Calculating the amplifier size needed for an audio system is the sum of many factors. Speaker efficiency, room size and how loud you like the sound relative to the other two factors make the determination. Loudness is very subjective, but there is a way to calculate the average amount of power to give a speaker to achieve a certain sound pressure level in a given space.
Find the cubic footage of the room. Measure the length and width, and height of the space using the measuring tape. Multiply the three measurements to determine the cubic footage.
Play a piece of music representative of the type routinely played in the system. Turn on the sound pressure level meter. Note the decibel reading on the display.
Examine the speaker manufacturer's specifications for speaker sensitivity. Note this for the equation.
Measure the distance from the main speakers to the primary listening position.
Enter these values into the advanced calculator, using this equation:
Measured sound pressure level - speaker sensitivity + 20 x (log) distance from speakers to seating position/reference distance (if different) + desired amplifier headroom.
Divide this value by 10 to get your amplifier wattage figure, per channel.