How to calculate the area of a circle
Updated on June 30, 2026 · by Rafael Rossi
A circle's area depends only on the radius. It's one of math's best-known formulas, showing up in pizzas, pipes, gardens and gears.
The formula
area = π × r²
Where π ≈ 3.14159 and r is the radius (center to edge).
Step-by-step example
Circle with radius 5:
- r²: 5 × 5 = 25
- Area: π × 25 ≈ 78.54
From the diameter
If you have the diameter (full width), the radius is half: r = d ÷ 2. A circle of diameter 10 has radius 5.
Area vs circumference
- Area = inner surface = π·r² (in units², like m²)
- Circumference = outline length = 2·π·r (in the radius's unit)
FAQ
Is a 16-inch pizza double a 8-inch one? No — it's four times the area. Since area depends on radius squared, doubling the diameter multiplies area by 4.
How to find the area of a half circle? Compute the full circle's area and divide by 2.