Ohm Law Calculator: Voltage, Current, Resistance and Power
Ohm Law Calculator: V, I, R and P
Enter any two of voltage, current, resistance and power, and the other two follow. The tool uses the last two boxes you typed in as the knowns, so changing your mind is just typing somewhere else.
The twelve formulas are all rearrangements of two facts: V equals I R, and P equals V I. Everything else, P equals I squared R, R equals V squared over P, follows by substitution, which is why knowing any two quantities pins down the other two.
How to use this tool
Type values into any two of the four boxes and the other two fill themselves in. The tool treats the last two boxes you touched as the knowns, so switching from a voltage and current problem to a power and resistance one is just typing in different boxes, no mode switch required.
The result line always states which two quantities it took as given, so there is never a question about which direction the calculation ran.
Two equations pretending to be twelve
Every Ohm law wheel you have ever seen, with its twelve formulas around the rim, is two facts in disguise: V equals I R, and P equals V I. Every other entry, P equals I squared R, R equals V squared over P, I equals the root of P over R, is one substitution away. Knowing that is more useful than memorising the wheel, because it means you can rebuild any formula at the panel with ten seconds of algebra.
It also explains why exactly two inputs are needed. Two equations, four unknowns: fix any two quantities and the system closes. Fix only one and the problem is underdetermined, which the tool tells you rather than guessing.
A worked check you can repeat on paper
A 12 V circuit drawing 2 A. Resistance is 12 over 2, which is 6 ohms, and power is 12 times 2, which is 24 W. Now come at the same circuit from the other side: given 24 W dissipated in 6 ohms, the current is the root of 24 over 6, exactly 2 A, and the voltage follows as 12. Every path through the wheel lands on the same four numbers, which is precisely how this tool was verified: all six possible input pairs, checked against the same known circuit.
Mistakes that catch people out
Mixing units without noticing. Milliamps with volts gives milliwatts, not watts, and kilohms flip it again. Fix: convert to base units before entering, or carry the prefixes deliberately.
Using the supply voltage across one component. In a series string, each resistor sees its share of the supply, not all of it. Fix: V in these formulas is the voltage across the element in question.
Applying DC resistance thinking to AC loads. Motors and anything with inductance draw current out of phase with voltage, and V times I gives apparent power, not real power. Fix: for AC machine loads, the power factor matters, which is the three phase tool next door.
Trusting a resistor label over a measurement. A 5 percent part can sit anywhere in a 10 percent wide band. Fix: for anything sensitive, measure.
How this was checked
The solver was verified on a single reference circuit, 12 V, 2 A, 6 ohms, 24 W, entered through all six possible input pairs. Each pair reproduced the other two quantities exactly, confirming the internal substitutions are consistent rather than merely plausible.
Related Calculators and Reference
The reason conductor size matters to these numbers is resistance over distance, which the Voltage Drop Calculator quantifies. AC power with a power factor lives in the Three Phase Power Calculator, and the watts computed here convert to other energy units in the Energy Unit Converter. For AC loads, where power factor and three phase change the answer, the Watts to Amps Calculator does the same arithmetic and adds the breaker size a continuous load needs. For the parts around these numbers, the Resistor Color Code Calculator reads a resistor from its bands, the LED Resistor Calculator sizes the series resistor for an LED, the Voltage Divider Calculator gives the output of a two resistor divider, and the Capacitor Calculator handles series and parallel capacitance and RC time constants. For alternating current, the Complex Number Calculator turns resistance into impedance with a phase angle.