Skip to main content

RMS, Peak, and Average Values – Understanding AC Measurements

When you measure “120V AC” from a wall outlet, it may sound straightforward—but the voltage actually reaches about 170 volts at its highest point. So why do we call it 120V AC?

The answer lies in three different ways of measuring AC voltage:

  • Peak value
  • Average value
  • RMS value

Peak Voltage​

Peak voltage is the maximum instantaneous voltage reached during one cycle of the AC waveform.

For household AC in the United States:

  • Peak voltage ≈ 170V

This value occurs only briefly at the top of each sine wave and represents the maximum voltage stress on components.

Peak Voltage

Average Voltage​

Average voltage is found by averaging all voltage values over one complete cycle.

For a normal sine wave:

  • The positive half-cycle cancels the negative half-cycle
  • Average voltage over one full cycle = 0V

Because of this, average voltage is not useful for standard AC analysis. It is mainly used for rectified waveforms.

Average Voltage Cancels

RMS Voltage (Root Mean Square)​

RMS voltage represents the effective value of an AC voltage. It is the equivalent DC voltage that would produce the same heating effect in a resistive load.

RMS accounts for the fact that both positive and negative half-cycles deliver power.

RMS Heating Effect

RMS Formula for a Sine Wave​

RMS=Peak2≈Peak1.414\text{RMS} = \frac{\text{Peak}}{\sqrt{2}} \approx \frac{\text{Peak}}{1.414}

Example​

If the peak voltage is 170V:

RMS=1701.414≈120V\text{RMS} = \frac{170}{1.414} \approx 120V

This is why household power is rated as 120V AC.

Why RMS Matters​

  • Power calculations use RMS values
  • Multimeters display RMS voltage
  • Appliance and component ratings are based on RMS

Using peak values instead of RMS would significantly overestimate power.

Quick Reference (Sine Waves)​

  • RMS = 0.707 × Peak
  • Peak = 1.414 × RMS
  • Average = 0.637 × Peak (half-wave rectified only)

Peak RMS Average Comparison

Key Takeaway​

When AC voltage is specified, it almost always refers to RMS voltage unless stated otherwise. RMS is the standard used for real-world AC power and circuit design.