How do I calculate the inductance of an iron core torroid inductor?

Question

If the design of an inductor calls for 80 turns of .5mm wire around a 40mm OD iron torroidal core, how do I calculate the inductance in mH or uH?

This isn't a homework problem, I'm building a circuit that calls for this inductor and I want to know what the resulting inductance is so that I don't have to build the inductor myself. I want to purchase one pre-wound.

Answer 1

(Inductance) = A / B
where
A = (0.4 * pi) * (number of turns)^2 * (area of wire) * 10E-8
and
B = ((length of gap) - (core mean length)/(relative permeability))

Using wire area in metres^2, lengths in metres, and relative permeability in henries per metre ... we get inductance in henries.