RMS to Watts Calculator
Find power from RMS voltage and resistance.
For a resistive load (like a heater), AC power is the same formula as DC.
How the Math Works
The RMS to Watts Calculator uses the fundamental relationship between power, voltage, and resistance in electrical circuits. Root Mean Square (RMS) voltage represents the effective value of a varying electrical signal, equivalent to the DC voltage that would produce the same power dissipation. The formula W = V² ÷ R derives from Ohm's Law and the power equation, where squaring the RMS voltage and dividing by the resistance yields power in watts. This calculation assumes a purely resistive load with no reactive components.
Practical Applications
To use this calculator, enter the measured RMS voltage across your device and the device's resistance rating. For example, if you have an AC power source measuring 120V RMS and your appliance has a resistance of 60 ohms, the calculation yields 240 watts (120² ÷ 60). This is particularly useful for determining the power consumption of heating elements, incandescent light bulbs, or any purely resistive load where you need to calculate energy usage from electrical measurements.
Day-to-Day Use
This calculation helps you understand energy consumption and costs in everyday situations. When selecting an appropriate fuse, circuit breaker, or wire gauge, knowing the actual power dissipation prevents overheating and potential fire hazards. It also enables accurate estimation of electricity costs by calculating how much power your appliances use, and helps verify whether a device operates within safe electrical limits. Whether you're installing new equipment, troubleshooting electrical issues, or simply budgeting for energy expenses, this tool provides essential information for safe and efficient electrical usage.
Worked example
120 V, 10 Ω → 1,440 W.
FAQ
AC or DC?
Both — RMS voltage is the equivalent DC value for power.