changes the circumference of a radius-7 circle from 43.98 to 43.96 — close, but not exact.
How do I measure a circle in real life?
Wrap a tape around it for the circumference, then divide by π for the diameter. Or measure straight across the widest point for the diameter and halve it for the radius.
Are circumference and perimeter the same thing?
Yes, for a circle. Circumference is just the special name for the perimeter of a circle.
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