"How many watts do I need?" depends on three things: how efficient the speaker is (its sensitivity, in dB at 1 watt / 1 meter), how far away the listeners are, and how loud it needs to be. This calculator uses the standard free-field formulas so you can size an amplifier or check what a speaker can do.
Indoors, wall reflections usually make it somewhat louder than the outdoor result, so the calculation is a safe starting point. Keep the amplifier's RMS power within the speaker's RMS rating, and set gain properly to avoid clipping – see how to set amplifier gain. For outdoor paging, read choosing a PA horn speaker for outdoor use.
It depends on speaker sensitivity and distance more than on crowd size. Enter your speaker's sensitivity, the distance to the back of the audience and a target level; the calculator gives the RMS watts each speaker needs.
Sensitivity is the sound level a speaker produces with 1 watt at 1 meter, in dB. Every 3 dB more sensitivity means half the amplifier power for the same loudness.
Loudness in decibels is logarithmic. Twice the power is +3 dB, which is a noticeable but small change; listeners perceive roughly +10 dB as twice as loud.
Use continuous (RMS) ratings for both amplifier and speakers. Peak or maximum ratings are short bursts and are not comparable between brands.
More free audio tools: Speaker impedance calculator · Amp wattage & SPL calculator · Speaker wire gauge calculator
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These calculators run entirely in your browser; nothing you type is sent or stored. Results are estimates based on standard formulas; always check your amplifier and speaker manuals.