How to Calculate Watts or Wattage

Written by martin adamovic
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How to Calculate Watts or Wattage
Calculating wattage is simple, (electricity image by Mat Hayward from

Calculating watts or wattage is easiest by using the formulas established by Ohm's law. Ohm's law explains the relationship that exists between current, voltage, and resistance and outlines a series of formulas that allow you to easily calculate wattage using a combination of the voltage, resistance, and current values of the device or circuit. Even if you have never worked with electricity, you can quickly calculate the wattage of a device if you can do some very simple arithmetic as outlined by Ohm's formulas.

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  1. 1

    Determine the voltage, current, and resistance for the device. These values are measured in volts, amps, and Ohms, respectively. You can find these values on the label itself or in the manual that comes with the device. In most cases you will only be able to determine two out of the three values. Depending on the two values you have you can calculate watts using one of the following methods.

  2. 2

    Calculate watts using volts and amps by multiplying the two values together. The product of the voltage and the current is power in watts.

  3. 3

    Calculate watts using Ohms (resistance) and amps (current), by multiplying the square of the amps value by Ohms. Simply multiply amps by itself and then multiply that value by Ohms.

  4. 4

    Calculate watts using ohms (resistance) and voltage (volts) by dividing volts by ohms to calculate amps (current). Multiply amps by volts to calculate watts.

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