Circle Area Calculator
Find a circle area from its radius.
Also returns circumference and diameter.
Inputs in mm, cm, m, km, inches, feet, yards, or miles.
A = π × r²
Area scales with the square of radius. Doubling the radius quadruples the area. A 16-inch pizza has four times the surface of an 8-inch pizza, not twice. That fact alone is worth knowing before you order.
A few real-world circle areas to anchor your intuition:
| Object | Diameter | Area |
|---|---|---|
| Standard dinner plate | 11 in | 95 sq in |
| 12-inch pizza | 12 in | 113 sq in |
| 16-inch pizza | 16 in | 201 sq in |
| Hula hoop | 36 in | 1,018 sq in (7 sq ft) |
| Backyard kiddie pool | 5 ft | 19.6 sq ft |
| Standard above-ground pool | 24 ft | 452 sq ft |
| Boxing ring | 20 ft | 314 sq ft |
If you only know the diameter: r = d/2, then plug into A = π × r². Or skip a step: A = π × d² / 4.
If you only know the circumference: r = C / (2π), then A = π × r² works out to A = C² / (4π).
Worked example — calculating fabric for a round tablecloth:
You want a tablecloth that drapes 12 in below the edge of a 60-in diameter round table. Total cloth diameter = 60 + 2 × 12 = 84 in (radius 42 in). Area = π × 42² ≈ 5,542 sq in ≈ 38.5 sq ft. From standard 60-in width fabric, you need at least two cuts and a seam down the middle — or order from a wide bolt to get a single piece.
Why a 14-inch pizza is more than 25% bigger than a 12-inch:
12-inch pizza: π × 6² = 113 sq in. 14-inch pizza: π × 7² = 154 sq in. The 14-inch is 36% bigger by area. Two friends, one with a 12-inch and one with a 14-inch, are eating very different amounts of pizza.
Quick reference values:
- Radius 1: area π ≈ 3.14
- Radius 2: area 4π ≈ 12.57
- Radius 3: area 9π ≈ 28.27
- Radius 5: area 25π ≈ 78.54
- Radius 10: area 100π ≈ 314.16
Memorize a few of these and you can sanity-check any circle area in your head.