Ohm's Law Calculator
V = I × R. Enter any two and get the third, plus the power. And the part most calculators get wrong: on AC, volts ÷ amps is impedance, not resistance.
How we calculated this
What this page answers
Ohm's law ties three quantities together:
- V — volts, the push
- I — amps, the flow
- R — ohms, the resistance to that flow
Know any two and the third follows: V = I × R, I = V ÷ R, R = V ÷ I. Add power and you get P = V × I, P = I² × R, P = V² ÷ R.
The AC trap — this is the one that matters
On a DC circuit, volts ÷ amps is resistance. Straightforward.
On AC it is not. The result is impedance (Z): resistance plus the opposition from anything inductive or capacitive on the circuit, such as a motor winding or ballast. Resistance and impedance are different quantities and are not interchangeable.
The resistive part is roughly R = Z × power factor. A load pulling 10 A at 120 V with a power factor of 0.8 has an impedance of 12 Ω, but a resistive part nearer 9.6 Ω.
This calculator labels which quantity it is giving you every time. Many Ohm's law calculators simply print “resistance” and move on.
Three-phase: why this tool says no
Ask for ohms on a three-phase circuit and this calculator declines, on purpose. A single resistance figure depends on whether the load is wye or delta connected, and that changes the answer. This form does not ask, and many people wiring a motor do not know off the top of their head. Printing a number anyway would be a guess dressed up as a result.
Three-phase power — volts, amps, watts, and kVA — works normally in the Power Calculator.
Worked example
A 120 V circuit is pulling 10 A on DC. R = V ÷ I = 120 ÷ 10 = 12 Ω, and P = V × I = 1,200 W.
With the same numbers on single-phase AC at 0.8 power factor: Z = 120 ÷ 10 = 12 Ω, resistive part ≈ 12 × 0.8 = 9.6 Ω, and real power = 120 × 10 × 0.8 = 960 W. Same volts, same amps, different answers — that is the AC trap in one line.
What this doesn't do
Ohm's law describes a circuit; it does not size a conductor. Wire size depends on ampacity (heat) and voltage drop (distance), neither of which is in this equation. For those, use Wire Size, Ampacity Check and the Voltage Drop calculator.
Want to know why you should trust this result? Read how VoltDrop checks its numbers.
Frequently asked questions
- What is Ohm's law?
- Voltage equals current times resistance: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I. Know any two and you have the third.
- How do I calculate resistance from volts and amps?
- Divide volts by amps. 120 V at 10 A is 12 Ω on DC. On AC, that same division gives impedance, not resistance.
- Is Ohm's law the same for AC?
- The relationship holds, but the ohms are impedance rather than plain resistance. Impedance includes inductive and capacitive opposition, not just resistance. The resistive part is roughly impedance × power factor.
- How do I find watts from ohms?
- Use P = I² × R if you know current, or P = V² ÷ R if you know voltage.
- Why won't this calculate ohms for a three-phase circuit?
- Because the answer depends on whether the load is wye or delta connected, and this form does not ask. Rather than guess, it declines. Three-phase power calculations work normally.
- Does Ohm's law tell me what size wire to use?
- No. Wire size comes from ampacity and voltage drop over distance. Ohm's law does not consider either.