Circle Measurements Calculator

What Is a Circle Measurements Calculator?

A circle is fully described by four values: radius, diameter, circumference, and area. The Circle Measurements Calculator takes any one of them and returns the other three, so you never need to rework the formulas by hand. It is useful for geometry homework, fabrication, and quick design checks.

How the Math Works

All four values share one unknown: the radius. The calculator first solves for the radius, then derives the rest:

$$r = \frac{d}{2}, \quad r = \frac{c}{2\pi}, \quad r = \sqrt{\frac{A}{\pi}}$$

$$d = 2r, \quad c = 2\pi r, \quad A = \pi r^{2}$$

Worked Example

Start with a radius of 5 cm. The diameter is then 10 cm and the circumference is about 31.4159 cm. The area is:

$$A = \pi \times 5^{2} \approx 78.5398 \text{ cm}^{2}$$

Entering an area of 78.5398 instead returns a radius of 5 cm, because the radius is the square root of area divided by pi.

Related free tools

The Circle Length Calculator focuses on circumference, the Circle Perimeter Calculator adds semicircle arcs, and the Circle Calculator offers a broader set of circle properties.