Calculate voltage, current, resistance, and power from any two known values. Enter any two of the four quantities and the calculator solves for the other two instantly.
Ohm's Law is the most fundamental equation in electrical engineering and electronics. Discovered by Georg Simon Ohm in 1827, it describes the relationship between voltage, current, and resistance in an electrical circuit. Combined with the power equation (Watt's Law), these four quantities — voltage (V), current (I), resistance (R), and power (P) — form the foundation for understanding and designing every electrical circuit.
| Quantity | Symbol | Unit | Analogy |
|---|---|---|---|
| Voltage | V | Volts (V) | Water pressure |
| Current | I | Amperes (A) | Water flow rate |
| Resistance | R | Ohms (Ω) | Pipe restriction |
| Power | P | Watts (W) | Work done by water |
Think of electricity like water flowing through a pipe. Voltage is the pressure pushing water through the pipe. Current is how much water flows per second. Resistance is how narrow the pipe is (restricting flow). Power is the total work the water can do — and it depends on both how much pressure and how much flow there is.
Series resistance adds directly: R_total = R1 + R2 + R3. Current is the same through each resistor, but voltage divides proportionally. Parallel resistance uses the reciprocal formula: 1/R_total = 1/R1 + 1/R2 + 1/R3. Voltage is the same across each branch, but current divides. For two parallel resistors: R_total = (R1 × R2) / (R1 + R2). Parallel resistance is always less than the smallest individual resistor.