Diameter to Radius Calculator
Enter a circle's diameter to find the radius, or enter the radius to find the diameter. The circumference and area are computed automatically whichever direction you go.
I know the
r = d / 2
- 1
Diameter (input)
10 - 2
Radius = diameter ÷ 2
10 ÷ 2 = 5
How does this calculator work?
Diameter and radius are related by d = 2r (or r = d/2). Enter either one to get the other, plus the circumference (πd) and area (πr²). The calculator works in both directions — just select whether you're starting from diameter or radius.
Formula
How this is calculated
The radius of a circle is the distance from its centre to any point on its edge; the diameter is the full width of the circle through its centre, equal to twice the radius: d = 2r, or equivalently r = d / 2. Every circle property expressed in terms of one can be rewritten in terms of the other — it is a matter of preference or context.
The circumference (perimeter of the circle) is C = πd = 2πr, where π ≈ 3.14159. The area enclosed by the circle is A = πr² = π(d/2)². These formulas have been known since antiquity; Archimedes bounded π between 223/71 and 22/7 around 250 BC.
This calculator uses JavaScript's built-in Math.PI constant (accurate to 15–16 significant figures) and applies no rounding until the display stage. Inputs must be positive real numbers and represent the same length unit — the output is in the same unit (squared for area).
Frequently asked questions
Yes, by definition. For any circle, the diameter passes through the centre and its length is exactly twice the radius. This holds in Euclidean geometry regardless of size or units.
Multiply the diameter by π: C = πd ≈ 3.14159 × d. This is the simplest form of the circumference formula and directly uses the diameter rather than the radius. Equivalently, C = 2πr if you start with the radius.
Any consistent unit — centimetres, inches, metres, feet, etc. The radius and diameter will be in the same unit you enter; the circumference will also be in that unit; the area will be in that unit squared. Make sure all inputs use the same unit.
Also known as
TG we-Calculate Editorial Team. (2026). Diameter to Radius Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/diameter-to-radius-calculator
TG we-Calculate Editorial Team. "Diameter to Radius Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/diameter-to-radius-calculator.
TG we-Calculate Editorial Team, "Diameter to Radius Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/diameter-to-radius-calculator
@misc{wecalculate_diameter_to_radius_calculator, title = {Diameter to Radius Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/diameter-to-radius-calculator}}, year = {2026}, note = {TG we-Calculate} }
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