How do I find the area of a circle?

Use A = πr², where r is the radius. If you only know the diameter, halve it first (r = d/2). For example, a circle of radius 5 has area π·25 ≈ 78.54.

What is the circumference formula?

C = 2πr = πd. A circle with radius 5 has a circumference of 2π·5 ≈ 31.42.

How do I get the radius from the area?

Rearrange A = πr² to r = √(A/π). This calculator does it automatically when you choose 'Area' as the known value.

Circle Calculator

Free circle calculator. Enter any one of radius, diameter, circumference or area and instantly get the other three, using d = 2r, C = 2πr and A = πr². Perfect for geometry homework and design work.

From radius, diameter, circumference or area, find all the other circle properties.

d = 2 r, C = 2 π r, A = π r^{2}

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