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 formula | Formula Image | Straight text formula |
---|---|---|

$L={N}^{2}\left(\frac{{d}^{2}}{18d+40b}\right)$ | L = N^2[d^2 / (18d + 40b)] | |

$N=\frac{\text{exception 25:}}{d}$ | N = [(L*(18d + 40b))^0.5] / d |

Where

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: |

Inductance: