If you do your own crossovers and need to calculate the correct Inductor or Capacitor for your system, then check out Pacific Accessory Corporation's (PAC) online calculators.

http://www.pac-audio.com/crossover/

Not for everyone but might help some.

-Ozzy

I would warn people though, that this is nice just to get a general idea of how these components work, but to effectively build a passive crossover that's even somewhat close to reality, you need far, far more sophisticated software and accurately measured drivers.

Even with all of the zobel impedance matching circuits that you can throw at a driver, phase issues and non-linear impedances result in the designed crossover behaving nothing like you expect it to.

The formula for the Zobel Filters is wrong.