You don’t need a math degree—just a simple formula. For a standard AC sine wave (like your home outlet), the RMS voltage is peak voltage divided by the square root of 2. Square root of 2 is about 1.414, so it’s easy: RMS = Peak / 1.414.
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Let’s do a quick example from your kitchen. If you know your outlet’s peak voltage is 170 volts, divide that by 1.414. You get very close to 120 volts RMS—the number on your toaster’s tag. See? It’s just a friendly adjustment.
For a more exact method (like if you’re tinkering with a multimeter), you can measure several equally spaced points on the wave, square each one, average those squares, then take the square root. That’s the “Root Mean Square” process—square, average, square root—but honestly, most people just use the peak-to-RMS shortcut.
