How do you find radius from circumference?
Divide by 2π.
Radius of a Circle Calculator
Back-solve for radius when you know circumference, area, or diameter.
Choose what you know
Select input type and enter the known positive value.
About this calculator
How to use Radius of a Circle Calculator, plus the formulas and logic behind the results.
Tool description
Radius relates to diameter by halves, to circumference by C/(2π), and to area by √(A/π).
How to use
- Select circumference, area, or diameter.
- Enter the known positive value.
- Select Calculate.
- Read radius (and related measures).
Formulas & reference
r = C / (2π)r = √(A / π)r = d / 2
Radius recovery
- C = 2π → r = 1
- A = π → r = 1
- d = 10 → r = 5
- Keep units consistent
Calculation logic
Applies the inverse relation for the selected known quantity using M_PI.
Inputs must be positive.
Radius of a Circle Calculator FAQ
Common questions about this calculator and how to use the results.
How do you find radius from area?
Take the square root of area divided by π.
How do you find radius from diameter?
Halve the diameter.
Can area be zero?
No—enter a positive area.
What π is used?
M_PI.
Does this work for spheres?
Radius meaning is the same, but sphere area/volume formulas differ.