Pa System Size Calculator
Calculate PA system wattage and speaker placement for events from venue size and audience. Enter values for instant results with step-by-step formulas.
Formula
Required Watts = 10^((Target SPL + Distance Loss - Sensitivity) / 10)
Where Target SPL is the desired sound pressure level at the farthest listener in dB, Distance Loss is calculated using the inverse square law (20 log10 of distance), and Sensitivity is the speaker reference efficiency (typically 97 dB/1W/1m). The result is multiplied by a headroom factor of 1.5x.
Worked Examples
Example 1: Wedding Reception - Indoor Hall
Problem: A 150-person wedding in a 250 m2 indoor hall needs a PA for a DJ set. What system is required?
Solution: Target SPL for DJ = 100 dB\nMax distance = sqrt(250 / pi) = 8.9 m\nSPL loss over distance = 20 x log10(8.9) = 19.0 dB\nRoom absorption loss = 3 dB (indoor)\nRequired SPL at source = 100 + 22 = 122 dB\nAbove reference sensitivity (97 dB) = 25 dB\nRequired watts = 10^(25/10) = 316 watts RMS\nWith 1.5x headroom = 474 watts\nSpeakers needed = 2 (minimum pair)\nSubwoofers = 2 (DJ ratio)
Result: 2 main speakers (500W each) + 2 subwoofers | Total: ~1,500W system
Example 2: Outdoor Concert - 500 People
Problem: An outdoor concert venue of 800 m2 for 500 people with live bands. Calculate PA requirements.
Solution: Target SPL for live music = 105 dB\nMax distance = sqrt(800 / pi) = 16.0 m\nSPL loss = 20 x log10(16) = 24.1 dB\nOutdoor absorption = 6 dB\nRequired SPL at source = 105 + 30.1 = 135.1 dB\nAbove reference = 38.1 dB\nRequired watts = 10^(38.1/10) = 6,457 watts RMS\nWith 1.5x headroom = 9,686 watts\nSpeakers needed = 20\nSubwoofers = 10
Result: 20 main speakers + 10 subwoofers | Total: ~20,000W system | 4 x 20A circuits
Frequently Asked Questions
How do I determine the right PA system wattage for my event?
The right PA wattage depends on three primary factors: venue size, event type, and whether the space is indoor or outdoor. Speech events require approximately 85 dB SPL at the farthest listener, while live music needs 100 to 110 dB. Start by calculating the maximum distance from the speakers to the farthest audience member, then account for sound pressure loss over distance using the inverse square law. Every doubling of distance reduces SPL by 6 dB. Add headroom of 3 to 6 dB to prevent distortion during peaks. A general rule is 5 to 10 watts per person for speech events and 25 to 50 watts per person for live music in an indoor venue.
Do I need subwoofers for my PA system and how many?
Subwoofers are essential for DJ sets, live music with bass instruments, and any event where full-range reproduction below 80 Hz is needed. They are generally not required for speech-only events like conferences or lectures. The standard ratio is one subwoofer for every two main speakers for live music, and a 1.5-to-1 ratio for DJ and electronic music events where deep bass is critical. Subwoofers should be placed on the ground, ideally against a wall or in a corner to take advantage of boundary reinforcement, which can add 3 to 6 dB of output. Cardioid subwoofer arrangements, using front-facing and rear-facing subs with delay, help reduce bass bleed onto the stage and improve low-frequency control in the audience area.
How does indoor versus outdoor affect PA system requirements?
Outdoor events require significantly more power than indoor events because there are no walls or ceiling to contain and reflect sound energy back toward the audience. Indoors, room reflections can add 3 to 6 dB of apparent loudness, effectively doubling the perceived volume compared to the same system outdoors. Outdoor systems typically need 2 to 4 times the wattage of indoor systems for comparable coverage. Wind can carry sound away from the audience, and temperature gradients cause sound to refract upward on hot days. Outdoor events also lack the natural reverberation that makes indoor sound feel full, so more speakers spread across the area provide better results than fewer, more powerful speakers concentrated at one location.
What electrical power supply do I need for a PA system?
PA systems draw significant electrical current, and inadequate power supply is a common cause of hums, buzzes, and system failures at events. Calculate total power consumption by adding the wattage of all amplifiers, then divide by voltage (typically 120V in North America or 240V elsewhere) to determine amperage draw. A 5000-watt system draws approximately 42 amps at 120V or 21 amps at 240V. Standard circuits provide 15 or 20 amps each, so you may need multiple dedicated circuits. Never share circuits with lighting dimmers or motors, which introduce electrical noise. For outdoor events without venue power, generator sizing should be 1.5 to 2 times the calculated power draw to handle startup surges and maintain clean sine wave output.
How do I prevent feedback in a PA system?
Feedback occurs when amplified sound from the speakers reaches the microphone at sufficient level to create a self-reinforcing loop. The primary prevention method is to position speakers in front of the microphone pickup pattern, so sound travels away from the mic toward the audience. Use directional (cardioid or supercardioid) microphones that reject sound from the rear and sides. Keep the gain structure optimized: set each component in the signal chain to its ideal operating level rather than cranking one stage to compensate for another. Graphic or parametric equalizers can notch out specific feedback frequencies, and modern digital processors include automatic feedback detection and suppression. Ringing out the system during soundcheck by slowly raising gain until feedback appears, then cutting that frequency by 3 dB, is standard practice.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.