Calculations to give the inductance of a single-layer close-wound air-cored coil are extremely complex, but a good approximation is given by the following formula provided that these assumptions are adhered to:

  • Wire diameter is less than 10% of the coil diameter.
  • Wire is close wound in a single layer.
  • The coil's length is 0.4 to 3 times the diameter.

The formula is often called Wheeler's Formula

MathML formulaFormula ImageStraight text formula
L = N 2 d2 18d+40b One Layer Inductor Formulae. L = N^2[d^2 / (18d + 40b)]
N = L18d+40b d N = [(L*(18d + 40b))^0.5] / d


L is inductance in µH,

d is coil diameter in inches,

b is coil length in inches,

N = number of turns

Coil Diameter:
Coil Length:
Number of Turns:  


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