Circumference to Diameter Calculator
Enter exactly one of circumference, diameter, or radius. The page fills the other two using Math.PI from this browser. Area is not on this spoke; use the circle calculator for area.
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How to convert circumference and diameter
- Decide which single length you already know: circumference, diameter, or radius.
- Enter that one positive number. Leave the other two fields blank.
- Read diameter, circumference, and radius in the result rows. π comes from Math.PI in this browser.
- Clear the default radius of 1 before you type a circumference or diameter, so exactly one field stays filled.
- Open the full circle calculator when you also need area from the same length.
Circumference and diameter
Circle linear measures linked by π
| Circumference | C = 2πr = πd |
|---|---|
| Diameter | d = 2r = C / π |
| Radius | r = d / 2 = C / (2π) |
| π source | JavaScript Math.PI in this browser |
| Default example | Radius 1 gives diameter 2 and C = 2π |
| Input rule | Exactly one of C, d, or r must be filled |
What circumference and diameter measure
Circumference is the length of the closed curve around a circle. Diameter is the longest chord through the center. Radius is half that diameter. The three lengths lock together through π, so any one fixes the other two on a true circle.
How C, d, and r relate
C = πd and C = 2πr. Solving for diameter uses d = C / π. Solving for radius uses r = C / (2π) or r = d / 2. The page uses Math.PI, not a hand-typed 3.14, so results match other IEEE floating-point tools in the same browser.
How this spoke solves the missing lengths
Count how many of circumference, diameter, and radius are finite numbers. Exactly one must be present. The engine recovers radius first, then prints diameter and circumference. Zero or negative lengths raise an error. Filling two fields at once is rejected so the system stays determined.
When this conversion helps
Tape a pipe circumference and recover diameter for fittings, turn a track length into a radius for layout, or check a textbook that states only one of the three. Ellipses and stadium ovals are not circles and do not follow C = πd.
Limits
Perfect circles only. No arc length for a partial sweep, no chord tables, and no area on this page. IEEE floating point applies. CZNull does not receive the numbers.