Ohm's Law Calculator

The three core Ohm's Law formulas relate voltage (V), current (I), and resistance (R): V = I × R (voltage equals current times resistance), I = V ÷ R (current equals voltage divided by resistance), and R = V ÷ I (resistance equals voltage divided by current). Combined with power formulas — P = V × I, P = I² × R, and P = V² ÷ R — any two known values let you solve for the rest.

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points, with resistance as the constant of proportionality. The formula is V = I × R, where V is voltage in volts, I is current in amps, and R is resistance in ohms. Rearranged, this also gives I = V / R and R = V / I, letting you solve for any one quantity when the other two are known.

Electrical power is calculated as P = V × I (voltage times current), measured in watts. Combining this with Ohm's law (V = I × R) gives two more useful forms: P = I² × R (power from current and resistance) and P = V² / R (power from voltage and resistance). These let you find power directly from whichever two quantities you already know, without first solving for the third.

Given voltage (V) and current (I), resistance is R = V / I and power is P = V × I. For example, with a 12-volt supply and 2 amps of current, resistance is 12 / 2 = 6 ohms, and power is 12 × 2 = 24 watts. This is one of the most common real-world scenarios, since voltage and current are usually the easiest quantities to measure directly with a multimeter.

Given current (I) and resistance (R), voltage is found using Ohm's law: V = I × R. Power is then P = I² × R, or equivalently P = V × I once voltage is known. For example, 2 amps flowing through a 6-ohm resistor produces a voltage of 2 × 6 = 12 volts and a power dissipation of 2² × 6 = 24 watts.

Given resistance (R) and power (P), current is found by rearranging P = I² × R to get I = √(P / R). Once current is known, voltage follows from Ohm's law: V = I × R. For example, with 24 watts dissipated across a 6-ohm resistor, current is √(24 / 6) = √4 = 2 amps, and voltage is 2 × 6 = 12 volts.

Ohm's law and its power equations let engineers and hobbyists size resistors, predict heat dissipation, and avoid damaging components before building a circuit. Knowing the power a resistor must dissipate (P = I²R or P = V²/R) determines what wattage rating the physical resistor needs — undersizing it causes overheating or failure. It's also the basis for calculating current draw in LED circuits, voltage drops across wires, and battery life estimates in simple DC circuits.