Rt = R1 + R2 + R3 ...

Parallel wiring can be found with this formula:

Rt = 1 / ( 1/R1 + 1/R2 + 1/R3 ... )

Of course, this can only tell you the simple part of the impedance, and doesn't calculate the complex part. Impedance of a speaker is actually not a single value, but rather has peaks and dips. Actual impedance can be measured with a sine wave signal generator and an AC multimeter using a series resistor as a voltage divider. An easier way to do it is with a computerized measurement system like Speaker Workshop. That will measure impedance charts and much more.

• ZMAX - Maximum impedance, impedance at resonance
  
• T/S Measurements