How to use it
Type any two of the four values: voltage (V, in volts), current (I, in amperes), resistance (R, in ohms) and power (P, in watts). The tool uses the two you edited most recently and fills in the other two, shown in green. Results use engineering units, so 0.02 A is shown as 20 mA and 4,700 ohms as 4.7 kΩ. The lower panel adds up resistors: typing several values gives the total when they are wired in series (one after another) or in parallel (side by side).
The formulas
Ohm’s law says that the voltage across a resistor equals the current through it times its resistance: V = I × R. Electrical power is voltage times current: P = V × I, which can also be written P = I² × R or P = V² / R. Every pair of the four values can be turned into the other two with these. For example, 12 V across a 4 ohm resistor gives 3 A and 36 W. Resistors in series add: R = R1 + R2 + …. In parallel, the reciprocals add: 1/R = 1/R1 + 1/R2 + …, so two equal resistors in parallel make half the resistance.
Questions
Does it work for AC? It works for the RMS values of simple resistive loads such as heaters and kettles. Motors, coils and capacitors need more than Ohm’s law. What size resistor does an LED need? Subtract the LED’s own voltage from the supply, then divide by the LED current; see the LED example. This tool does not give safety advice: always follow the ratings on your components. To read a resistor’s color bands, use the Resistor Color Code Calculator.