Calculate circle circumference, radius, diameter, and area from any one known value in mm, cm, m, in, or ft, with unit options for each input.
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Circle Length Formula
“Circle length” is another name for circumference: the distance around the edge of a circle. The calculator uses one of four formulas depending on which value you provide.
From radius: C = 2πr From diameter: C = πd From area: C = 2π·√(A/π) From length: r = C/(2π), d = C/π, A = C²/(4π)
- C — circle length (circumference)
- r — radius
- d — diameter (d = 2r)
- A — area enclosed by the circle
- π — pi, ≈ 3.14159265
All inputs are converted to meters internally, then results are converted back to the unit you selected. The circle is assumed to be a perfect (planar) circle. For ellipses or arcs, these formulas do not apply.
Reference Values
Quick lookup for common radii. Use it to sanity-check the calculator output.
| Radius | Diameter | Length (C) | Area |
|---|---|---|---|
| 1 cm | 2 cm | 6.283 cm | 3.142 cm² |
| 10 cm | 20 cm | 62.83 cm | 314.2 cm² |
| 0.5 m | 1 m | 3.142 m | 0.785 m² |
| 1 m | 2 m | 6.283 m | 3.142 m² |
| 5 m | 10 m | 31.42 m | 78.54 m² |
| 12 in | 24 in | 75.40 in | 452.4 in² |
Common unit conversions for circle length:
| From | Multiply by | To get |
|---|---|---|
| m | 100 | cm |
| m | 3.2808 | ft |
| in | 2.54 | cm |
| ft | 12 | in |
Worked Examples
Example 1 — From radius. A circle has radius 7 cm.
C = 2π(7) = 14π ≈ 43.98 cm.
Example 2 — From diameter. A pipe has a diameter of 4 in.
C = π(4) = 4π ≈ 12.57 in.
Example 3 — From area. A circular pad has area 50 m².
r = √(50/π) ≈ 3.989 m, so C = 2π(3.989) ≈ 25.07 m.
Is circle length the same as perimeter? Yes. For a circle, the perimeter, circumference, and “circle length” all refer to the distance around the boundary.
How accurate is the result? Results use double-precision π. Rounding in the display is to 4–5 significant figures. Use “Show work” to see the unrounded values in meters.
