Circle Length Calculator

What Is a Circle Length Calculator?

A circle's length is its circumference: the distance all the way around its outer edge. The Circle Length Calculator takes one known value (radius, diameter, or circumference) and returns the other two. It is handy for wheels, pipes, rings, and any round object where you can measure just one dimension.

How the Math Works

Three relationships connect every circle. The diameter is twice the radius, and the circumference is pi times the diameter:

$$d = 2r$$

$$c = 2\pi r = \pi d$$

Rearranging the last formula gives the radius when the circumference is known:

$$r = \frac{c}{2\pi}$$

Worked Example

A circular tabletop has a radius of 5 cm. Its diameter is 10 cm, and its circumference is:

$$c = 2 \times \pi \times 5 \approx 31.4159 \text{ cm}$$

If you measured a circumference of 31.4159 cm instead, dividing by 2 pi returns a radius of 5 cm.

Related free tools

For all four circle values at once, try the Circle Measurements Calculator. The Circle Perimeter Calculator handles perimeter and semicircle arcs, and the Area of a Circle Calculator covers surface area.