Choose the two values you know, enter them with any unit prefix, and the calculator solves the other two using Ohm's law and the power equations.
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Ohm's law states that voltage equals current times resistance: V = I × R. Combining it with the power formula P = V × I gives equations that relate all four quantities, so any two known values determine the other two. For example, P = V² / R, P = I² × R, I = √(P / R) and V = √(P × R). The calculator picks the right pair of equations for the values you know and converts all prefixes to base units first, so you can mix milliamps and kilohms freely.
A 12 V supply across a 470 Ω resistor draws 25.5 mA and dissipates 0.306 W. That needs at least a 1/2 W resistor for a safety margin.
Pick a resistor rated for at least twice the power the calculator shows, so it runs cool and lasts.
V = I × R. Voltage (volts) equals current (amps) times resistance (ohms).
P = V × I. Using Ohm's law it can also be written as P = I² × R or P = V² / R.
Yes. Each value has its own prefix selector, and everything is converted to base units before solving.
For purely resistive AC loads, use RMS voltage and current. Circuits with capacitors or inductors need impedance and power factor, which this calculator doesn't model.
A dash means the result is undefined for your inputs, such as a zero resistance when dividing by it.
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