Calculators

Circle

Enter a circle's radius and get its area, circumference and diameter at once. Uses π to full precision for geometry, crafts and engineering.

Try it now

💡 Good to know: π's digits never repeat or end, yet only ~39 of them suffice to compute the observable universe's circumference to atomic precision.

Worked example

Area: 78.539816 · Circumference: 31.415927 · Diameter: 10

How it’s solved

Worked on the example above, step by step — follow along and you can do it on paper next time, no tool required.

A circle is set by its radius r. Area grows with the square of r; the circumference grows straight with r.
Area = π × r²
π × 5² = 78.539816
Circumference = 2 × π × r
2 × π × 5 = 31.415927
Diameter = 2 × r
2 × 5 = 10

Learn the method

Inputs

Radius
5

Frequently asked

What are the formulas?

Area = πr², circumference = 2πr, diameter = 2r.

Can I start from the diameter?

Halve the diameter to get the radius, then enter that here.