Amplifier Power Calculation

Estimate amplifier output power from a voltage measurement across a known resistive load. Enter RMS, peak or peak-to-peak voltage and select the load impedance. Results update automatically.

Measured AC voltage (V) Measure across the load. Voltage measurement Peak-to-peak (Vpp) Peak (Vpk) RMS (Vrms) Choose the reading from your meter or scope. Load resistance 2 ohms 4 ohms 8 ohms 16 ohms 32 ohms Custom value A non-inductive dummy load is preferable.

Calculated output

Average sine-wave output power

6.25 W

RMS voltage

7.07 V

Peak voltage

10 V

Peak-to-peak voltage

20 V

RMS load current

884 mA

Output level (dBW)

7.96 dBW

Output level (dBm)

37.96 dBm

RMS voltage for 1 W at this load

2.83 V

Measurement note: This calculator assumes a sinusoidal signal and a resistive load. For a meaningful clean-power measurement, check the oscilloscope waveform for clipping. A loudspeaker's actual impedance varies with frequency, so a nominal 4, 8 or 16-ohm speaker is not a precise resistive test load. Use a suitably power-rated dummy load. An earth-referenced oscilloscope ground clip must only be connected to a point known to be safe to ground.

How the calculation works

For a sine wave: Vrms = Vpeak / square root of 2, and Vpeak = Vpp / 2.

Output power: P = Vrms squared / R. RMS current = Vrms / R.

Decibels: dBW = 10 x log10(power in W). dBm = 10 x log10(power in mW). At zero watts, neither logarithmic reading is defined.

Example: 20 V peak-to-peak across an 8-ohm load gives 7.07 V RMS, 884 mA RMS and 6.25 W.