Calculate the Root Mean Square (RMS) voltage for different AC waveforms.
Last updated: March 2026 | By ForgeCalc Engineering
Root Mean Square (RMS) is a statistical measure of the magnitude of a varying quantity. In electrical engineering, RMS voltage is the DC equivalent voltage that would produce the same heating effect (power dissipation) in a resistor as the AC voltage.
When you measure AC voltage with a standard multimeter, it typically displays the RMS value. For example, a standard 120V outlet in the US has an RMS voltage of 120V, but its peak voltage is actually around 170V.
Where:
• V_rms is the Root Mean Square voltage
• V_pk is the peak voltage (maximum amplitude)
• √2 ≈ 1.414
RMS is used because it directly relates to power. A 120V RMS AC source and a 120V DC source will both heat a light bulb to the same brightness.
Peak-to-Peak (V_pp) is the total voltage difference between the positive peak and the negative peak. For a symmetrical sine wave, V_pp = 2 × V_pk.
For a perfect square wave that switches between +V and -V, the RMS voltage is simply equal to the peak voltage (V_pk), because the magnitude is constant.
Standard multimeters assume a perfect sine wave and just multiply the average by 1.11. A 'True RMS' meter actually calculates the square root of the mean of the squares, providing accurate readings for distorted or non-sinusoidal waves.
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