Circle Diameter Calculator — From Radius, Area or Circumference
Choose what you know — radius, area, or circumference — enter a value, and instantly get the diameter and every other circle measurement.
Known measurement
d = 2r — the longest straight line through the circle center
- 1
Radius (given)
7 - 2
Diameter = 2 × r
2 × 7 = 14
How does this calculator work?
Diameter d = 2r. From circumference: d = C/π. From area: d = 2√(A/π). Select your known measurement, enter a value, and the calculator returns diameter, radius, area, and circumference using standard circle formulas.
Formula
How this is calculated
The diameter of a circle is the chord passing through the center — twice the radius (d = 2r). Every circle property is a function of r alone: area A = πr², circumference C = 2πr. Inverting these relationships lets you recover r from any one of them: r = C/(2π) from circumference, r = √(A/π) from area, r = d/2 from diameter.
This calculator accepts any single measurement, derives the radius using the correct inverse formula, then computes all four properties in one step. The result uses π to full floating-point precision (about 15 significant digits), so rounding to the precision of your original measurement is appropriate for practical work.
Note that area and circumference scale differently with r: area grows as r² while circumference grows as r. Doubling the radius quadruples the area but only doubles the circumference.
Frequently asked questions
The radius is the distance from the center to the edge of the circle. The diameter is the full width through the center — exactly twice the radius. Engineers often specify pipe and bolt sizes by diameter; mathematical formulas usually use radius.
Divide by π: d = C / π. This follows directly from C = πd rearranged. For example a circumference of 31.416 gives a diameter of approximately 10.
Use d = 2√(A / π). Start with A = πr², solve for r = √(A / π), then d = 2r. For example an area of 78.54 gives r = √(78.54/3.14159) ≈ 5, so d = 10.
Also known as
TG we-Calculate Editorial Team. (2026). Circle Diameter Calculator — From Radius, Area or Circumference [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/circle-diameter-calculator
TG we-Calculate Editorial Team. "Circle Diameter Calculator — From Radius, Area or Circumference." TG we-Calculate. 2026. https://we-calculate.com/calculator/circle-diameter-calculator.
TG we-Calculate Editorial Team, "Circle Diameter Calculator — From Radius, Area or Circumference," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/circle-diameter-calculator
@misc{wecalculate_circle_diameter_calculator, title = {Circle Diameter Calculator — From Radius, Area or Circumference}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/circle-diameter-calculator}}, year = {2026}, note = {TG we-Calculate} }
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