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Ohm's Law Calculator: Voltage, Current, Resistance and Power
Fill in exactly two of the four boxes (voltage, current, resistance, power) and leave the other two empty. The calculator solves for the missing pair using Ohm's law and the power law and shows which formula it used.
Ohm's law and the power law
Ohm's law says that the current through a conductor is proportional to the voltage across it: V = I × R, with voltage in volts, current in amperes and resistance in ohms. The power law says that electrical power in watts is P = V × I. Substituting one into the other gives the useful forms P = I² × R and P = V² ÷ R.
Between them, the four quantities form a set in which any two determine the other two. That is why this calculator asks for exactly two inputs. The twelve formulas that appear on the well-known Ohm's law wheel are simply the six pairs written out in all their rearrangements.
Which formula for which pair
| You know | Resistance | Power |
|---|---|---|
| V and I | V ÷ I | V × I |
| V and R | (from V ÷ I) | V² ÷ R |
| I and R | (given) | I² × R |
| V and P | V² ÷ P | (given) |
| I and P | P ÷ I² | (given) |
| R and P | (given) | (given) |
For example, a 12 V supply across a 6 ohm load carries 2 A and dissipates 24 W. A load rated at 100 ohms and 4 W can only be fed with 20 V and 0.2 A before it reaches its rating.
Where the simple law stops working
- Non-linear parts. Diodes, LEDs and transistors do not have a constant resistance, so resistance computed from one operating point is valid only at that point.
- Temperature. Resistance of most metals rises with temperature. A lamp filament, for example, has a much lower resistance when cold than when lit.
- AC circuits. With inductors and capacitors, impedance and power factor replace plain resistance. For a purely resistive AC load the same formulas work with RMS values.
- Units. Convert milliamps and kilohms before entering them. 20 mA is 0.02 A, and 4.7 kΩ is 4,700 Ω.
Estimating and safety
This tool is for estimating and learning. Mains-voltage wiring, battery banks and any circuit that can deliver high current or heat can cause fires, shock and injury. Fuse and breaker ratings, wire sizes and installation methods are set by local electrical codes, and a qualified electrician or engineer must decide them for a real installation. Use the results to understand a circuit, check homework or choose components for low-voltage experiments, and always allow a safety margin when the answer is a component rating. A resistor dissipating 4 W calculated, for instance, should be a part rated well above that, because parts run hot close to their limit.
Worked example
A 12 V supply drives 2 A through a load. What are the resistance and the power?
Inputs
| Voltage (V) | 12 V |
|---|---|
| Current (I) | 2 A |
| Resistance (R) | Ω |
| Power (P) | W |
Result
R = 6.0000 Ω, P = 24.0000 W
| Voltage | 12.0000 V |
|---|---|
| Current | 2.0000 A |
| Resistance | 6.0000 Ω |
| Power | 24.0000 W |
| Formulas used | R = V ÷ I and P = V × I |
- The relationships hold for resistive loads in DC circuits and for the RMS values of AC circuits with resistive loads.
Estimating note: Quantities and ratings here are estimates for planning and learning. Products, local codes and site conditions differ, so check manufacturer data and local regulations, and use a qualified professional for electrical, structural or safety-critical work.
Frequently asked questions
What is Ohm's law?
Ohm's law states that voltage equals current multiplied by resistance, written V = I × R. Rearranged, current is voltage divided by resistance and resistance is voltage divided by current.
How do I find power from voltage and resistance?
Square the voltage and divide by the resistance, using P = V² ÷ R. A 12 volt supply across a 6 ohm load, for example, gives 144 divided by 6, or 24 watts.
Why do I have to leave two boxes empty?
Any two of voltage, current, resistance and power determine the other two. Entering three or four values could contradict each other, so the calculator asks for exactly two.
Does Ohm's law work for AC?
It works for resistive loads using RMS values. With motors, coils or capacitors, impedance and power factor also matter, so the simple formulas give only part of the picture.
What units does this calculator use?
Volts, amperes, ohms and watts. Convert milliamps, kilohms and kilowatts first, because mixing prefixes is the most common cause of wrong answers by a factor of a thousand.
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References and sources
- NIST: SI units for electrical quantities, the volt, ampere, ohm and watt. Link