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Ohm's Law Calculator

Calculate voltage, current, resistance, and power using Ohm's Law. Enter any two known values and the calculator solves for the third plus power.

How to Use Ohm's Law

What Is Ohm's Law?

Ohm's Law is the most fundamental relationship in electrical engineering. It states that the voltage across a conductor is directly proportional to the current flowing through it, with the constant of proportionality being resistance. Expressed as V = I × R, this simple equation allows you to calculate any one of the three electrical quantities — voltage, current, or resistance — when the other two are known.

Named after German physicist Georg Simon Ohm, who published the relationship in 1827, Ohm's Law underpins nearly every aspect of electrical design, from sizing wire to selecting fuses and designing circuits. Mastering it is essential for anyone working with electricity.

The Ohm's Law Formulas

Voltage: V = I × R (Voltage = Current × Resistance)

Current: I = V / R (Current = Voltage / Resistance)

Resistance: R = V / I (Resistance = Voltage / Current)

Power (basic): P = V × I (Power = Voltage × Current)

Power (current form): P = I² × R

Power (voltage form): P = V² / R

Where V = Voltage (Volts), I = Current (Amperes), R = Resistance (Ohms), P = Power (Watts)

These six formulas form what is commonly called the Ohm's Law Wheel or Ohm's Law Triangle. By knowing any two of V, I, and R, you can solve for the third and also calculate power dissipation.

Worked Example

Scenario: A 12V battery powers a circuit with a 10Ω resistor. Calculate the current and power.

  1. Known: V = 12V, R = 10Ω, I = unknown
  2. Current: I = V / R = 12 / 10 = 1.2A
  3. Power: P = V × I = 12 × 1.2 = 14.4W
  4. Verification: P = I² × R = (1.2)² × 10 = 1.44 × 10 = 14.4W ✓

This means the resistor will dissipate 14.4 watts of power as heat, so a resistor rated for at least 15W or higher should be selected for safe operation.

Practical Tips

  • Unit consistency is critical. Always use Volts (V), Amps (A), and Ohms (Ω). If values are given in milliamps or kilo-ohms, convert them first: 1 mA = 0.001A, 1 kΩ = 1000Ω.
  • DC vs AC: Ohm's Law applies directly to DC circuits and AC circuits with purely resistive loads (heaters, incandescent lights). For AC circuits with inductive or capacitive loads, replace resistance with impedance (Z) and account for the power factor.
  • Resistive vs non-linear loads: Ohm's Law only applies to linear (ohmic) devices where resistance is constant. Devices like LEDs, diodes, and fluorescent bulbs have non-linear voltage-current relationships and do not obey Ohm's Law directly.
  • Power dissipation matters: Always calculate power to verify that components are rated for the expected dissipation. Exceeding power ratings leads to overheating and component failure.

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Frequently Asked Questions

What is Ohm's Law?
Ohm's Law (V = I × R) describes the relationship between voltage, current, and resistance in an electrical circuit. Voltage (V) is measured in volts, current (I) in amperes, and resistance (R) in ohms. When any two values are known, the third can be calculated using one of the three formulas: V = I × R, I = V / R, or R = V / I.
Does Ohm's Law apply to AC circuits?
Ohm's Law applies to AC circuits with purely resistive loads, such as heaters and incandescent light bulbs. For AC circuits with inductive loads (motors, transformers) or capacitive loads, you must use impedance (Z) instead of resistance and account for the phase angle between voltage and current (power factor). The generalized form is V = I × Z.
What is the power formula and how does it relate to Ohm's Law?
The basic power formula is P = V × I (watts = volts × amps). When combined with Ohm's Law, you get two additional forms: P = I² × R and P = V² / R. These are all algebraically equivalent and allow you to calculate power dissipation when you know any two of V, I, or R. Power tells you how much energy a component converts to heat per second.
Can Ohm's Law be used for non-ohmic devices like LEDs?
No. Ohm's Law only applies to linear (ohmic) devices where resistance remains constant regardless of voltage or current. LEDs, diodes, transistors, and fluorescent bulbs are non-ohmic — their voltage-current relationship is not linear. For LEDs, you typically use a series resistor with Ohm's Law to limit current, but the LED itself does not follow V = I × R.