Radius Calculator
Find a circle's radius from its diameter, circumference or area.
Change the values and press Calculate to work out your own figures.
Enter any one of the radius, diameter, circumference or area.
Show the working
- r = d ÷ 2
- d = 2r, C = 2πr, A = πr²
- Radius = 5 in
How to use it
Enter any one of the radius, diameter, circumference or area.
Enter the measurements (decimals, fractions such as 3/4, or mixed numbers such as 1 1/2) and, if you like, their unit; the answer is then also given in other units. Show the working gives the formula.
Key facts
r = d ÷ 2 = C ÷ 2π = √(A ÷ π)
Three ways
Halve the diameter, divide the circumference by 2π, or take the square root of the area divided by π.
Questions
What is the radius of a circle with area 100?
√(100 ÷ π) ≈ 5.641895835.
And from a diameter of 10?
5.
Formulas
r = d ÷ 2 = C ÷ 2π = √(A ÷ π)
Sources
Limitations
- All measurements in one calculation use the same unit; the answer is also given in other units.
- Measurements up to 10¹⁵.