Calculate voltage, current, resistance or power using Ohm\ Law — enter any two values to instantly find the other two with full formula working.
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Ohm\ Law states that the current (I) through a conductor is proportional to the voltage (V) across it, with resistance (R) as the constant of proportionality: V = I × R. Named after Georg Ohm (1827), it is one of the most fundamental relationships in electrical engineering and applies to most resistive elements at constant temperature.
The four basic electrical relationships: V = I × R (voltage), I = V/R (current), R = V/I (resistance), P = V × I = I²R = V²/R (power). Knowing any two of the four quantities (V, I, R, P) allows you to calculate the other two. This calculator accepts any two inputs and calculates the remaining values.
Voltage (V) in volts (V), Current (I) in amperes (A), Resistance (R) in ohms (Ω), Power (P) in watts (W). Common prefixes: milliamps (mA = 0.001 A), kilohms (kΩ = 1,000 Ω), milliwatts (mW), kilowatts (kW). Always check your units before calculating — mixing mA and A is a common error.
For DC (direct current) circuits, Ohm\ Law applies straightforwardly. For AC (alternating current), resistance is replaced by impedance (Z), which includes resistance (R) and reactance (X) from capacitors and inductors: Z = √(R² + X²). The basic V = IZ relationship still holds, but calculations involve complex numbers.
Ohm\ Law is used to: calculate the correct resistor value in LED circuits (to limit current and prevent burnout), size fuses and circuit breakers, design voltage dividers, calculate power dissipation in components, troubleshoot electrical faults and design power supplies. Every electrical engineer applies it daily.
Not all materials follow Ohm\ Law (i.e. not all are "Ohmic"). Ohmic materials have constant resistance regardless of voltage. Non-Ohmic devices include diodes (only conduct in one direction), transistors, LEDs and thermistors (resistance changes with temperature). These require different models for accurate analysis.
Current, not voltage, kills. As little as 10 mA causes muscular paralysis; 100–200 mA through the chest can cause ventricular fibrillation and death. The body\ resistance varies from 1,000 Ω (wet skin) to 100,000 Ω (dry skin). At 240V with wet hands: I = 240/1000 = 0.24A = 240 mA — potentially lethal. Always treat mains voltage with extreme respect.
Kirchhoff\ Voltage Law (KVL) states that the sum of all voltages around a closed loop equals zero. Kirchhoff\ Current Law (KCL) states that the sum of currents entering a node equals the sum leaving it. These two laws, combined with Ohm\ Law, form the complete toolkit for analysing any resistive DC circuit.
