About the circle calculator
Every circle measurement is linked through π. Know one value and you can find the rest — handy for round tables, garden beds, pipes, and pizza comparisons.
How to use this calculator
- Choose what you know.
- Enter the value.
- Read the radius, diameter, circumference, and area.
Worked examples
- r = 5 ft
- C = 2π × 5 ≈ 31.42 ft
- A = π × 5² ≈ 78.54 sq ft
Circumference 31.42 ft, area 78.54 sq ft.
Measurement guidance
Measuring a large circle
Wrap a string or tape around it to get the circumference, then use that as your known value.
Frequently asked questions
Is a 16-inch pizza twice a 12-inch one?
In area it’s about 1.78 times larger (16² ÷ 12²), not just 1.33.
What value of π is used?
Full double-precision π (3.14159265…).