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Ohm's Law Calculator

Ohm's Law defines the relationship between voltage, current, and resistance in an electrical circuit: V = I x R. A circuit with 2 amps flowing through 12 ohms of resistance has a voltage of 24 volts. Select which variable to solve for, enter the two known values, and the calculator computes the third instantly. A second section calculates electrical power using P = V x I.

Quick Answer

A 12-ohm resistor with 2 amps of current flowing through it has a voltage drop of 24.00 V and dissipates 48.00 W of power.

Ohm's Law (V = IR)

Electrical Power (P = VI)

Common Examples

Input Result
I = 2 A, R = 12 ohms V = 24.00 V
V = 120 V, R = 60 ohms I = 2.00 A
V = 9 V, I = 0.5 A R = 18.00 ohms
V = 120 V, I = 10 A P = 1,200.00 W
V = 5 V, I = 0.02 A P = 0.10 W

How It Works

This calculator uses two fundamental electrical formulas.

Ohm’s Law

V = I x R

Where:

  • V = voltage in volts (V), the electrical potential difference
  • I = current in amperes/amps (A), the flow of electric charge
  • R = resistance in ohms (the Greek letter omega), the opposition to current flow

Rearranged forms:

  • Voltage: V = I x R
  • Current: I = V / R
  • Resistance: R = V / I

Electrical Power

P = V x I

Where P is power in watts (W). Power can also be expressed using Ohm’s Law substitutions:

  • P = I^2 x R (substituting V = IR)
  • P = V^2 / R (substituting I = V/R)

Understanding the Relationships

Voltage is the “pressure” that pushes electrical current through a circuit. Resistance opposes that flow. Higher resistance means less current for the same voltage. Power measures the rate of energy transfer or consumption.

Common Values

  • Standard US household outlet: 120 V
  • Standard European household outlet: 230 V
  • USB port: 5 V
  • Car battery: 12 V
  • AA battery: 1.5 V

Worked Example

A 60-watt light bulb operates on a 120 V circuit. The current drawn is I = P / V = 60 / 120 = 0.50 A. The resistance of the filament is R = V / I = 120 / 0.50 = 240 ohms. Alternatively, using Ohm’s Law: if a 12-ohm resistor carries 2 A of current, the voltage across it is V = 2 x 12 = 24 V, and the power dissipated is P = 24 x 2 = 48 W.

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Frequently Asked Questions

What is Ohm's Law?
Ohm's Law is a fundamental principle of electrical circuits discovered by Georg Simon Ohm in 1827. It states that the voltage across a conductor is directly proportional to the current flowing through it, with resistance as the proportionality constant: V = IR. It applies to resistive (ohmic) materials where resistance remains constant regardless of voltage.
What is the difference between voltage, current, and resistance?
Voltage (V) is the electrical potential difference that drives current through a circuit, measured in volts. Current (I) is the flow rate of electric charge, measured in amperes. Resistance (R) is the opposition to current flow, measured in ohms. A common analogy: voltage is like water pressure, current is like the flow rate of water, and resistance is like the pipe diameter.
Does Ohm's Law apply to all components?
Ohm's Law applies to ohmic (linear) components like resistors, where resistance stays constant. Non-ohmic components like diodes, transistors, and LEDs do not follow Ohm's Law because their resistance changes with voltage or current. However, the formula V = IR can still be used instantaneously at a specific operating point.
What is a watt?
A watt (W) is the SI unit of power, equal to one joule per second. In electrical terms, one watt equals one volt times one ampere. A 100-watt light bulb converts 100 joules of electrical energy into light and heat every second. One kilowatt (kW) equals 1,000 watts.
How do I calculate electricity cost from power?
Electricity is typically billed in kilowatt-hours (kWh). Multiply the power consumption in kilowatts by the number of hours to get kWh. For example, a 100 W device running for 10 hours uses 0.1 kW x 10 h = 1 kWh. At $0.12 per kWh, that costs $0.12.