Ohm's Law and Power Calculator
Enter any two of voltage, current, resistance and power to get the other two.
Formulas
V = I·R and P = V·I = I²R = V²/R. Knowing the power a resistor dissipates tells you which rating to buy (1/4 W, 1/2 W and so on). A common rule is to allow at least twice the calculated power.
Frequently asked questions
How do I choose a resistor's power rating?
Pick a rating at least twice the power the resistor dissipates. If it dissipates 0.1 W, a 1/4 W (0.25 W) resistor is a good choice.
Are there parts that don't follow Ohm's law?
Yes. Diodes, LEDs and transistors are non-linear, so current is not proportional to voltage and V = IR does not apply directly. Use the voltage-current curve in the datasheet instead.