CircleLab

Circle Circumference

The distance around a circle, from its radius.

Radius

Circumference

31.416

C = 2π · r

Area: 78.54

What you entered

Radius: 5

1
Diameter
2 × 5= 10
2
Circumference
2π × 5= 31.4159
3
Area
π × 5²= 78.5398

Result

Circumference: 31.416

A circle with radius 5 has a circumference of 31.416 units.

Circle circumference and area by radius

Circumference is 2 pi r, and the diameter column is simply twice the radius. Values are rounded to two decimals.

Radius	Diameter	Circumference	Area
1	2.00	6.28	3.14
2	4.00	12.57	12.57
3	6.00	18.85	28.27
5	10.00	31.42	78.54
7	14.00	43.98	153.94
10	20.00	62.83	314.16
12	24.00	75.40	452.39
15	30.00	94.25	706.86
20	40.00	125.66	1256.64

Every value is in the same unit family as the radius: a radius in cm gives a circumference in cm and an area in square cm. Notice the r=2 row, where circumference and area both read 12.57 - a coincidence at that radius only, because 2*pi*r equals pi*r^2 exactly when r=2. Area grows with the square of the radius while circumference grows linearly, so doubling the radius doubles the circumference but quadruples the area. That is why the 20 row has twice the circumference of the 10 row and four times the area.

Circumference vs. area

Circumference grows linearly with radius (double r, double C) while area grows with the square of r — a useful distinction when you're buying trim (linear, circumference) versus flooring (area).

Real uses

Fencing a circular pool, sizing a belt or chain that wraps a wheel, or figuring out how far a wheel of known diameter travels in one rotation all come down to this formula.

Frequently asked questions

How do I calculate circumference from diameter?

Circumference = π × diameter. A wheel with diameter 26 inches has circumference π × 26 = 81.68 inches. That's how far it rolls in one full rotation. If you have the radius, use C = 2πr (same thing, since diameter = 2r).

How far does a wheel travel in one revolution?

Exactly one circumference. A bicycle wheel with a 700mm diameter travels π × 700 = 2,199 mm ≈ 2.2 meters per revolution. Multiply by the number of revolutions to get total distance. This is how odometers and bike computers work.

What is the difference between circumference and perimeter?

Circumference is the perimeter of a circle — the total distance around it. 'Perimeter' is the general term for the boundary of any shape, while 'circumference' specifically refers to circles and ellipses.