his is an popular coil geometri used in todays wireless charger circuits.
The formula used in this calculation is based on the
Harold A. Wheeler approximations
for air core flat spiral coil inductor.

…where:

1 inch = 0,0254m=2,54cm = 25,4mm.
This formula applies at ‘low’ frequencies (<30MHz)
using enameled copper wire. Some people call it “magnet
wire”.