Inductance of a Solenoid
This is a single purpose calculation which gives you the inductance value when you make any change in the parameters.
Small inductors for electronics use may be made with air cores. For larger values of inductance and for transformers, iron is used as a core material. The relative permeability of magnetic iron is around 200.
This calculation makes use of the long solenoid approximation. It will not give good values for small air-core coils, since they are not good approximations to a long solenoid.