How to use this calculator
- Enter or choose i know the.
- Enter or choose value.
- The area updates as you type. Use Copy result or Share to keep it.
Method
d = 2r; C = 2πr = πd; A = πr²; r = C ÷ 2π = √(A ÷ π)
- r
- radius
- d
- diameter
- C
- circumference
- A
- area
- π
- 3.14159265…
Worked example
- Diameter = 2 × 5 = 10.
- Circumference = 2π × 5 = 10π ≈ 31.415927.
- Area = π × 5² = 25π ≈ 78.539816.
A circle of radius 5 has diameter 10, circumference 31.42 and area 78.54 square units.
What the result means
Area grows with the square of the radius: doubling the diameter of a pizza gives four times as much pizza.
Assumptions and limits
- A perfect circle in a flat plane.
- Results are rounded to 6 decimal places.
Questions
Why does 3.14 give a slightly different answer?
3.14 is π rounded to two decimals, about 0.05% low. This calculator uses π to double precision.
How do I get the area from the diameter?
Halve the diameter to get the radius, then A = πr², or directly A = πd² ÷ 4.