Ohm's Law Calculator
Calculate voltage, current, resistance, and power. Enter any two values — Ohm's Law and the power formula solve the other two instantly.
V
A
Ω
W
Enter any 2 values to calculate the other 2
Key Formulas
V = I × R
I = V / R
R = V / I
P = V × I
P = I² × R
P = V² / R
Common Circuit Examples
| Scenario | V (Volts) | I (Amps) | R (Ω) | P (Watts) |
|---|---|---|---|---|
| LED (standard red) | 2 | 0.020 | 100 | 0.040 |
| 9 V battery + 100 Ω resistor | 9 | 0.090 | 100 | 0.81 |
| USB 5 V phone charger (1 A) | 5 | 1 | 5 | 5 |
| USB-C laptop charger (20 V, 3 A) | 20 | 3 | 6.667 | 60 |
| 12 V car circuit (2 A load) | 12 | 2 | 6 | 24 |
| Incandescent bulb 100 W at 230 V | 230 | 0.44 | 529 | 100 |
| Microwave 1000 W at 230 V | 230 | 4.348 | 52.9 | 1,000 |
Frequently Asked Questions
Ohm's Law states that the voltage (V) across a conductor equals the current (I) through it multiplied by its resistance (R): V = I × R. Proposed by Georg Ohm in 1827, it is the foundational relationship in electronics. If you double the voltage across a fixed resistor, the current doubles. If you double the resistance at constant voltage, the current halves.
Power (P) in watts can be calculated three ways using Ohm's Law: P = V × I (voltage times current), P = I² × R (current squared times resistance), or P = V² / R (voltage squared divided by resistance). All three formulas give the same result for a given circuit. This calculator solves all four quantities (V, I, R, P) whenever any two are known.
Voltage is measured in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W). For large resistances use kΩ (kilohms = 1,000 Ω) or MΩ (megohms = 1,000,000 Ω). For small currents use mA (milliamps; 1 mA = 0.001 A). Simply convert to base units (V, A, Ω, W) before entering values in this calculator.
You need exactly two known values from voltage (V), current (I), resistance (R), or power (P) to solve the other two. With V and I given, the calculator computes R = V/I and P = V×I. With I and R given, it computes V = I×R and P = I²×R. Any valid pair of positive values works — the calculator shows which outputs were derived (highlighted) versus which were provided (muted).