Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, given by the formula V = I times R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. It is the foundational relationship in basic circuit analysis.

How do I use this calculator to find resistance?

Select Solve for Resistance from the dropdown, enter the voltage (V) and current (I), and the calculator returns R = V divided by I. For example, 12 V and 3 A gives 4 ohms.

How is electrical power related to Ohm's Law?

Once you know voltage and current you can calculate power using P = V times I. You can also express power purely in terms of resistance: P = I squared times R, or P = V squared divided by R. This calculator shows the power result alongside the Ohm's Law solution.

Ohm's Law Calculator

Ohm's Law is the cornerstone of electrical circuit analysis. It states that the voltage across a conductor equals the current through it multiplied by its resistance: V = I times R. This simple relationship lets you find any one of the three quantities when you know the other two, and it also unlocks the power equation P = V times I, giving you all four key circuit values from just two inputs. You choose which quantity you want to solve for using the dropdown menu, enter the two known values, and the calculator instantly returns the unknown along with the electrical power dissipated by the component. Voltage (V) is measured in volts, current (I) in amperes, resistance (R) in ohms, and power (P) in watts. For example, if a resistor has 12 V across it and 3 A flowing through it, you can verify the resistance is 4 ohms and the power dissipated is 36 W. Engineers, electricians, electronics hobbyists, and students use this relationship constantly when designing circuits, selecting components, or diagnosing faults. The tool handles the three common solve modes: voltage from current and resistance, current from voltage and resistance, and resistance from voltage and current. It guards against division by zero and shows a clear message when inputs are missing or invalid. These results apply to purely resistive DC circuits; reactive AC components involving capacitance or inductance require impedance analysis beyond this basic form of the law.

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Solve for

Voltage (V)

Current (I)

Resistance (R)

Ω

3.00 A

current

Voltage12.00 V

Current3.00 A

Resistance4.00 Ω

Power36.00 W

How it works

The calculator applies Ohm's Law in the direction you choose. Solving for current: I = V divided by R. Solving for voltage: V = I times R. Solving for resistance: R = V divided by I. Once two of the three electrical quantities are known, power is calculated as P = V times I. The input field for the quantity being solved is disabled automatically so you cannot enter a conflicting value. Division by zero is caught and flagged. All results are rounded to two decimal places.

Worked example

A 12 V battery powers a circuit through a 4-ohm resistor. Select "Solve for Current", enter V = 12 and R = 4. The current is I = 12 divided by 4 = 3.00 A. Power is P = 12 times 3 = 36.00 W. These match the default values pre-filled above, so you can confirm the output without changing anything.

Related calculators

Electrical Power Calculator: find power using P=VI, P=I²R, or P=V²/R directly.
Watts to Amps Calculator: convert power and voltage to current for DC or AC circuits.
Series and Parallel Resistance Calculator: combine multiple resistors before applying Ohm's Law.
Percent Error Calculator: check how close your measured circuit values are to theory.

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