What does it mean when it says 18 watts x 4 RMS?

i'm looking for a stereo for my '05 ford ranger edge and almost all of the stereo's that i've looked at say in the discription: "18 watts x 4 RMS". what does this mean?


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18 watts RMS is what the stereo will put out continuosly to all 4 speakers.

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The output of the stereo's amplifier is 18 watts x 4 (channels). RMS means "Root Mean Square". This is an industry standard by which amplifier output is measured.

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RMS stands for "root mean squared"
it is an acronym that represents the amount of constant power (in wattage) that the average head unit can produce continually.
this is not the maximum power output!

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18 watts X 4 RMS means that the stereo gives you 4 channels of speaker capability!
Those 4 channels are usually for the: Left Front Speaker-18 watts
Right Front Speaker-18 watts
Left Rear Speaker-18 watts
Right Rear Speaker-18 watts
And the 18 watts RMS is the amount of clear power per speaker the stereo can give!

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RMS = Root Means Squared. So 18 watts x 4 RMS means, that the internal amplifier that the head unit has built into it, will supply 4 speakers a minimum of 18 watts per speaker. Now, if the head unit had a constant supply of 13.6 volts, you can double the output of the internal head unit amps wattage. Which means each speaker will then be getting 36 watts x 4 RMS. This is only if you can supply a consistant 13.6 volts. Since your alternator is regulated, it only charges up to a 13.6 volt charge, then shuts off. Then when the voltage drops again, it charges back up the car battery. Your amp that is built into the head unit is non-regulated. So the more voltage you apply to it the more wattage it can produce. Of course, the internal amp has bult in protection, that will keep it from running away by over powering it. It will also thermally shut itself off if it gets too hot. This is called a thermal shut down or thermal overload. Most of the time you can tell when this is happening when the music "clips", cuts on and off. You just need to turn off the amp and let it cool down. Most amps have "heatsinks". Heat sinks dissappate the heat that the amp has built up, but sometimes not fast enough. Some amps have built in fans, that do the same thing.

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If you are just going to run the factory speakers in the vehicle, then the current output of the head unit will be fine. If you are looking for a cleaner sound, I would suggest upgrading the speakers to aftermarket ones that the stores sell. And if you decide to go this route, you might find that the head unit isn't strong enough to push the new aftermarket speakers. Then you will need to add an external amplifier to give more power to the aftermarket speakers.

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Hope this clarifies alittle more.
Good Luck!
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