Calculate electrical values using Ohm's Law (V=IR) and power equations. Find voltage, current, resistance, or power from any two known values.
Enter any two values to calculate the others
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Ohm's Law and the power equations are the foundation of electrical engineering. Enter any two of the four values — voltage (V, volts), current (I, amperes), resistance (R, ohms) or power (P, watts) — and the calculator solves for the other two instantly, with a worked step-by-step solution you can follow or cite.
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance: V = I × R. It rearranges to I = V ÷ R and R = V ÷ I, so knowing any two values gives the third. Combining it with the power equation P = V × I (equivalently P = I²R or P = V²/R) lets you find all four quantities — voltage in volts, current in amperes, resistance in ohms and power in watts. Ohm's Law applies to DC and purely resistive AC circuits.
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Voltage is measured in Volts (V), current in Amperes (A), resistance in Ohms (Ω), and power in Watts (W). The calculator handles conversions between milli, kilo, and mega prefixes.
For AC circuits, Ohm's Law uses impedance (Z) instead of resistance, which includes capacitive and inductive reactance. This calculator is designed for DC and purely resistive AC circuits.
Calculate the power dissipation using P = I²R or P = V²/R, then choose a resistor rated for at least 2x the calculated power for safety margin.
All formulas are mathematically equivalent. V = IR can be rearranged to I = V/R or R = V/I. Power formulas P = VI, P = I²R, and P = V²/R all give the same result.
Divide the voltage by the resistance: I = V ÷ R. For example, 12 V across a 4 Ω resistor gives I = 12 ÷ 4 = 3 A. To find voltage instead use V = I × R, and for resistance use R = V ÷ I.
The Ohm's Law wheel (or pie chart) arranges all twelve forms of the Ohm's Law and power equations around V, I, R and P so you can pick the formula that uses the two values you already know. This calculator does that automatically — enter any two values and it applies the correct formula.