Ohm's Law Calculator
Solve for voltage, current, resistance, or power in a simple DC circuit using Ohm's Law.
The formulas
Ohm's Law: V = I × R (voltage = current × resistance). Power: P = V × I (power = voltage × current).
Worked example
A 12V supply across a 4Ω resistor draws a current of 12 ÷ 4 = 3A, and dissipates power of 12 × 3 = 36W.
How the three modes work
- Pick what to solve for and the other two boxes appear. Voltage = current × resistance, current = voltage ÷ resistance, and resistance = voltage ÷ current.
- Power is worked out from the final voltage and current (P = V × I) and shown beneath every answer.
- Inputs are volts, amps and ohms, and results are rounded to three decimals with the unit attached (V, A, Ω or W).
- Dividing by zero is refused: a resistance of 0 in current mode gives "Resistance can't be zero", and a current of 0 in resistance mode gives "Current can't be zero".
- Negative numbers are not rejected. −12 V across 4 Ω returns −3.000 A and 36.000 W, because a negative current times a negative voltage is a positive power.
More worked examples
Mains-style load: 230 V across 46 Ω gives a current of 5.000 A and a power of 1150.000 W. Small signal: 0.02 A through 470 Ω gives 9.400 V and 0.188 W. Unknown resistor: 9 V with 0.015 A flowing gives 600.000 Ω and 0.135 W.
There is no unit drop-down, so convert before typing. 20 mA is entered as 0.02, and 4.7 kΩ as 4700. Results this small can also be rounded away: 4 V across 10,000 Ω is really 0.0004 A, which the three-decimal display shows as 0.000 A. If that happens, solve with larger numbers or scale the problem in your head.
What it does not cover
It handles one resistance at a time. For resistors in series or parallel, combine them into a single equivalent value first. Wire gauge, temperature effects and anything involving AC impedance are outside the calculation.
Frequently asked questions
Does this work for AC circuits?
This calculator uses the basic DC form of Ohm's Law. AC circuits with reactance (from capacitors or inductors) need impedance instead of plain resistance, which this tool doesn't model.
Why is power always shown?
Once voltage and current are known (directly or derived), power follows immediately from P = V × I, so it's included alongside every result at no extra input cost.
How do I enter milliamps or kilohms?
Convert to the base unit first. Milliamps are divided by 1,000 (35 mA becomes 0.035) and kilohms are multiplied by 1,000 (2.2 kΩ becomes 2200).
Why does a very small current show as 0.000 A?
Answers are rounded to three decimal places. Anything below 0.0005 A rounds to zero on screen even though the underlying value is not zero.
Can the power result be negative?
Power is voltage times current, so it only goes negative if exactly one of those is negative. With two negatives, or two positives, it is positive.