Radius of a Circle Calculator
The radius of a circle is the distance from its centre to any point on its edge. Given the area, circumference, or diameter, this calculator derives the radius instantly using the inverse of the standard circle formulas.
Solve radius from
sq units
Distance from the centre to the edge of the circle
- 1
Divide area by π
78.54 ÷ π = 25.000058 - 2
Radius = √(A / π)
√25.000058 = 5
How does this calculator work?
Radius from area: r = √(A/π). Radius from circumference: r = C/(2π). Radius from diameter: r = d/2. Select which measurement you have, enter the value, and all other circle properties (diameter, circumference, area) are computed immediately from the derived radius.
Formula
How this is calculated
All three circle formulas share the radius r as the single defining measurement. The area A = πr² inverts to r = √(A/π); the circumference C = 2πr inverts to r = C/(2π); and the diameter d = 2r gives r = d/2. Select which measurement you know and enter its value — the other three are then computed from the derived radius.
The calculator uses π to full double-precision floating-point accuracy (≈ 3.14159 265 358 979). Real measurements always carry unit labels, so make sure the input is in consistent units (e.g., all in centimetres) — the radius and diameter will be in the same unit, and area will be in square of that unit.
This is a pure geometric calculation with no physical assumptions. Any shape in which area or perimeter is known and the shape is a perfect circle will give the correct radius.
Frequently asked questions
Rearrange the area formula A = πr² to get r = √(A/π). For example, if the area is 50 cm², the radius is √(50/π) ≈ 3.99 cm.
The radius is the distance from the centre to the edge. The diameter passes through the centre edge-to-edge (d = 2r). The circumference is the total distance around the circle (C = 2πr ≈ 6.283r). All three are derived from the same single value r.
The calculator works in any consistent unit. If you enter the area in cm² it returns the radius in cm. Enter the circumference in metres and get the radius in metres. The label "units" is generic — substitute your actual unit throughout.
Also known as
TG we-Calculate Editorial Team. (2026). Radius of a Circle Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/radius-of-a-circle-calculator
TG we-Calculate Editorial Team. "Radius of a Circle Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/radius-of-a-circle-calculator.
TG we-Calculate Editorial Team, "Radius of a Circle Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/radius-of-a-circle-calculator
@misc{wecalculate_radius_of_a_circle_calculator, title = {Radius of a Circle Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/radius-of-a-circle-calculator}}, year = {2026}, note = {TG we-Calculate} }
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