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Resistor Power Calculator

Enter the resistance and either voltage or current to find the power dissipated.

How it is calculated

Resistance R is entered in ohms and must be above zero. In the default voltage mode you give the voltage V across the resistor in volts, and the tool computes P = V² ÷ R and I = V ÷ R. In current mode you give the current I in amperes, and it computes P = I² × R and V = I × R. Power comes out in watts, voltage in volts and current in amperes, each to four decimals. The result is the heat the resistor must dissipate. Compare it with the resistor's rated wattage, leaving a margin, since a part running at its full rating gets very hot. The tool needs the voltage across the resistor, not the supply voltage, when other parts share the circuit.

Worked example

A 100 Ω resistor has 12 V across it. Current: 12 ÷ 100 = 0.1200 A. Power: 12² ÷ 100 = 144 ÷ 100 = 1.4400 W. A common quarter-watt part would be heavily overloaded, and even a 1 W part is below the load, so you would need a resistor rated at 2 W or more. In current mode, 100 Ω carrying 0.12 A gives the same: 0.12² × 100 = 1.44 W.

Try this example →

Common mistakes

  • Entering the supply voltage when the resistor is in series with an LED or another part. Use only the voltage across the resistor itself, otherwise the power is overstated.
  • Giving the resistance in kilohms or milliamps in amperes. Enter 4.7 kΩ as 4700 and 20 mA as 0.02.
  • Choosing a resistor whose rating just equals the result. Running continuously at the full rating is hot; leave headroom.
  • Forgetting the mode. If you fill in the current field but leave the voltage mode selected, the tool has no voltage and returns nothing.
  • Squaring the wrong quantity: power grows with the square of voltage or current, so doubling the voltage on the same resistor quadruples the heat.

About this calculator

Power dissipated in a resistor follows Joule's law, calculated either from voltage as P = V² / R or from current as P = I² × R — both give the same wattage for a given resistance, since V = IR links them. Only voltage or only current is needed alongside the resistance value; the calculator fills in whichever formula applies.

Electronics engineers and hobbyists check this before finishing a circuit to make sure a resistor's power rating (commonly 1/8W, 1/4W, 1/2W, or 1W for through-hole types) is high enough for the current it will actually carry — undersizing it causes the resistor to overheat, discolor, or fail outright.

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Comments (1)

  • Editorial team

    Two identical resistors in series share the heat equally, so each one only has to survive half of the total power. Work out the total for the combined resistance, divide by two, and compare with each part's rating. That is one way to build a higher-wattage resistor from small parts. What are you powering, an LED, a shunt or a load resistor, and how did you choose the rating?

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