When should I use P = V^2/R versus P = I^2*R versus P = V*I?

Use whichever matches the two quantities you know. If you know V and R, use V^2/R. If you know I and R, use I^2*R. If you know V and I, use V*I. All three are equivalent by Ohm's law.

What does the result represent physically?

P in watts is the rate at which electrical energy is converted to heat in the resistor. A 10 Ohm resistor with 10 V across it dissipates P = 100/10 = 10 W continuously.

How do I use this to choose the correct resistor power rating?

Calculate P = V^2/R for your circuit conditions, then select a resistor rated for at least twice that power to provide a safety margin. For 1 W calculated dissipation, use a 2 W rated resistor.

Ohmic Power Calculator

Determine electrical power dissipated by a resistor using P = V²/R. Enter the voltage across and resistance of the component to find power in watts for circuit design and safety analysis.

By AbraCalc Editorial Team · Reviewed by AbraCalc Methodology Review · Last reviewed: 2026-06-25

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How to use this tool

Enter voltage (v) and resistance (r) in the fields above.
Results update instantly as you type — or click Calculate.
Read your power and the full breakdown beneath it.

Formula

P = V² / R

How it works

Square the voltage and divide by the resistance to obtain power dissipated as heat.

Worked example

Common mistakes to avoid

Using current I instead of voltage V — this formula P = V^2/R is for voltage and resistance; if you have current, use P = I^2*R instead.
Mixing up formulas P = V^2/R and P = V/R — the second is not a real power formula; power always involves voltage squared or current squared.
Entering resistance in kilohms without converting — 1 kOhm = 1000 Ohm; using kOhm directly understates power by 1000.

Key terms

Frequently asked questions

When should I use P = V^2/R versus P = I^2*R versus P = V*I?: Use whichever matches the two quantities you know. If you know V and R, use V^2/R. If you know I and R, use I^2*R. If you know V and I, use V*I. All three are equivalent by Ohm's law.
What does the result represent physically?: P in watts is the rate at which electrical energy is converted to heat in the resistor. A 10 Ohm resistor with 10 V across it dissipates P = 100/10 = 10 W continuously.
How do I use this to choose the correct resistor power rating?: Calculate P = V^2/R for your circuit conditions, then select a resistor rated for at least twice that power to provide a safety margin. For 1 W calculated dissipation, use a 2 W rated resistor.