The ARRL handbook for radio communications (Ward 2011) gives guidance on designing with ferrite cored inductors:

Ferrite cores are often unpainted, unlike powdered-iron toroids. Ferrite toroids and rods often have sharp edges, while powdered-iron toroids usually have rounded edges.

Because of their higher permeabilities, the formulas for calculating inductance and turns require slight modification. Manufacturers list ferrite A_{L}values in mH per 1000 turnssquared. Thus, to calculate inductance, the formula isL=A

_{L}xN^{2}/1000000where:

L = the inductance in mH

A_{L }= the inductance index in mH per 1000 turns-squared, and

N = the number of turns.Example: What is the inductance of a 60-turn inductor on a core with an A

_{L }of 523? (See the chapter Component Data and References for more detailed data on the range of available cores.)L=A

_{L}xN^{2}/1000000=523×60^{2}/1000000=1.88e6/1e6=1.88mH

Lets follow the example through. Continue reading ARRL guidance on design of ferrite cored inductors