How to calculate wattage?

The wattage rating on your amplifier is the maximum amount of electrical power it will provide to the speaker. The wattage or amperage label on the back of your amp is the amount of electrical power that the amp will draw from the wall outlet.

You can't always correlate watts and volume, as some guitar amps seem louder than others, but you can use watts to compare broadly: 10-30W is a small amp, 30-75W is a medium-sized amp, and more is a big amp. Sizes for PA and Bass amps are different.

Does that help?

Yeah, that sure does.
But how can I find the wattage rating on my amplifier?...how much it really provides to the speakers?

But how can I find the wattage rating on my amplifier?...how much it really provides to the speakers?

The simplest answer is "not easily" :-) If you Google something like "measure audio watts" you will find lot more explanation. There is a wikipedia answer on this as well.

Here's a good article on the subject, see Power Ratings about halfway down.

http://users.chariot.net.au/~gmarts/amppower.htm

This is a terrific site with lots of information about guitars and amps, great article on effects order.

Power (in watts) = Current (in amps) x Force (in volts). That's Ohm's law.

But amplifiers aren't continuous in their power output. Musical sounds are caused by vibrations... which are created in a circuit by varying the voltage... which means your amp never puts out the same amount of power from one microsecond to the next. If it did, you wouldn't be able to make music through it.

That's not something that marketing people can work with. They like "our amp puts out x power".

So they come up with different measurements, like "peak power" or "instantaneous power" or "musical power output". They don't mean much, because each number is specifically designed to make the advertiser's product look better than the others.

Most amp comparisons are done on RMS power (root-mean-square), which is meant to provide some measure of the average power out when the input is a pure sine wave. Music isn't pure sine waves, but its at least an apples-to-apples number for electrical power output under specific circumstances.

But electrical power ins't what you hear - what you hear is sound power, which is measured in dB (decibels). And electrical power gets converted to music by the speakers. Some speakers are more efficient than others - same power in, more sound out.

What confuses many people is the relationship between amp power and sound power. All other things being equal (speaker efficiency, resistance load, etc), if you want it to sound "twice as loud" you need about 10 times the watts.

There are formulae that an engineer can use to predict the output power given parameters such as the amplifying device (tube or transistor)'s amplification factor, the output load on it, class of operation, and operating voltages. Not simple. If you're familiar with the circuits you can often look up values for the same amplifying device (say, an EL84 tube) operating under typical conditions with the same voltage in a manual and know it'll be about the same, and there are formulae in the designer's handbooks for correcting that value if the voltages are a bit different. These calculations are based on further simplifying assumptions such as a pure sine wave input and operation at some low level of distortion.

Vink's right, the answer is "Not easily," unless you just want a comparison with something similar.

Easiest way is to look up the owner's manual, in which there will be a power rating.