Ohm’s Law Calculator — V = IR with Power
Ohm’s law links voltage, current and resistance: V = IR. Add the power relation P = VI and any two known values determine the other two. Enter whichever pair your problem gives you and this calculator completes the set.
These four quantities cover almost every basic circuit question — from sizing a resistor for an LED to checking the current drawn by a household appliance.
Use the calculator
Enter any TWO values — the calculator solves the other two.
The formula
V = IR and P = VI. Combining them also gives P = I²R and P = V²/R, which are handy when current or voltage is the quantity you do not know.
Worked example
Worked example: A 12 V battery is connected across a 4 Ω resistor. Current I = V/R = 12/4 = 3 A. Power dissipated P = VI = 12 × 3 = 36 W.
Frequently asked questions
What is Ohm's law in words?
The current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance, provided temperature stays constant: I = V/R.
Does Ohm's law apply to every component?
No. It applies to ohmic conductors (most metals at constant temperature). Filament lamps, diodes and thermistors are non-ohmic — their resistance changes, so V = IR still defines resistance at a point but it is not constant.
How do I calculate power if I only know voltage and resistance?
Use P = V²/R, which comes from substituting I = V/R into P = VI. This calculator does it automatically when you enter V and R.