Welcome to our watt calculator, where you can calculate the electrical power using the Watt's law and Ohm's law. If you're curious about the relation between power, voltage, current, and resistance, or you're wondering how to calculate watts, you've come to the right place! Keep reading to learn the basics of electrical power, including calculating watts using the watts formula.

Electric power and Watt's law: Calculating watts from volts and amps

Electric power is the rate of electric energy carried in a circuit. Devices like electric generators produce electric power, while appliances (loads) like electric motors and light bulbs consume it to produce light, heat, or mechanical work.

Watt's law connects electric power to the voltage and current in a circuit:

P=VIP = V \cdot I


  • PP - Electric power;
  • VV - Voltage, the electric potential difference between the two ends of a circuit; and
  • II - Current, the rate of flow of electric charge through the circuit.

The SI unit of power is Watt (W)(\text{W}), voltage is volts (V)(\text{V}), and current is amperes or amps (A)(\text{A}). Thus the power calculation involves calculating watts from volts and amps using the watts formula.

The term wattage refers to power measured in watts. One watt (1 W)(1\text{ W})is the electric power transferred by a voltage of one volt (1 V)(1 \text{ V}) through the flow of one-ampere (1 A)(1 \text{ A}) current.

Ohm's law and electric power

Ohm's law established a relationship between the voltage and current in a circuit by stating that they're directly proportional to each other:

VIV=IRV \propto I\\ V = I \cdot R

Where RR, the proportionality constant, is the resistance to the flow of current in a circuit. We measure resistance in ohms (Ω)(\Omega).

Combining this relation to Watt's equation, we get two new equations for electric power:

P=I2RP=V2RP = I^2 \cdot R \\[1em] P = \frac{V^2}{R}

Now you can find power from:

  • Voltage and current using Watt's law;
  • Voltage and resistance by combining Watt's equation and Ohm's law; or
  • Current and resistance by combining Watt's formula and Ohm's law.

To calculate power transferred through multiple resistors, head to our resistor wattage calculator.

How to calculate watts

Let's learn how to find watts with some examples:

  • A current of 0.5 A0.5 \text{ A} passes through a light bulb. The domestic voltage supply in the USA is 120 V120 \text{ V}. Let's use Watt's law to find the wattage of the light bulb:
P=VI=120 V0.5 AP=60 W\qquad \begin{align*} P &= V \cdot I\\ &= 120 \text{ V} \cdot 0.5 \text{ A}\\ P &= 60 \text{ W} \end{align*}
  • A voltage of 220 V220 \text{ V} drives current through a 100 Ω100 \text{ }\Omega resistor. The electric power in the circuit is:
P=V2R=2202100P=484 W\qquad \begin{align*} P &= \frac{V^2}{R}\\[1em] &= \frac{220^2}{100}\\[1em] P &= 484 \text{ W} \end{align*}

How to find watts using this watt calculator

This watt calculator is easy to use:

  • To calculate watts from volts and amps, enter the value of voltage and current.
  • Enter the values of voltage and resistance to calculate wattage from volts and ohms.
  • Input the current and resistance values to find electric power from amps and ohms.

This watt calculator is versatile enough for you to enter any two of the four parameters to find the remaining values! Use it to find volts or amps from the power consumed in watts!

Krishna Nelaturu
Resistance (R)
Voltage (V)
Current (I)
Power (P)
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