⚡Ohm's Law Calculator — Calculate voltage, current, resistance, and power (V=IR)

The Ohm's Law Calculator solves for any one of the four fundamental electrical quantities — voltage (V), current (I), resistance (R), and power (P) — when the other two are known. Select the unknown you want to find, enter the two known values with their units, and the calculator returns all four quantities with the formula substitution shown step by step.

Ohm's Law states that voltage equals current times resistance: V = I × R. This single relationship governs the behavior of resistive circuits and is the starting point for electrical engineering, electronics design, home wiring assessment, and physics coursework. Knowing how to apply it quickly is fundamental to working with any electrical system.

Power extends Ohm's Law through P = V × I = I² × R = V² / R. These three equivalent power formulas let you compute wattage from any two known quantities. If you know current and resistance but not voltage, you can still find power directly using P = I² × R without solving for voltage first.

Practical applications span a wide range. Electronics hobbyists use Ohm's Law to select the correct resistor value for an LED circuit (too little resistance and the LED burns out, too much and it barely glows). Electricians apply it to calculate wire sizing, breaker ratings, and motor current draw. Physics students use it to analyze series and parallel circuit problems. Solar installers apply it to match panel output to inverter input requirements.

This calculator includes unit conversion so you can work with millivolts, kilovolts, milliamperes, and kilohms without manual conversion. Voltage inputs can be in V or kV; current can be in A, mA, or μA; resistance can be in Ω, kΩ, or MΩ. Power output is returned in W, mW, kW, or MW depending on magnitude.

For students, this tool is useful for homework verification and understanding how changing one variable affects the others. In a fixed-resistance circuit, doubling the voltage doubles the current. In a fixed-voltage circuit, doubling the resistance halves the current. Testing these relationships live with the calculator builds circuit intuition faster than static examples alone.

Ohm's Law connects voltage, current, and resistance through equivalent formulas. Power adds a fourth dimension using the same variables.

Core Ohm's Law equations V = I × R (voltage from current and resistance) I = V / R (current from voltage and resistance) R = V / I (resistance from voltage and current)

Power formulas (three equivalent forms) P = V × I P = I² × R P = V² / R

Derived from power and one other quantity V = P / I and V = √(P × R) I = P / V and I = √(P / R) R = V² / P and R = P / I²

Worked example — basic circuit V = 12 V, R = 470 Ω I = 12 / 470 = 0.02553 A = 25.53 mA P = 12 × 0.02553 = 0.306 W

Worked example — LED resistor selection V_supply = 5 V, V_LED = 2 V, I_LED = 20 mA R = (V_supply − V_LED) / I = (5 − 2) / 0.020 = 150 Ω