Hi again,
The electrical calculations for a toroid are the same as for any other
core. The calculation for the core area is a little different of course
because the shape is different, however it's the same idea: you
simply calculate the cross sectional area.
To do this, recognize that the core has an inner diameter ID,
outer diameter OD, and height H.
The width W is calculated from W=(OD-ID)/2, and having now the
width and height we can calculate the cross sectional area:
A=W*H.
That's it.
The magnetic characteristics of a core depend highly on what
material it is made of and vary widely from as low as 100G to 20kG.
The only way to get this information is from the manufacturer or do
some electrical tests on the core.