Circle Area Calculator

Three Ways to Find Circle Area

Enter whichever measurement you have — radius, diameter, or circumference — and the area updates as you type. No need to convert units manually or rearrange formulas.

Radius (R) — distance from the center to any point on the circle. The fundamental measurement.

Diameter (d) — the longest distance across the circle, passing through the center. d = 2R.

Pi (π) — the ratio of circumference to diameter, approximately 3.14159. An irrational constant that appears in every circle formula.

Circumference (C) — the perimeter of the circle. C = 2πR = πd.

Circle Area Formulas

Three equivalent formulas. Pick the one that fits your data:

From radius:

S = πR²

From diameter:

S = ¼ πd²

From circumference:

S = C² / 4π

All three produce the same result — they are mathematically equivalent.