Know the radius, diameter, circumference or area of a circle? Get all four.
Area (A)
Radius (r)
Lengths of the circle
Radius (r)
Diameter (d)
Circumference (C)
Show the working
Working in metres (m) and square metres (m²); switch the result units as you like.
r = 0.05 m
d = 2r = 0.1 m
C = 2πr = 2 × π × 0.05 = 0.31415927 m
A = πr² = π × 0.05² = 0.0078539816 m²
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Formula
d = 2rC = 2πr = πdA = πr²r = C ÷ 2π = √(A ÷ π)
How it works
Every measurement of a circle follows from its radius, the distance from the centre to the edge. The diameter is twice the radius, the circumference (the distance around) is 2π times the radius, and the area is π times the radius squared.
π (pi) is the ratio of any circle’s circumference to its diameter, about 3.14159. It is irrational, so its digits never end; this calculator uses the full double-precision value. If your class says to use 3.14, your answer may differ slightly in the later decimal places.
Working backwards is just as easy: divide the circumference by 2π to get the radius, or divide the area by π and take the square root. Use any consistent unit — if the radius is in centimetres, the area comes out in square centimetres.
Frequently asked questions
Why is area in square units but circumference isn’t?
Circumference is a length (a distance around), so it has the same unit as the radius. Area multiplies two lengths (r × r), so its unit is squared — cm², m², ft².
If I double the radius, what happens to the area?
It quadruples, because area depends on r². The circumference only doubles.