Original poster: "Godfrey Loudner" <ggreen-at-gwtc-dot-net> Such an engine already exist as WinXformer.zip at http://www.tesla-2-dot-org/~mikeh. Godfrey Loudner >Where did you get that formula? How does it include frequenct into the >equation? I'd like to use in my transformer calculator I'm developing but it >needs to change with frequency.