Amplifier Power Output Calculator
Electrical power into a resistive load follows Ohm's-law power identities: P = V² ÷ Z, P = I² × Z, and P = V × I, where V and I are RMS values and Z is the load impedance in ohms.
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How to use this calculator
- Enter any two of: RMS voltage across the load, RMS current, and load impedance (ohms). The tool solves for power and the missing quantity.
- Use RMS values, not peak. If you measured a peak (crest) voltage, divide by 1.414 first to get RMS for a sine wave.
- Read off the RMS power output in watts, plus the power level in dBW and any value you left blank.
How it works
Electrical power into a resistive load follows Ohm's-law power identities: P = V² ÷ Z, P = I² × Z, and P = V × I, where V and I are RMS values and Z is the load impedance in ohms. Given any two of voltage, current and impedance the third and the power are fully determined, which is why this calculator lets you enter any two.
The power level in dBW is 10 × log₁₀(P), a logarithmic view handy for comparing amplifiers. This model treats the speaker as a plain resistor equal to its nominal impedance. Real loudspeaker impedance rises and falls with frequency, and program material is not a steady sine wave, so the figure is a nominal continuous rating rather than an exact match to a datasheet's 'music' or 'peak' power number.
Worked example
28.3 V RMS into an 8-ohm speaker. An amplifier delivers 28.3 V RMS across an 8-ohm load. Enter voltage = 28.3 V and impedance = 8 ohms. P = V² ÷ Z = 28.3² ÷ 8 = 800.89 ÷ 8 = 100.11 W. So the amp is putting out about 100 W RMS into 8 ohms, a level of 20.00 dBW, drawing 3.538 A RMS.
Common mistakes
- Mixing peak and RMS. Feeding a peak voltage into a formula that expects RMS overstates power by 2×. For a sine wave, RMS = peak ÷ 1.414.
- Using speaker sensitivity or a marketing 'PMPO' number as impedance or power — those are different quantities. Impedance is the nominal ohms (4, 6, 8...) printed on the driver.
- Assuming power simply halves when you switch from 4 to 8 ohms at the same voltage. At a fixed voltage, P = V² ÷ Z, so doubling the impedance halves the power — but a real amp's voltage often sags into lower impedances, so measure, don't assume.
Frequently asked questions
Do I enter peak or RMS voltage?
RMS. The formulas P = V² ÷ Z and P = V × I only give true (average) power when V and I are RMS values. For a sine wave, RMS = peak ÷ √2 ≈ peak ÷ 1.414. If you have peak-to-peak, divide by 2.828 to get RMS.
Why does the same amp make more watts into 4 ohms than 8 ohms?
At a given output voltage, power is V² ÷ Z, so halving the impedance from 8 to 4 ohms doubles the power — in the ideal case. In practice the amplifier's voltage drops a little under the heavier 4-ohm load, so the real increase is usually less than exactly double.
Is this the RMS or peak power rating?
It's continuous RMS power into a resistive load, the figure most makers quote as 'RMS watts'. Peak, program, and 'PMPO' ratings use short bursts or different reference points and are typically larger; they are not what this calculator returns.
Can I use this for the total power of a stereo amp?
This gives power per channel into one load. For a two-channel amp driving two speakers, calculate one channel and double it — but check the maker's rating, since many amps can't sustain full rated power on both channels at once.
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