Enter any two of voltage, current, resistance or power. FigureDesk solves for the other two and shows the exact formula used.
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Ohm's law (V = I × R) and Joule's law (P = V × I) relate voltage, current, resistance and power. Any two of the four values fix the other two — this calculator picks the correct one of the twelve standard derivations automatically.
V = I × R
P = V × I
(combined: P = I² × R = V² / R)
Given a voltage of 12 V and a current of 2 A:
Ohm's law states that voltage equals current times resistance (V = I × R) for a resistive circuit. Combined with Joule's law (P = V × I), any two of voltage, current, resistance, and power determine the other two.
V, I, R, and P are related by two independent equations, so fixing any two of them mathematically determines the other two — there is no freedom left for the remaining values.
Only for purely resistive AC loads, where reactance is zero. For circuits with inductance or capacitance, the relationship between voltage and current also depends on phase angle, which this calculator does not model.
This calculator solves Ohm's and Joule's law for a single V/I/R/P relationship — it doesn't sum resistors for you. For resistors in series, add their resistances directly (R_total = R1 + R2 + ...) and enter that total. For resistors in parallel, first combine them (1/R_total = 1/R1 + 1/R2 + ...), then enter the combined resistance here to solve for current, voltage, or power.
Mixing units is the most common one — entering milliamps where the calculator expects amps, or kilohms where it expects ohms, silently produces a result that's off by a factor of 1,000. Convert to base units (volts, amps, ohms, watts) before entering a value, and sanity-check the result's order of magnitude against what you expect.
A faster path from watts to amps when you already know the system type and power factor.
A faster path from amps to watts when you already know the system type and power factor.
Once you know the current, look up conductor sizing properties for that circuit.
A direct application of Ohm's law: size a current-limiting resistor for an LED.
A direct application of Ohm's law: split a voltage across two resistors.
FigureDesk results are advisory and do not replace a licensed electrician, a qualified engineer, or the applicable local electrical code. Verify any electrical work against your local code and with a qualified professional before relying on it.