How this calculator works
For a sine wave, the voltage rises to a peak of root two times its RMS value. Power in a resistor is proportional to voltage squared, so peak instantaneous power is twice the average, or RMS, power. That factor of two is what this page applies. In audio, the term RMS watts usually means continuous average power measured with a sine test tone, while peak watts is the largest instantaneous power. Music with a high crest factor can produce peak power far above the continuous rating, and the exact ratio depends on the waveform.
Formula
For a sine wave, P(peak) = 2 x P(RMS) and P(RMS) = P(peak) / 2. The same result follows from P = V²/R: V(peak) = sqrt(2) x V(RMS), so V(peak)²/R = 2 x V(RMS)²/R.
Worked example
100 RMS watts is 200 peak watts. 50 peak watts is 25 RMS watts. A 20 W RMS amplifier has a 40 W sine-wave peak, before any supply sag is considered.
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Frequently asked questions
Is RMS watts a real unit?
It is a convention, not a separate unit. The watt is already an average power. The label RMS watts tells you the power was measured with an RMS voltage into a known load, usually a sine wave.
Does the factor of two apply to music?
No. A sine wave has a crest factor of about 1.414 in voltage, which squares to a power factor of two. Music, speech and percussion have different and changing crest factors, so the peak power of a musical signal can exceed twice its long-term average.
Why do some amplifiers list peak power much higher than RMS?
Peak power describes a very short transient, and it can be inflated by measuring with a low duty cycle or a loose distortion limit. RMS, or continuous, power is the more useful comparison between amplifiers.