The Circumference Of A Circle Calculator

The formula this calculator uses

circumference = 2 x pi x radius

C

Circumference, the distance around the circle

r

Radius, from centre to edge

d

Diameter, twice the radius

How to check the result by hand

1. 1

   Find the radius or the diameter

   They are related by a factor of two, so either works. If you measured across the circle, you have the diameter and can skip straight to multiplying by pi.

2. 2

   Check both measurements use the same unit

   Pi has no units, so the answer inherits whatever unit the radius was in. Mixing centimetres and metres is the usual source of an answer out by a factor of a hundred.

3. 3

   Double the radius

   A radius of seven gives a diameter of fourteen. This step exists only to convert to diameter, which is the quantity pi actually relates to.

4. 4

   Multiply by pi

   Fourteen times 3.14159 is about 43.9823. Using 3.14 instead gives 43.96, which is fine for a rough estimate but drifts as the circle gets bigger.

5. 5

   Decide whether the answer is plausible

   The circumference must be a little more than three times the diameter, always. If your answer is less than the diameter, or more than four times it, something has gone wrong.