OpenHomeAV

Amplifier Power Calculator

How many watts do you need? Enter speaker sensitivity, distance and impedance to get the amplifier power for reference peaks and the amps that deliver it.

Published

Speakers of 88 dB sensitivity, 3.5 m from your seat, need 614 W per channel into 8 Ω for 105 dB peaks (RP22 level 2 for screen speakers). Surround and height speakers at the same distance need 308 W for their 102 dB. No AV receiver with at least 5 channels reaches 614 W into 8 Ω; 2 power amplifiers do (2 rated by the maker, all channels driven).

Speaker impedance

Sensitivity as the maker quotes it. Distance is from your seat to the front speakers.

Units

Power needed

614 W per channel into 8 Ω

  • Screen speakers, 105 dB peaks614 W
  • Surrounds and heights, 102 dB peaks308 W

Watts at the speaker. The sub is not counted: it has its own amplifier (see the Subwoofer Placement Calculator).

What changes the number

  • With 91 dB speakers308 W
  • Sitting 2.5 m away314 W
  • For RP22 level 1 (102 dB)308 W

Amplifiers that deliver it

measured A bench test (the source is named). rated by the maker The manufacturer's figure, often with only 2 channels driven; expect less with every channel busy.

AV receivers with at least 5 channels
AmplifierChannelsWatts into 8 ΩHow the figure was obtainedNoteCheck
None in the catalogue.
Power amplifiers
AmplifierChannelsWatts into 8 ΩHow the figure was obtainedNoteCheck
Emotiva XPA-DR11650 W0.1% THDrated by the maker rated by the maker, all channels driven
Wharfedale PRO DP-4100N MK241000 W1% THDrated by the maker rated by the maker, all channels driven

Makers rate power at different distortion limits (shown under the watts where the record states it), so a bigger number is not always a bigger amplifier.

Two catalogue amps can bridge pairs of channels for more power; set that up in the Cinema Designer.

Continue in Cinema DesignerOpens a 5.1 room with your seat 3.5 m from the centre speaker, every speaker but the sub at 88 dB and 8 Ω; no amplifier is set, so the designer uses its default 150 W per channel at 8 Ω, ready for every seat, each channel's RP22 level and more amplifiers.

How many watts do I need for a home cinema?

This amplifier power calculator answers one question: how many watts per channel does your system need to reach the peak level you want at your seat? Three things set the answer. Speaker sensitivity is how loud the speaker plays for a given input, quoted in dB at 2.83 V, 1 m. Distance is how far you sit from the speaker. The target is the peak level you want, which the calculator takes from the RP22 levels. Every 3 dB of sensitivity halves the watts you need, and every halving of the distance quarters them.

Enter your numbers and the page gives the watts per channel, the amplifiers in the catalogue that deliver it, and, if you name your own amp, the level it reaches at your seat. The sums are the Cinema Designer’s own, so the two always agree.

What reference level means

Film mixes are made for a reference level of 85 dB at the seat, with peaks 20 dB above it, so 105 dB. RP22 turns that into four levels for the screen speakers: 102, 105, 108 and 111 dB. Surround and height speakers are held 3 dB lower, at 99, 102, 105 and 108 dB. Level 2, 105 dB, is the reference. Listening 10 dB below reference needs a tenth of the power, which is why a system that cannot reach 105 dB can still sound loud in a living room.

Reading amplifier power figures

A power figure means little without how it was made. An all-channels-driven figure has every channel working at once. A 2-channel figure drives two and leaves the rest idle, so a receiver rated that way will give less with a full surround mix. A measured figure comes from a bench test with the source named; a rated figure is the maker’s claim. This calculator marks each amplifier as measured, rated by the maker, or partly measured, counts them apart in the answer, and never ranks them as the same kind of number.

The catalogue shows why. The Denon AVC-X8500HA has a measured 131 W with 5 channels driven at 8 ohms, and a measured 225 W at 4 ohms with only 2 channels driven. Blending the two would suggest more power between the loads than it can give a full mix, so between 4 and 8 ohms the calculator holds the 8 ohm figure. At exactly 4 ohms the only 4 ohm figure is the 2-channel test, and the calculator uses it and says so.

Impedance: 8, 6 and 4 ohms

A lower impedance draws more current. Amplifiers differ in how much more they deliver, so the calculator reads the figure each one publishes at 8 and at 4 ohms. At a load in between it interpolates when both figures were made the same way, and holds the 8 ohm figure when they were not. It never scales a figure up past what is published, because an amplifier is current limited into low loads. A figure you type by hand follows the designer’s 1/Z rule, scaled from the load you rated it at.

What this calculator assumes

One speaker in free space, sound falling with the square of distance, no gain from the room and no loss from power compression. Sensitivity is used as the Cinema Designer uses it. The subwoofer is left out: it has its own amplifier and needs room modes, not the inverse square law, so see the Subwoofer Placement Calculator. Real rooms add some level and real amplifiers clip, so treat the answer as the watts to aim for, not a guarantee.

When to go further

The Cinema Designer takes the same layout and amplifier and works out every speaker at its real distance from every seat, each channel’s RP22 level, several amplifiers at once and bridged channels. The button below opens it with your numbers already in place. For the level your speakers reach without naming an amplifier, the speaker SPL calculator is the simpler tool.

Frequently asked questions

How many watts do I need for home theater?
It depends on speaker sensitivity and how far you sit. For reference peaks of 105 dB, 88 dB speakers 3.5 m away need 614 W per channel. The same room with 95 dB speakers needs 123 W, because every 3 dB of sensitivity halves the watts. Sitting 1 m closer cuts the figure to 314 W. The calculator above gives the figure for your speakers and seat, for the RP22 level you choose.
Is 100 watts per channel enough?
For 88 dB speakers 3.5 m away, 100 W reaches 97.1 dB peaks, 7.9 dB short of 105 dB. That is fine for comfortable listening, and short of reference peaks. With 95 dB speakers the same 100 W reaches 104.1 dB. The answer is the speaker and the distance as much as the amplifier.
Does a 4 ohm speaker need more power?
For the same level, yes in watts. Sensitivity is quoted at 2.83 V, which is 1 W into 8 ohms but 2 W into 4 ohms, so a 4 ohm speaker with the same quoted sensitivity takes twice the watts for the same level. Most amplifiers deliver more watts into 4 ohms than into 8, but rarely double, and some publish no 4 ohm figure. The calculator uses each amplifier's own published figure at your load and never scales it up past what is published.
What does "all channels driven" mean?
It means the power figure was measured with every channel of the amplifier working at once, as in a film with a full mix. Many makers rate an AV receiver with only 2 channels driven, which is easier on the power supply and gives a bigger number. The calculator marks each figure with how many channels were driven and counts all-channels figures apart.
Measured or rated power: which should I trust?
A bench test by a third party is evidence, and the source is named in the table. A rated figure is the maker's claim, often at lower distortion limits and with fewer channels driven. Where a published figure is the only one at a load, the calculator uses it and says so. When one figure is measured and the other rated, the amplifier is marked partly measured, and the two are never treated as the same kind of number.
Do I need a separate power amplifier?
When the watts you need are within a receiver's all-channels figure, no. At 95 dB, 3.5 m and 8 ohms, 4 AV receivers in the catalogue reach the 123 W needed. At 88 dB the need is 614 W, and no receiver in the catalogue reaches it, while 2 power amplifiers do. A power amplifier also lets you add channels, and the Cinema Designer can bridge pairs of channels on the amps that allow it.
How much power do surround and height speakers need?
Less. RP22 sets the peak level for surrounds and heights 3 dB below the screen speakers (99, 102, 105 and 108 dB against 102, 105, 108 and 111 dB for levels 1 to 4), which halves the watts at the same distance. In the default room that is 308 W against 614 W. The calculator shows both.