Fill in any two of voltage, current, resistance and power — the other two are calculated instantly.
Ohm's law relates voltage (V), current (I) and resistance (R) in a simple equation: V = I × R. The power equation adds a fourth variable: P = V × I. Together, these two equations let you find any two unknowns from any two knowns — twelve combinations in all. This calculator handles all of them.
The law describes the behaviour of linear (ohmic) resistors: components where the current is proportional to the voltage. Most metals, resistors and heating elements follow it closely at constant temperature. Semiconductors, LEDs and batteries do not — their resistance changes with voltage, temperature or state of charge, and Ohm's law gives an approximation at best.
Practical uses include sizing a resistor for an LED (given the supply voltage and the desired current), checking whether a fuse is rated for the circuit (power = voltage × current), and estimating the current draw of an appliance from its wattage and the mains voltage. Everything here is DC; for AC circuits, impedance replaces resistance and the calculations involve phase angles.
V = I × R. I = V / R. R = V / I. P = V × I = I²R = V²/R.
Only as an approximation with purely resistive loads (heaters, incandescent bulbs). For AC circuits with inductors or capacitors, impedance replaces resistance and the phase angle matters.
Volts measure the electrical pressure (potential difference). Watts measure the rate of energy use (power). A higher voltage at the same current means more power.
Because there are four variables and two independent equations. Two knowns determine the other two uniquely. With fewer, the system is underdetermined; with more, it may be inconsistent.
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