PA System Size for Audience Calculator (Watts, SPL, Speakers & Subs)

How it works

watts ≈ audience × W/person (speech 1.5, acoustic 4, band 8, DJ 12) × venue (indoor 1, marquee 1.5, outdoor 2) · SPL needed at 1 m = target + headroom (6–10 dB) + 20 log₁₀(distance) − room gain · max SPL per top = sensitivity + 10 log₁₀(watts) · n tops add 10 log₁₀(n) · subs ≈ tops × 0.5–1.5 for music

A PA has one job — to deliver a target loudness with headroom to the farthest listener — and two ways of estimating what that takes: the rental-house rule of so many watts per person scaled for content and venue, and the physics of sensitivity, power and the inverse-square loss over distance. Speech needs little and tolerates modest speakers; amplified music needs ten times the power and subwoofers to carry the low end, and outdoors doubles everything because there are no walls to return energy. The larger answer of the two methods is the safe one, and beyond about forty metres no amount of power at the stage substitutes for delayed speakers placed among the crowd.

Worked example

Three hundred people indoors hearing a full band to 25 m: the rule says 300 × 8 W ≈ 2,400 W, or about 6 tops of 500 W; the SPL method needs 100 + 10 + 28 − 3 = 135 dB at 1 m, and 98 dB tops at 500 W peak at 125 dB each, so 10 of them. Ten tops and 10 subs is the answer — or, more realistically, fewer higher-output boxes (a 102 dB, 1,000 W top needs only 4) — giving about 100 dB average at the back, no delay towers, and roughly 6 kW of average draw across four circuits.

Frequently asked questions