圆周长计算器 — 公式

## How to Calculate the Circumference of a Circle

### Formula

**C = 2 × π × r**

Or equivalently: **C = π × d** (where d is the diameter)

Where:
- **C** = Circumference
- **π** ≈ 3.14159
- **r** = radius
- **d** = diameter (2r)