Free Calculator

Speaker Amplifier Power Calculator

How many watts your speakers actually need to hit your target SPL at the listening chair, and whether the amp you have can clear it without clipping. Runs the inverse square law across speaker sensitivity, impedance, room gain, crest factor, and sub crossover so the wattage you see is the wattage your amp must deliver.

My rigs

Save speaker+amp+room setups here. Tap any saved rig to load it instantly when you swap gear.

Required power per channel - -
Drive difficulty
- -
Amp headroom - Pick an amp to see if it has enough watts.

Your speakers

load
dB / W / m

Bookshelf: 84-88. Floor-stander: 87-92. Horn: 95+.

Nominal impedance

Listening goal

target
Listening level
dB SPL
What you listen to
dB

Room & amp

drive
Room size
dB
W / channel

Power needed as you change distance

live

The inverse square law: every time you double the distance from the speaker, the power needed jumps 4x. The orange curve is the watts you need at this speaker + target SPL; the vertical line is your seat; the dashed line is your amp's continuous output. Where the amp line crosses the curve is the furthest you can sit before the amp runs out of clean watts.

Show the math

Sizing the amp for the speaker, room, and target SPL

The math behind the number

Required amplifier wattage per channel is P = 10^((target - sensitivity + 20·log10(distance) - room_gain + crest - stereo - sub) ÷ 10). Every doubling of distance costs 4x the watts. Every 3 dB drop in sensitivity also doubles your power need. A stereo pair sums about +3 dB at the chair; crossing over a sub at 80 Hz removes the bottom octaves and trims another 3-6 dB off the main amp's load.

Crest factor is the most under-counted figure. Music averages 10-20 dB below its peaks; a 100 W amp running 100 W average will clip every transient. Set the genre pill honestly and the calculator returns the wattage your amp must deliver cleanly, not the long-term average.

Room gain, impedance, and why specs lie about it

Manufacturers measure sensitivity at 1 m in anechoic conditions. Your room is not anechoic. Small rooms add 6 dB of low-frequency boundary gain; medium rooms about 3 dB; large or treated rooms behave closer to free-field. The room-size pills set sensible defaults; tweak the custom field for your specific space.

Impedance changes which wattage figure on the amp's spec sheet matters. A 4 Ω speaker asks for roughly 1.4-2x the current of an 8 Ω speaker at the same SPL. Always match the amp's rated output at the speaker's impedance, not the headline 8 Ω number. The amp dropdown picks an amp's 4 / 8 Ω figures automatically.

Reference speakers, by what they need

Sensitivity is the difference between a flea-watt SET amp and a 200 W class D power amp - same room, same target. Plug any of these into the calculator and the amp-class verdict updates instantly.

SpeakerSensitivityImpedanceWhat drives it
KEF LS50 Meta85 dB/W/m8 Ω50-100 W class-AB; rewards high-current amps.
Klipsch RP-600M II96 dB/W/m8 Ω15-40 W on tubes; loves SET and PrimaLuna.
Neumann KH 120 II86 dB/W/mactiveSelf-powered; no external amp needed.
Q Acoustics 3050i91 dB/W/m6 Ω50-100 W integrated; easy across the board.
Klipsch Heresy IV99 dB/W/m8 Ω1-10 W. Flea-watt SET territory.
Magnepan LRS+86 dB/W/m4 Ω100+ W into 4 Ω. Panels eat current.
Klipsch La Scala AL5105 dB/W/m8 ΩUnder 2 W. Single-driver tube amps shine.
Wharfedale Linton 8590 dB/W/m6 Ω25-50 W push-pull tube or class-AB.

FAQ

Speaker amplifier power FAQ.

How sensitivity, impedance, distance, and room gain change the watts you need, plus how to choose between a 50 W and a 200 W amp for the same speaker.

  1. How do I calculate the amplifier power my speakers need?

    Enter speaker sensitivity in dB/W/m, listening distance, and target SPL at the chair. The calculator runs the inverse square law to find the wattage required, then adds a headroom factor for music transients. Match an amp whose continuous output meets or exceeds that figure into the speaker's nominal impedance.

  2. How does sensitivity reduce the amplifier wattage I need?

    Every 3 dB of speaker sensitivity halves the amplifier power required for the same SPL. An 89 dB/W/m speaker needs only half the watts of an 86 dB/W/m speaker. High-sensitivity horns at 95+ dB can reach reference levels on a single watt; insensitive 83 dB monitors will need triple-digit wattage in a large room.

  3. How much headroom should an amp have over my calculated power?

    Target 2-4x the calculated continuous power as peak amp capability. Music has 15-20 dB of crest factor; an amp that hits the calculated number on average will clip on transients. A 100 W RMS amp that doubles to 200 W into a 4-ohm speaker is ideal for 50 W average loads.

  4. Does room size and treatment change how much power I need?

    Yes. A small treated room adds 3-6 dB of boundary gain at low frequencies, effectively boosting your sensitivity. Large or anechoic rooms behave closer to free field and demand more power. The calculator includes a room-gain field so you can dial in your environment.

  5. What about clipping headroom and amplifier classes?

    Class A, AB, and D all clip ungracefully if pushed past their continuous rating. The "watts" in the calculator should be clean RMS into the speaker's impedance, not peak music power or PMPO marketing numbers. For the most demanding speakers (low sensitivity, low impedance), high-current amps that double their wattage at 4 ohms are the right pick.

  6. How many watts do I need for KEF LS50 Meta?

    The LS50 Meta is rated at 85 dB/W/m and a tough 4 Ω nominal. For 95 dB peaks at 2.5 m with 10 dB headroom, the calculator returns roughly 40-50 W per channel into 4 Ω. In practice you want an amp that doubles to that figure at 4 Ω, so a 25 W class-A or a 50 W class-AB integrated that confidently delivers 80-100 W at 4 Ω is the right pick.

  7. Do horns and high-efficiency speakers really need just a few watts?

    Yes. A 99 dB/W/m Klipsch Heresy IV hits the same SPL on roughly 1 / 16 the power of an 87 dB/W/m bookshelf. That is why flea-power SET amps (300B, 2A3 single-ended triodes at 8-25 W) work brilliantly with Heritage Klipsch, Avantgarde, and DeVore Fidelity, and fall apart with low-sensitivity studio monitors.