[TowerTalk] Determine inductance of a Torrid with N Turns

[Chuck Gooden]

Could someone please tell me how I can determine the inductance of N 
Turns of wire on a Fair-rite #2631803802 (2.4"x1.4"x0.5") Torid?

I am trying to use the formula on page 140 for the 2017 Fair-Rite 
catalog.  I think my trouble is with calculating the value for C1.   I 
believe this will be a ratio of the cross sectional area of the torid 
core and the circumference of the torid (l).

I have looked at other inductance equations on the net and I am also 
confused why the Fair-Rite uses 4 * PI, the other equations use 2 * PI.

I am working in the metric values given in the Fair-Rite catalog even 
though I gave the dimensions in inches above,  and I am using a value of 
1500 for Ui.

Thanks,

Chuck - K9LC