Ohm's Law Calculator
The foundation of electronics: enter any two of voltage, current, resistance and power, and the calculator finds the other two using Ohm's law (V = I × R) and the power law (P = V × I).
How to use this calculator
- Enter the two values you know — voltage, current, resistance or power.
- Leave the two you want to find at 0.
- Press Solve — the calculator computes the missing two.
Formula used
Ohm's law (V = I × R) links voltage, current and resistance; the power law (P = V × I) adds power. Together, knowing any two of the four determines the other two — the reason this single relationship underpins nearly all basic circuit work.
Example calculation
12 V across a circuit drawing 2 A: resistance = 12 ÷ 2 = 6 Ω, power = 12 × 2 = 24 W.
Every value is linked: double the voltage at fixed resistance and current doubles too, while power quadruples (since P = V²/R).
The relationship behind every circuit
Ohm's law is the single most useful equation in electronics: voltage equals current times resistance. It captures an intuitive idea — for a given resistance, more voltage pushes more current — and combines with the power law (power = voltage × current) to describe how energy flows and dissipates in a circuit.
The practical power of the "any two gives all four" property is that real problems rarely hand you the values you want directly. You might know a supply voltage and a component's power rating and need the current it'll draw, or a resistance and desired current and need the voltage. The calculator's twelve-way solving covers every such combination, and the alternate power forms (I²R for heat in a resistor, V²/R for a known voltage) are the ones that catch people out.
Why use this calculator?
- Solve any circuit-basics problem from just two known values.
- Get power automatically — the value that determines component ratings.
- See all four power formulas for choosing the right one.
Frequently asked questions
What is Ohm's law?
V = I × R: voltage equals current times resistance. Rearranged, current = voltage ÷ resistance, and resistance = voltage ÷ current. It describes how much current a voltage drives through a given resistance.
How do I calculate power in a circuit?
Power (watts) = voltage × current. If you don't know both, use P = I²R (current and resistance) or P = V²/R (voltage and resistance). All three give the same answer for a given circuit.
Why does power matter for choosing components?
Components dissipate power as heat, and every resistor has a wattage rating. Exceed it and the part overheats or fails. Calculating P = I²R tells you the minimum wattage rating a resistor needs for its position in the circuit.