RMS, peak and average voltage explained
Why AC voltage has several names, what RMS really measures, and how peak, peak-to-peak and average relate for a sine wave.
Why one AC voltage has many numbers
An alternating voltage is not a single value but a wave that rises to a crest, falls through zero, and dips to a trough many times a second. Because of that motion, engineers describe it with several figures. Peak is the highest instantaneous value, peak-to-peak is the distance between the top and bottom crests, and RMS is a kind of effective average. Each answers a different question, which is why a data sheet or a scope can quote numbers that look inconsistent until you know which is which.
What RMS actually measures
RMS stands for root mean square, and it is the value of steady direct voltage that would deliver the same average power to a resistor as the alternating waveform does. That is why mains supplies are rated in RMS: a 120 V RMS outlet heats a kettle element exactly as a 120 V battery would. For a sine wave the RMS is the peak divided by the square root of two, roughly 0.707 of the peak, and it is what an ordinary multimeter set to AC reports.
The sine-wave relations
For a pure sinusoid the four quantities lock together with fixed ratios. Peak equals RMS times the square root of two, peak-to-peak is twice the peak, and the average, meaning the full-wave rectified mean, is two times the peak divided by pi. Rearranged, RMS is about 0.707 of the peak and the rectified average is about 0.637 of the peak. Knowing any one value therefore fixes all the others, which is exactly what this calculator does in a single step.
Where the factors break down
Those neat ratios are a property of the sine shape alone. A square wave has an RMS equal to its peak, and a triangle wave has an RMS of the peak divided by the square root of three, so the same conversion factors do not apply. Real-world power often contains harmonics that distort the shape, which is why a true-RMS meter exists to measure effective value directly. Treat this tool as a sine-wave assistant, and reach for a true-RMS instrument when the waveform is not clean.