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Diameter of a Circle Calculator

Enter any one measure of a circle — radius, diameter, circumference or area — and get the other three instantly, with every formula shown with your numbers substituted in.

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Reading the known value
Solving for the radius
Computing the other measures

Result

This page opens the circle calculator with the circumference preselected as the known value and the diameter as the highlighted answer. Enter the distance around the circle and the tool divides by π to give the distance across it.

The identity behind it is the definition of π itself: the circumference of every circle is π times its diameter, so d = C ÷ π. A tree trunk measuring 94 around a tape is 94 ÷ π ≈ 29.92 across; a round table with a 470 circumference spans about 149.6. This is the standard trick for measuring things you cannot put a ruler through the middle of.

The radius and area are computed in the same pass, and every formula is printed with your numbers substituted so the working can be checked line by line. The math runs entirely in your browser.

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Accepted Formats & Rules

  • Solves from any of the four known values: radius, diameter, circumference or area.
  • Shows each formula with substitution, e.g. C = 2πr = 2 × π × 5 ≈ 31.415927.
  • Exact answers in terms of π (like A = 25π) whenever the radius or diameter is given.
  • Copy button grabs all four measures in one click.

Key Benefits

✓ One input is enough: give any single measure and the radius, diameter, circumference and area all appear at once.
✓ Every result comes with the formula written out with your numbers substituted — ideal for showing your work in homework.
✓ Runs entirely in your browser: no sign-up, no upload, instant results.

Use Cases

  • • Find how much edging or fencing a round flower bed needs from its measured diameter.
  • • Get the exact answer in terms of π, like A = 25π, for geometry homework — with every formula substituted step by step.
  • • Convert a tape measurement around a trunk, pipe or column into its diameter and cross-section area.