Ohm's Law Calculator
From the current and the resistance, get the voltage that drives the circuit and the power it dissipates — the two numbers behind every simple circuit.
Two inputs, two answers
Enter the current and resistance and the calculator returns the voltage (I × R) and the power (I² × R) at once.
Use SI units
Current in amperes (A), resistance in ohms (Ω) — then the voltage comes back in volts (V) and the power in watts (W).
What is Ohm's law?
Current and resistance in, voltage and power out
Ohm's law is the relationship that ties together the three core quantities of a simple electrical circuit: voltage, current, and resistance. It says the voltage across a component equals the current through it multiplied by its resistance, written V = I × R. Once you know the current and the resistance, the voltage is fixed — and so is the power the component turns into heat or work, P = I² × R. That makes current and resistance the two inputs you need for sizing LED resistors, checking wiring, picking battery packs, and almost any hands-on electronics task.
Enter the current in amperes and the resistance in ohms to get the voltage and power instantly.
Two short formulas, both built from the current (I) and the resistance (R).
voltage = I × RThe voltage is simply the current times the resistance (I × R). The power — the rate at which the component dissipates energy — is the current squared times the resistance (I² × R), which is the same as voltage times current (V × I). Because the current is squared in the power formula, the power grows much faster than the voltage as the current rises.
Suppose a current of 2 A flows through a resistance of 10 Ω.
Voltage
2 × 10 = 20 V — the voltage needed to drive that current.
Power (squared current)
2² × 10 = 4 × 10 = 40 W — the power dissipated by the resistance.
Cross-check
Power also equals V × I = 20 × 2 = 40 W — the same answer two ways.
The two outputs answer two different practical questions. The voltage (20 V for 2 A through 10 Ω) is the electrical "push" needed to force that current through the resistance — it tells you the supply or battery voltage your circuit needs. The power (40 W) is how fast the component converts electrical energy into heat or work — it tells you whether a resistor, wire, or trace can cope without overheating. The key insight is that voltage rises in step with current, but power rises with the square of it: for a fixed resistance, doubling the current from 2 A to 4 A doubles the voltage from 20 V to 40 V, yet quadruples the power from 40 W to 160 W. 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. Reach for the power figure whenever heat matters.
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. This tool also solves only the voltage and power from the current and resistance; to find the current or resistance instead, rearrange the formula (current = V ÷ R, resistance = V ÷ I). Keep the inputs in amperes and ohms so the answers come back in volts and watts.