Area of a Circle Calculator
Find the area of any circle instantly. Enter the radius, diameter, or circumference you know and this calculator computes the area along with all other circle measurements.
The area of a circle measures how much space the circle encloses. There are three common ways to calculate it depending on which measurement you have.
From the radius:
A = πr²
This is the most direct formula. If a circle has radius 7, its area is π(7²) = 49π = 153.938 square units.
From the diameter:
A = π(d/2)² = πd²/4
Since the diameter is twice the radius, divide it by 2 first. A circle with diameter 10 has area = π(5²) = 25π = 78.54 square units.
From the circumference:
A = C²/(4π)
First find the radius with r = C/(2π), then compute the area. If the circumference is 31.42, then r = 31.42/(2π) = 5.0, and A = π(25) = 78.54 square units.
Practical uses: Circle area calculations come up in pipe sizing, pizza comparisons, irrigation coverage, wheel and gear design, and any scenario where you need the surface area of a round object.