I am assuming you are designing a powdered iron core inductor for a switching power supply.
Basically you start from the required DC current and the required inductance. That will give you the energy the core needs to store. You then select a toroid that can store that energy. To obtain the inductance, use the AL of the core and calculate the number of turns. Note that the inductance will swing, depending on the saturation ratio. Take that into account, although this is generally a desirable effect, since the inductance is higher at low currents, allowing operation in continuous current mode down to lower currents.
Select wire size based on the DC current. Check the turns will fit in the toroid. Else, either select a larger core or thinner wire.
Calculate the total losses: core + DC. Check temperature rise and make sure it is below 40C (lower is better, since cores age).
If it's too high, you need to redo the selection.
Go to the Micrometals website and check it out. They also have software that will do the calculations for you.
https://www.micrometals.com/
Magnetics Inc. offer their Kool Mu cores. Check out their website for design guides. **broken link removed**