Ohm's Law Calculator — solve V, I, R and P
Enter any two of voltage, current, resistance or power and get the other two, with the worked formula and a matching SPICE netlist.
Example: At 12 V and 20 mA, resistance and power are 600 Ω and 240 mW.
Formula
V = I × R P = V × IWorked example
At 12 V and 20 mA, resistance and power are 600 Ω and 240 mW.
R = V / I
R = 12 V / 20 mA
R = 600 Ω
P = V × I
P = 12 V × 20 mA
P = 240 mW
Frequently asked questions
What is Ohm's law?
Ohm's law states that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance: V = I × R. Combined with P = V × I, any two of voltage, current, resistance and power determine the other two.
How do I calculate watts from volts and amps?
Multiply them: P = V × I. 12 V at 2 A is 24 W. If you have resistance instead of one of those, use P = V² / R or P = I² × R.
Why does the calculator ask for exactly two values?
Two quantities are enough to fix the other two. Supplying three over-constrains the problem, and if they disagree even slightly there is no single correct answer to return.
What resistor power rating do I need?
Take the calculated power and at least double it. A resistor dissipating 0.2 W should be specified as 0.5 W. Running a part near its rating makes it run hot, drift out of tolerance, and age fast.
Does Ohm's law work for AC circuits?
For purely resistive loads, yes, using RMS values. Once capacitance or inductance is involved you need impedance rather than plain resistance, and the current lags or leads the voltage.
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