Ohm's Law Calculator
Ohm's law ties voltage, current and resistance together, and the power formula adds watts. Give this any two of the four and it derives the rest — which is exactly how the law is used in practice, because you can almost never measure all four.
Fill In Any Two
Leave the other two blank. Exactly two values are needed — no more, no less.
The two formulas underneath
Ohm's law states that current through a conductor is proportional to the voltage across it and inversely proportional to its resistance: V = I × R. The power formula is separate and older: P = V × I. Combine them by substitution and you get the familiar variants — P = I²R and P = V²/R — which is why any two of the four quantities fix the other two. That is the whole trick behind this calculator, and behind the Ohm's law wheel printed in the back of electronics textbooks.
Why exactly two values
One value is not enough: a 12 V supply says nothing about current until something is connected. Three or four is over-determined — if you enter values that contradict each other there is no correct answer, only a choice of which input to ignore. Silently picking one would hide your mistake, so this tool asks for exactly two and tells you when you have given it more. If your four measured values disagree, that disagreement is the finding: either a meter is wrong or the circuit is not what you think it is.
Where the law stops working
Ohm's law describes ohmic materials — metals and carbon resistors at steady temperature. Diodes, transistors, fluorescent lamps and most semiconductors are non-ohmic: their resistance changes with the voltage applied, so a single R does not describe them. Incandescent bulbs are a classic trap, because a cold filament has a fraction of its hot resistance, which is why they draw a large inrush current at switch-on. Batteries have internal resistance, so terminal voltage sags under load.
AC is a different question
Everything above is direct current. In alternating-current circuits, capacitance and inductance add reactance, and the quantity that opposes current is impedance (Z), not plain resistance. Voltage and current can also fall out of step, so real power in watts is no longer simply volts times amps — it is volts times amps times the power factor. For a purely resistive AC load such as a heating element, the power factor is 1 and these formulas hold with RMS values. For motors, transformers and switch-mode supplies it does not, and using the DC formula will overstate the real power consumed.
Reading the energy line
Power is a rate; energy is power multiplied by time. The last output line converts watts into the kilowatt-hours you would be billed for if the load ran continuously for an hour, which is often the number you actually care about. A 60 W lamp left on all day is 1.44 kWh; multiply by your tariff to price it.
Frequently Asked Questions
Can I enter all four values?
Why does my bulb draw more current than this predicts?
Does this work for AC circuits?
What is the kWh line for?
Sources
Official publications only. Links open the original document in a new tab.
- National Institute of Standards and Technology The International System of Units (SI) Definitions of the ampere, volt and ohm
- National Institute of Standards and Technology Special Publication 811 — Guide for the use of the SI Correct use of unit symbols and prefixes