Last updated: 2026-06-16

RMS, Peak & Peak-to-Peak Voltage

An alternating voltage can be described in several ways, and they are easy to confuse. The same sine wave might be quoted as 120 V, 170 V, or 340 V depending on whether you mean its RMS value, its peak, or its full peak-to-peak swing. This calculator converts between all four common descriptions — RMS, peak, peak-to-peak, and the rectified average — for a pure sine wave.

Formula

For a sinusoidal voltage the relationships are:

These factors hold only for sine waves. Square, triangular, and other waveforms have different ratios between their RMS, peak, and average values.

Worked example

A mains supply is rated at 120 V RMS. Its peak is:

The full swing the wire actually sees is the peak-to-peak value:

and the rectified average is $V_{avg} = 2 \times 169.7 / \pi \approx 108.0\ \text{V}$.

A note on "average"

The plain average of a sine wave over one complete cycle is zero — the positive and negative halves cancel exactly. The average reported here is the full-wave rectified average, computed over the absolute value of the waveform. It is the figure that matters for rectifier outputs and for moving-coil meters that respond to rectified current.

Why it matters

Mains and signal voltages are almost always quoted as RMS because that is the value that determines power delivered to a resistive load: a 120 V RMS source heats a heater exactly as a 120 V DC source would. But insulation, capacitor voltage ratings, and oscilloscope readings all relate to the peak or peak-to-peak figures, which are noticeably larger. Keeping the three straight prevents both under-rating components and misreading waveforms.