Enter radius, diameter, area, or circumference — instantly get all other circle measurements with step-by-step formulas
Enter any one value — all others are calculated instantly.
r
Results
Enter a value to see all circle measurements
From Radius
d = 2r
C = 2πr
A = πr²
Find Radius
r = d / 2
r = C / (2π)
r = √(A / π)
About This Tool
A circle is defined by a single measurement — its radius. From that one number, you can calculate the diameter, circumference, and area. But what if you know the area and need the radius? Or you measured the circumference and need the diameter?
This calculator works in any direction. Enter whichever value you know — radius, diameter, circumference, or area — and all other measurements are computed instantly. The formulas are shown alongside the results so you can verify the math or use them in your own work.
Useful for geometry homework, engineering calculations, construction planning, or any time you need to go from one circle measurement to another without pulling out the formula sheet.
How to Use
1. Choose which value you know: Radius, Diameter, Circumference, or Area
2. Enter the value in any unit (the calculator is unit-agnostic)
3. All other circle measurements are computed instantly
4. Read the step-by-step formula for each result
5. Use "Copy Link" to share a specific calculation
Formula
Diameter = 2 × Radius
Circumference = 2 × π × Radius
Area = π × Radius²
Radius from Diameter: r = d / 2
Radius from Circumference: r = C / (2π)
Radius from Area: r = √(A / π)
Frequently Asked Questions
How do I find the area of a circle?
Multiply π (3.14159) by the radius squared: A = π × r². For a circle with radius 5, the area is π × 25 = 78.54 square units. If you know the diameter, halve it first to get the radius.
How do I find the circumference of a circle?
Multiply the diameter by π: C = π × d, or equivalently C = 2 × π × r. A circle with a 10 cm diameter has a circumference of 31.42 cm. This is the total distance around the circle.
How do I find the radius from the area?
Divide the area by π, then take the square root: r = √(A / π). For example, if the area is 100 cm², the radius is √(100 / 3.14159) = √(31.83) = 5.64 cm.
How do I find the diameter from the circumference?
Divide the circumference by π: d = C / π. If a circular pool has a circumference of 25 meters, its diameter is 25 / 3.14159 = 7.96 meters, and its radius is 3.98 meters.
What is pi (π)?
Pi (π) is the ratio of a circle's circumference to its diameter. It's approximately 3.14159265358979 and is an irrational number — its decimal representation never ends or repeats. This calculator uses the full-precision value of π available in your browser (15+ decimal digits).
How do I find the area of a semicircle?
Calculate the full circle area (π × r²) and divide by 2. A semicircle with radius 8 has area = π × 64 / 2 = 100.53 square units. The perimeter of a semicircle is π × r + 2r (the curved part plus the straight diameter).
What is the area of a circle with radius 10?
A = π × 10² = π × 100 = 314.16 square units. The circumference is 2 × π × 10 = 62.83 units, and the diameter is 20 units.