Circle Calculator
How to Calculate Circle Properties
From any one known value (radius, diameter, or area), you can find all the others.
The key formulas
- Radius (r): distance from center to edge
- Diameter (d): d = 2r (straight across through center)
- Circumference (C): C = 2πr or C = πd (perimeter of the circle)
- Area (A): A = πr² (space inside the circle)
Step-by-step
- Select which value you know (radius, diameter, or area).
- Enter that value. The calculator converts it to radius first.
- From radius, all other values are computed instantly.
Worked example
If radius = 5: Diameter = 10, Circumference = 2 × π × 5 ≈ 31.42, Area = π × 25 ≈ 78.54.
Real-world uses
Calculating pipe capacity, circular garden beds, pizza area comparisons, wheel circumference for speedometer calibration, and land surveying.