With direct current, it’s easy to determine the voltage that’s present between two points in an electronic circuit: You simply measure the voltage with a voltmeter. With alternating current, however, measuring the voltage isn’t so simple. That’s because the voltage in an alternating current circuit is constantly changing.
There are actually three ways you can measure voltage in an AC circuit. The three ways are:
Peak voltage: A measurement of the largest voltage present between 0 V and the highest point on the AC cycle. It's the maximum voltage that the AC voltage attains.
Peak-to-peak voltage: The difference between the highest and lowest peaks of the AC voltage. In most AC voltages, the peak-to-peak voltage is double the peak voltage.
RMS voltage: The average voltage of the circuit; also called the mean voltage. RMS stands for root mean square, but that’s important only if you're studying for an exam or something. RMS voltage is far and away the most common way to specify the voltage of an AC circuit.
For example, when we say that the voltage at a household electrical outlet is 120 VAC, what we really mean is that the RMS voltage is 120 V.
If the AC voltage follows a true sine wave, the RMS voltage is equal to 0.707 times the peak voltage. Or to turn it around, the peak voltage is equal to about 1.4 times the RMS voltage. Thus, the actual peak voltage at a household electrical outlet is about 168 V.
The true RMS voltage is a bit tricky to calculate, since it involves some fairly complicated math. RMS is calculated by sampling the actual voltage in very small time increments. Then, the sample voltages are squared, the squares of the voltages are added up, and the average of all the squared values is calculated. Finally, the square root of the average is calculated. This is the actual RMS value.
For a true sine wave, the preceding calculation turns out to be very close to multiplying the peak voltage by 0.707. For AC voltages that aren't true sine waves, however, the actual RMS value can be different than the “multiply by 0.707” shortcut would indicate.
Nearly all AC voltmeters report the RMS voltage, but only more expensive AC voltmeters calculate the actual RMS by sampling the input voltage and doing the sum-of-the-squares thing. Inexpensive voltmeters simply measure the peak voltage and multiply it by 0.707. Fortunately, this is close enough for most purposes.