What it works out
Enter a radius or a diameter and you get the other one, the circumference and the area. Use it for round columns, footings, tanks or a patio. It takes a length measurement only, so enter the size with a unit such as feet or inches. The area comes out in square units and the circumference in length units.
The formula
- Area
- π × radius²
- Circumference
- π × diameter
- Radius
- diameter ÷ 2
In words: Area = pi times radius squared; circumference = pi times diameter; radius = diameter divided by 2
Worked example
A round footing 6 ft across
- Diameter 6 ft
- Radius = 6 ÷ 2 = 3 ft
- Circumference = π × 6 = 18.85 ft
- Area = π × 3² = 28.27 ft²
Enter a radius instead and you get the diameter the same way.
Good to know
- Length measurements only: enter feet, inches or another length unit.
- Full circles only, not arcs.
- A calculator, not a building-code check: confirm local rules with your building authority.
Questions
Radius or diameter?
Either. Enter the one you know and the tool gives the other.
What about the area of a ring?
Subtract the inner circle's area from the outer circle's area.