Flux Density Inside A Long Coil:
Assume that you have a long coil of wire, generally termed as a solenoid, with n turns and whose length is s meters. Assume that this coil carries a direct current of I amperes and has a core whose permeability is µ. The flux density B is in teslas within the core, supposing that it is not in a state of saturation, can be found by using this formula as follows:
B = 4π x 10-7 (µnI/s)
A fine estimation is
B = 1.2566 x 10-6 (µnI/s)