The following formula calculates the inductance of an air cored single layer coil.

L= ( *d*^{2} x *n*^{2})
_{/}
(18 x *d* + 40 x *len* )

where:- L is the inductance in micro Henrys,

*d* is the coil diameter (cntr-cntr) in inches (form diameter plus the wire),

*len* is the coil winding length in inches

and *n* is the number of turns.

"Wire length" allows for approximately a 2 inch tail at each end