Ohm's Law Calculator
Enter any two values
| Quantity | Formula Used |
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How the Solver Works
Ohm's Law states that voltage equals current multiplied by resistance: V = I × R. Combined with the power formula P = V × I, these two equations link four quantities — voltage (V), current (I), resistance (R), and power (P) — closely enough that any two of them fix the other two. This calculator fills in whichever two fields you leave blank by first deriving the two you entered into a matching pair, then applying the standard derived identities: R = V² / P, R = P / I², V = √(P × R), I = √(P / R), P = I² × R, and P = V² / R, alongside the core V = I × R relationship. Enter exactly two values and the other two populate automatically; the table above shows which formula produced each result.
A Common Mix-Up: Power Doesn't Need Both V and I
Many people assume you must know both voltage and current to find power, but any two of the four quantities are enough — for example, resistance and current alone give you power directly via P = I² × R, with no voltage figure required. This matters when a component is labeled with only a wattage and a resistance (common on resistors and heating elements), since you can still back out the current and voltage it will draw.
Related Tools
If you're identifying a resistor's resistance value from its color bands rather than solving a circuit equation, use the resistor color code calculator instead. For sizing wire and estimating losses over a length of cable once you know your circuit's current, see the voltage drop calculator.
Frequently Asked Questions
What if I only know resistance and power, not voltage or current?
That's fine — any two of the four quantities are enough. With resistance (R) and power (P) known, voltage is found from V = √(P × R) and current from I = √(P / R). You don't need to measure voltage or current directly to get a complete picture of the circuit.
Why does the calculator show an error for some input combinations?
A couple of the derived formulas involve division by a value that could be zero (for example I = V / R when resistance is 0, or R = P / I² when current is 0), or a square root of a negative number when resistance or power is entered as negative. In real circuits resistance and power are non-negative, so double-check your entered values if you see this message.