Circle Area Calculator
Calculate the surface area, boundary circumference, diameter, or radius of any circle. Input any parameter to solve for all others instantly.
A circle represents all points in a two-dimensional plane that are at a fixed distance (the radius) from a central point. The constant ratio of any circle's circumference to its diameter is represented by the Greek letter π (Pi), approximately equal to 3.14159.
Multiply the radius by itself, then multiply by Pi. This is the standard definition equation.
Square the diameter, multiply by Pi, then divide by 4. Useful because diameter is easier to measure directly.
Square the outer boundary circumference, then divide by 4 times Pi.
US Quarter Coin
Diameter: 0.95 in (2.43 cm)
Area ≈ 0.72 in² (4.62 cm²)
Standard Compact Disc (CD)
Diameter: 4.72 in (12.0 cm)
Area ≈ 17.5 in² (113.1 cm²)
NBA Basketball Hoop
Diameter: 18.0 in (45.7 cm)
Area ≈ 254.5 in² (1,641.7 cm²)
Large Pizza (14-inch)
Diameter: 14.0 in (35.6 cm)
Area ≈ 153.9 in² (993.1 cm²)
| Radius (cm) | Diameter (cm) | Circumference (cm) | Area (cm²) |
|---|---|---|---|
| 1 | 2 | 6.283 | 3.142 |
| 2 | 4 | 12.566 | 12.566 |
| 5 | 10 | 31.416 | 78.540 |
| 10 | 20 | 62.832 | 314.159 |
| 15 | 30 | 94.248 | 706.858 |
| 20 | 40 | 125.664 | 1256.637 |
| 50 | 100 | 314.159 | 7853.982 |
| 100 | 200 | 628.319 | 31415.927 |