Ohm's Law Calculator

Ohm's law relates voltage (V), current (I), and resistance (R) in a simple resistive circuit: V = I × R. Given any two values, you can solve for the third — this is the foundation of basic DC circuit analysis.

Voltage is in volts, current in amperes, and resistance in ohms. For AC circuits with pure resistance, RMS values use the same formula. Capacitors, inductors, and non-linear components need more advanced models.

Example: 12 V across a 4 Ω resistor draws I = V ÷ R = 3 A. Power dissipated is V × I = 36 W — check resistor wattage ratings in real designs.

Power in a resistive load also follows P = V × I = V²/R = I²R. When a datasheet lists max power but you only measured voltage, rearrange to find current or resistance before confirming the part is safe.

Series resistances add directly (R_total = R1 + R2). Parallel branches combine as 1/R_total = 1/R1 + 1/R2 — reduce a network to one equivalent resistance before applying Ohm's law to the whole circuit.

Worked example with typical bench values: 12 V supply and 4 Ω load → 3 A current and 36 W dissipation. A ¼ W resistor would overheat instantly; choose a part rated above the calculated power or increase resistance to cut current.

LED circuits illustrate why you rearrange Ohm's law before picking parts. A 12 V bench supply feeding a 2 V forward-voltage LED leaves 10 V across the series resistor. Targeting 20 mA, R = 10 V ÷ 0.02 A = 500 Ω — verify the resistor dissipates I²R ≈ 0.2 W, not a tiny 1/8 W part.

Wire resistance becomes non-negligible on long low-voltage runs. Copper at room temperature adds roughly 0.017 Ω per meter per mm² of cross-section — thin extension cords can drop measurable voltage under load even when the load itself is only a few ohms. RV and van builders often upsize cable after seeing more than three percent drop on a twelve-volt fridge circuit.

With defaults set to solve current, 12 V and 6 Ω give I = 2 A and P = 24 W. Switch the solve mode to resistance when you know supply voltage and desired current — that is how bench power supplies are matched to unknown loads. Confirm fuse and wire gauge ratings exceed calculated amperage before leaving a continuous bench test powered overnight.