A few things to note before I begin:

R = Resistance (in ohms)
V = Voltage (in volts)
I = Current (in amps)

To find the total resistence (in ohms)

Parallel:

1/Rt = 1/R1 + 1/R2 + 1/R3 .....continued for as many speakers as you have

Series:

Rt = R1 + R2 + R3 .....continued for as many speakers as you have

So, in simple terms for our purposes, we'll assume all speakers have the same impedance rating. Assuming that, we can use the following simpler formulas.

Parallel:

divide the resistence of the subs by how many you have.

example: if you are using two 4 ohm subs, divide 4 ohms by 2 to get the total resistance of 2 ohms. If you increase this to four 4 ohm subs, then you divide 4 ohms by 4 to get total of 1 ohm.

Series:

add the values for each sub to get the total. Two 4 ohm subs would add to a total of 8 ohms. Four 4 ohm subs would add up to 16 ohms.

Special note: DVC subs

You can also consider using DVC (Dual Voice Coil) subs. These have 2 pairs of terminals per sub. Each speaker can be wired to 2 channels on your amp, or you can wire the speaker in either series or parallel and it will act like a single sub (of different impedance, of course)

Say you have a pair of 4 ohm DVC subs. If you wire each sub to itself in parallel, that makes it a single 2 ohm speaker. Then wire the two subs together in series, and you have a single 4 ohm set-up. Illustrated below are some examples.

To determine the load on your electrical system (in amps)
(Special note: these formulas assume amplifier effiecieny of 100%. No amp EVER could achieve such efficiency, due to heat loss, distortion, and other factors. Even the best amplifiers manage 80% or less. Consider your amps efficiency before coming to any conclusions. Also, be sure to leave some amperage left over "for the car" too! you car has other electrical needs besides just the radio, ya know!)

If you know the voltage, and the wattage, use this formula to determine how many amps it will use:

Amps = Watts / Volts


so, if your running an amp that puts out 500 watts at 14.4 volts, divide 500 by 14.4 to get your answer of 34.7 amps.
This same formula works to determine what size alternator you need, as well as what size wire to use.
(note: remember, due to the efficiency of the amp, some power will be lost to heat and other distortion, so it will take more than 34.7 amps to get 500 *CLEAN* watts out to your speakers. In this example, lets assume a typical 70% efficiency. 34.7 times 70% (34.7 divided by .7) equals 49.5 amps)

Or, lets say you know the size of your alternator, and the system voltage, and you want to know how powerful an amp you can use:

Watts = Amps * Volts


so, say your car has an 80 amp alternator, and runs at 14.4 volts. Multiply 14.4 times 80 to get 1152 continuous watts, not counting amplifier efficiency and the car's other electrical needs.
(note: you *can* run higher than this, but anything higher than your alternator's rating will drain off your battery. Kill your battery while driving, and you'll be finishing your trip Flintstone-style)

And, as long as we're talking formulas, the last in this series helps you determine voltage. I really dont see why youd be wondering this, voltage should always be constant in car stereo, but, i wouldnt be right if i didnt give you this last one anyway.

Voltage = Watts / Amps

Also, be sure to check out our car stereo club on Yahoo! Devil Bass


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