You enter any two of the four quantities — voltage, current, resistance, or power — and the tool derives the other two using Ohm's Law (V = IR) and the power equation (P = VI). It identifies which two values you've provided, applies the appropriate rearrangement of the formulas, and displays all four. It assumes a simple DC resistive circuit with no reactance, capacitance, or inductance.
The basic V = IR relationship applies to AC circuits with purely resistive loads (heaters, incandescent bulbs). For circuits with capacitors or inductors, you need impedance (Z) instead of simple resistance, which accounts for phase differences between voltage and current.
Ohm's Law and the power equation give you two independent relationships among four variables. Two equations with two unknowns are fully solvable, so any pair of known values determines the other two.
The calculator uses the first two non-zero values it finds (in the order voltage, current, resistance, power) and derives the rest. If the extra values you entered are inconsistent, the calculated results will override them.
If you know the voltage and the current a device needs, Ohm's Law tells you the resistance to use. The power result tells you the wattage rating the resistor needs to handle without overheating — always choose a resistor rated above the calculated power.
This calculator solves for a single resistive element. For series circuits, add resistances (R_total = R1 + R2). For parallel, use 1/R_total = 1/R1 + 1/R2. Apply Ohm's Law to the total resistance and the source voltage to find current.
Estimate only. Results reflect your inputs and standard formulas. Double-check important decisions independently.