Ohm's Law Calculator
Pick the two quantities you know — voltage, current, resistance, or power — and the calculator solves for the other two. Every rearrangement of V = I × R and P = V × I in one place.
Any two, the other two
Choose which pair you know, enter them, and the calculator returns the remaining two quantities at once.
Use SI units
Voltage in volts (V), current in amperes (A), resistance in ohms (Ω), power in watts (W) — keep the inputs coherent and the answers come back coherent.
What is Ohm's law?
Any two of four quantities give you the rest
Ohm's law is the relationship that ties together the core quantities of a simple electrical circuit: voltage, current, and resistance, with electrical power following directly from them. It says the voltage across a component equals the current through it times its resistance, written V = I × R, while the power is P = V × I. Knowing any two of the four quantities — voltage (V), current (I), resistance (R), and power (P) — fixes the other two. This calculator implements the full "Ohm's law wheel": pick the pair you have, and it rearranges the formulas to solve for everything else.
Select which two values you know, enter them, and get the remaining two quantities instantly.
Two relationships generate all the rearrangements: voltage equals current times resistance (V = I × R), and power equals voltage times current (P = V × I). From whichever pair you enter, the calculator solves for the rest.
V = I × R P = V × I R = V ÷ I I = V ÷ R V = √(P × R)Each of the six pairs has its own pair of formulas. From voltage and current: R = V ÷ I and P = V × I. From current and resistance: V = I × R and P = I² × R. From resistance and power: V = √(P × R) and I = √(P ÷ R). Because power involves the square of current (or voltage), it grows much faster than the linear quantities as the circuit's drive rises.
Suppose you know the supply voltage is 12 V and the circuit draws a current of 2 A, and you want the resistance and the power.
Resistance
R = V ÷ I = 12 ÷ 2 = 6 Ω — the resistance the components present.
Power
P = V × I = 12 × 2 = 24 W — the power the circuit dissipates.
Cross-check
Power also equals I² × R = 2² × 6 = 4 × 6 = 24 W — the same answer two ways.
The two computed quantities answer the questions your known pair left open. The voltage is the electrical "push" driving the circuit; the current is how much charge actually flows; the resistance is how strongly the components oppose that flow; and the power is how fast electrical energy turns into heat or work. The key insight is that voltage and current scale linearly with one another at fixed resistance, but power scales with the square of either: for a fixed resistance, doubling the current doubles the voltage yet quadruples the power. That is exactly why an undersized resistor can survive a small current but burn out when the current climbs, and why wiring is rated by the current it must carry. Whenever heat matters, read the power figure first.
The formulas are exact for ohmic components, but a couple of practical points are worth keeping in mind.
Ohmic components and SI units
Ohm's law holds for components whose resistance is constant — resistors, wires, heating elements. Diodes, LEDs, and transistors are non-ohmic, so the current does not simply track the voltage and these formulas only approximate them. Some pairs need a non-zero divisor: solving for resistance from voltage and current requires a current above zero, and solving from power requires a non-zero quantity to divide by, so those combinations return no result. Keep the inputs in volts, amperes, ohms, and watts so the answers stay coherent.