How this calculator works
An ohmic resistor has a voltage drop proportional to current at a fixed temperature. Combining that proportionality with electrical power gives a complete set of relationships among four quantities. Two independent positive values define the operating point. For a pure resistor, the same relations hold for AC RMS voltage and current; reactive loads need impedance and power factor.
Formula
V = I R; P = V I = I² R = V²/R.
Worked example
A 12 V drop at 2 A means 6 ohms and 24 W. A 4 ohm resistor dissipating 16 W carries 2 A and has an 8 V drop.
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Frequently asked questions
Why require two independent values?
The four displayed quantities are linked by two equations. Two values determine the other two; entering more can create contradictory data.
Does this work for an LED?
An LED is nonlinear. Use its specified forward voltage at the intended current and calculate its series resistor separately.
Are these signed voltages?
This page uses magnitudes for a passive load. Signs depend on the chosen reference direction.