Last updated: November 19, 2025
Ohm's law calculator explained
Ohm's law links voltage (V), current (I), and resistance (R). This calculator solves for any one of those quantities and also computes power , covering the common relationships needed when designing or troubleshooting circuits.
How the conversion works
Core equations:
Power is . The calculator keeps units consistent, so entering milliamps or kilohms automatically converts to amperes and ohms before multiplication.
Units and conversions
| Quantity | Units | Notes |
|---|---|---|
| Voltage | V, mV, kV | Converted to volts. |
| Current | A, mA, μA | Converted to amperes. |
| Resistance | ohm, k-ohm, M-ohm | Converted to ohms. |
| Power | W, mW, kW | Computed from V and I. |
Worked examples
- LED circuit
Desired current 20 mA from a 5 V microcontroller pin. Required resistor:
Power dissipated W, so a 1/4 W resistor suffices.
- DC motor draw
Motor draws 3 A from a 12 V supply.
Tips and pitfalls
- Double-check unit prefixes; mixing mA with k-ohm without conversion yields incorrect voltages.
- Account for tolerance; choose resistor wattage at least 2x the calculated dissipation.
- For AC circuits with reactance, use impedance () instead of pure resistance; this simple calculator assumes DC or purely resistive loads.
- Measure actual current with a multimeter when dealing with nonlinear loads such as LEDs or motors.