Ohm's Law Calculator

Enter any two of voltage, current, and resistance to instantly calculate the third value plus power. A free Ohm’s Law calculator based on V=IR for electronics.

How to Use

  1. Enter known values

    Input any two of voltage (V), current (A), and resistance (Ω).

  2. Calculate

    Click the Calculate button to find the unknown value.

  3. View results

    See the calculated voltage, current, resistance, and power (W).

What is Ohm's Law?

Ohm's Law is the most fundamental rule of electrical circuits: the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. It was published in 1827 by the German physicist Georg Ohm, and the unit of resistance, the 'ohm (Ω)', is named after him.

The law ties together three quantities -- voltage (V, volts), current (I, amperes), and resistance (R, ohms) -- in a single equation: V = I × R. As a result, once you know any two of the three values, the remaining one is determined automatically.

In practice it is used every day for selecting LED protection resistors, checking the power consumption of appliances, sizing wire gauges, and designing electronic circuits. However, it holds exactly only for ohmic (linear) components whose resistance stays nearly constant. It cannot be applied directly to nonlinear devices such as diodes or transistors, whose resistance changes with the applied voltage.

Calculation Formula

Depending on the two values you enter, Ohm's Law and the power formula expand as follows.

  • V = I × R (voltage = current × resistance)
  • I = V / R, R = V / I
  • P = V × I = I² × R = V² / R (power, in watts)

Example: Connecting a 4Ω resistor to a 12V supply gives a current of I = 12 / 4 = 3A and a power of P = 12 × 3 = 36W. Conversely, if a 2A current flows through 6Ω, then V = 2 × 6 = 12V and P = 2² × 6 = 24W.

Frequently Asked Questions

What is Ohm's Law?
Ohm's Law is a fundamental rule of electrical circuits expressing the relationship voltage (V) = current (I) × resistance (R). It was published in 1827 by the German physicist Georg Ohm; current is proportional to voltage and inversely proportional to resistance.
Can I find the rest from just two values?
Yes. Since V, I, and R are all tied together in V = I × R, entering any two of them determines the third. This calculator supports three modes: voltage + current, voltage + resistance, and current + resistance.
How is power (watts) calculated?
Power is calculated as P = V × I (voltage × current), with the unit watt (W). Substituting Ohm's Law, it can also be found as P = I² × R or P = V² / R, so power is computed automatically from just the two values you enter.
What are the units of voltage, current, and resistance?
Voltage uses the volt (V), current the ampere (A), and resistance the ohm (Ω) as base units. One ampere is the flow of one coulomb of charge per second, and one ohm is the resistance when one ampere flows under one volt.
Which circuits does Ohm's Law apply to?
It applies to linear (ohmic) components whose resistance stays constant, such as ordinary resistors, wires, and heating elements. It cannot be used directly for nonlinear devices like diodes, transistors, or LEDs, whose resistance changes with voltage.
What is the difference between series and parallel connections?
In series, the total resistance is the sum of each resistance (R = R₁ + R₂ + …) and the current is the same everywhere. In parallel, the reciprocal of the total resistance is the sum of the reciprocals (1/R = 1/R₁ + 1/R₂ + …) and the voltage is the same everywhere.
Why add a resistor to an LED?
An LED is a nonlinear device whose current surges with even a small rise in voltage, so connecting it directly would burn it out from overcurrent. That is why a protection resistor is placed in series, calculated as R = (Vs − Vf) / I -- the supply voltage minus the LED forward voltage, divided by the desired current.
Does temperature affect resistance?
Yes. The resistance of most metal conductors increases as temperature rises. In precision circuits this change can make actual operation differ from values calculated at room temperature, so the temperature coefficient is taken into account as well.
2026 physical constants

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