How to use the Circle Circumference Calculator
Enter the radius. If you measured the diameter, halve it first.
The circumference formula
Circumference is 2 × π × radius, or π × diameter. A circle with a 5 cm radius has a circumference of 2 × 3.14159 × 5 ≈ 31.42 cm. The answer uses the same unit as the radius.
A practical example
A bike wheel with a 0.35 m radius travels 2π × 0.35 ≈ 2.2 m in one full turn, so 1,000 turns cover about 2.2 km. The same idea gives the length of trim around a round table or edging around a circular bed.
The formula
C = 2πr
Examples
- Radius 5
Circumference: 31.41592654
- Radius 10.5
Circumference: 65.97344573
- Radius 0.3
Circumference: 1.88495559
Circumference by radius
| Radius | Circumference |
|---|---|
| 1 | 6.2832 |
| 2 | 12.5664 |
| 3 | 18.8496 |
| 4 | 25.1327 |
| 5 | 31.4159 |
| 6 | 37.6991 |
| 7 | 43.9823 |
| 8 | 50.2655 |
| 9 | 56.5487 |
| 10 | 62.8319 |
| 12 | 75.3982 |
| 15 | 94.2478 |
| 20 | 125.6637 |
| 25 | 157.0796 |
| 50 | 314.1593 |
| 100 | 628.3185 |
Limitations and assumptions
The circumference comes back in the same unit as the radius.
Frequently asked questions
What is the circumference of a circle with radius 7?
About 43.98, since 2 × π × 7 = 14π.
How do I find the radius from the circumference?
Divide the circumference by 2π. A 31.42 cm circumference gives a radius of 5 cm.
What is the difference between circumference and area?
Circumference is the distance around the circle, in ordinary units. Area is the space inside, in square units.