The circumference formulas
Circumference is the distance around a circle. You can find it from either the radius or the diameter:
C = 2 × π × radius | C = π × diameter
Both give the same answer, since the diameter is twice the radius. For a radius of 5, the circumference is 2 × π × 5 ≈ 31.42.
Circle area
The area uses the radius squared:
Area = π × radius²
For a radius of 5, the area is π × 25 ≈ 78.54.
Quick reference
- Diameter = 2 × radius
- Radius = diameter ÷ 2
- Circumference ≈ 3.14159 × diameter
- Area ≈ 3.14159 × radius²
Frequently asked questions
How do you calculate circumference?
Multiply the diameter by π (about 3.14159), or multiply the radius by 2π. The circumference is the distance all the way around a circle. For a radius of 5, the circumference is 2 × π × 5 ≈ 31.42.
What is the difference between radius and diameter?
The radius is the distance from the center of a circle to its edge. The diameter is the full distance across the circle through the center, which is exactly twice the radius. So diameter = 2 × radius.
How do you find the area of a circle?
Square the radius and multiply by π: Area = π × radius². For a radius of 5, the area is π × 25 ≈ 78.54. This calculator shows the area alongside the circumference.
What is π (pi)?
Pi is the ratio of any circle's circumference to its diameter — the same constant for every circle, approximately 3.14159. It is an irrational number, so its decimals never end or repeat. This calculator uses a high-precision value of π.
Note: All results share the same unit as your input — feet in, feet out. Decimal results are rounded for display.