If it is an air-cored inductor, calculate the number of turns from its
measured dimensions.

This won't work with a ferrite core because its material permeability is not
known. Although if the core is a simple rod the effective permeability is
roughly 25 regardless of material permeability.

With a high permeability core, 100 or more, effective permeability becomes a
function only of the very long 'air gap'. So inductance stops increasing
with increasing core material permeabilty.

But why would you want to know the number of turns if the coil is already
wound!
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Reg.