Ohm's Law and Circuits

About this tool

This circuit calculator has two parts. The Ohm's law tab takes any two of voltage, current, resistance and power and finds the other two, with the formula and the working. The Resistors tab adds up a list of resistors in series and in parallel. Unit prefixes such as mA, kΩ and kW are built in, so you can type values as they appear in the question.

Tips

• In series, resistances add: 2 Ω + 3 Ω = 5 Ω. In parallel, the reciprocals add: 1/R = 1/6 + 1/3 = 1/2, so 6 Ω and 3 Ω in parallel give 2 Ω.
• The total resistance in parallel is always smaller than the smallest single resistor. If your answer is bigger, check the last step: you still need to flip 1/R.
• With 12 V across 4 Ω, the current is 3 A and the power is 36 W. VI, I²R and V²/R all give the same 36 W, which makes a quick check.

FAQ

Which two values do I need?

Any two of V, I, R and P. Every pair is enough to find the other two, because the formulas link all four. The working shows which formula was used for each answer.

Which unit prefixes can I use?

Voltage in mV, V or kV, current in µA, mA or A, resistance in Ω, kΩ or MΩ, and power in mW, W or kW. The working converts everything to base units first.

How is the current found in the Resistors tab?

The calculator divides the voltage you enter by the total resistance of the combination, using I = V / R.