Circumference to Diameter Converter
Divide any circumference by π and you have the diameter. Enter the distance around your circle — a pipe, a trunk, a ring — and get the diameter, radius, and area.
Example: with Circumference (C) 36 · Unit (optional) inches (in) → Diameter (d): 11.46 in.
Computed by the calculator below using its default values. Change any input to see your own numbers.
📏 Measure anything faster with a laser tape
Check it outDiameter from circumference: divide by π
Circumference and diameter sit in the same ratio for every circle: C ÷ d = π. So to find the diameter from the circumference, divide by π ≈ 3.14159: d = C ÷ π. A pipe with a 36 in circumference is 36 ÷ 3.14159 ≈ 11.46 in across, and a 6 in circumference works out to a 1.91 in diameter.
That fixed ratio makes diameter vs circumference easy to sanity-check: the circumference is always about 3.14 times the diameter, so the diameter is always the smaller number. Need the radius instead? Halve the diameter: r = C ÷ (2π), so 36 ÷ 6.2832 ≈ 5.73 in. The enclosed area follows from the circumference too: A = C² ÷ (4π) ≈ 103.13 sq in for the 36 in example.
How it’s calculated
d = C ÷ π, with π ≈ 3.14159 (JavaScript’s full-precision Math.PI). Radius r = d ÷ 2 and area A = C² ÷ (4π) are derived from the same circumference, so all three outputs describe one consistent circle. Values round only at display (2 decimals); the unit menu just labels the results.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Common mistakes
- Multiplying by π instead of dividing — the diameter is always about 3.14× smaller than the circumference.
- Measuring with a loose or angled tape — a sagging wrap inflates the circumference and the computed diameter.
- Mixing units — a circumference in inches gives a diameter in inches; convert before dividing, not after.
Frequently asked questions
How do you find the diameter from the circumference?
Divide the circumference by π ≈ 3.14159. A circle 36 in around has a diameter of 36 ÷ π ≈ 11.46 in.
What diameter is a 6 inch circumference?
6 ÷ π ≈ 1.91 in — handy for rings, dowels, and small pipes.
What is the difference between diameter and circumference?
The diameter is the straight line across the circle through its center; the circumference is the distance around it. They always satisfy C = πd, so the circumference is about 3.14 times longer.
How do I get the radius from a circumference?
r = C ÷ (2π). For C = 36, that’s 36 ÷ 6.2832 ≈ 5.73.