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C =         R
     ----------------
        9 X 10^9 

where R is the radius in meters and C is Farads.

The capacitance is simply a function of the surface area.

I didn't have a formula for calculating the area of a toroid, so I modified 
the formula for the area of a cylinder, "bent" it into a ring, and added the 
area of the center disk.

I had a table of actual values for several toroids of different chords and 
diameters.  I then played with fudge factors in the formula until it would 
yield results which matched the table to within 5%.

A friend of mine later took my crude formula and reduced it, but I never 
bothered to change the original.  If it ain't broke, I don't fix it.

My formula appears to work on small toroids all the way up to at least 8 to 
12 inch chords and up to at least 8 foot diameters. I suspect that the error 
of the formula would increase beyond these sizes.  However, the largest 
toroid I've built so far is only 62 inches in diameter, and I don't plan on 
going much larger in diameter than that.

So, as you can see, I'm not a mathematician or a scientist.  I just make it 
work.

I hope this answers your question.

Bert