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There are three ways to measure voltage in an AC circuit:

  • Peak voltage: A
    measurement of the
    largest voltage.

  • Peak-to-peak voltage: The difference between the highest and lowest peaks of the voltage reading.

  • RMS voltage: The average voltage. 
    RMS stands for root mean square. RMS voltage is the most common way to calculate the voltage of an AC circuit.

DCAClab calculates RMS by sampling the input voltage and doing the sum-of-the-squares, simulating the high quality voltmeters.

Notice the dashed orange line in the oscolicope, its the RMS value of voltage. A good article that explains the math behind RMS can be found here.