Ohm's Law Formula

May 8, 2026

Ohm’s Law Formula

Ohm’s law describes the relationship between voltage, current, and resistance in an electrical circuit. Expressed as V = I × R, it’s one of the most essential formulas in electronics and electrical engineering. Whether you’re designing a circuit, troubleshooting an electrical problem, or calculating power consumption, Ohm’s law provides the foundation.

The Ohm’s Law Formula: V = I × R

The three variables in Ohm’s law are:

V – voltage, measured in volts (the electrical potential difference)
I – current, measured in amperes (the flow of electric charge)
R – resistance, measured in ohms (opposition to current flow)

The formula states that voltage equals current multiplied by resistance. This means:

Higher current or resistance results in higher voltage (if the other variable is fixed)
Doubling the resistance doubles the voltage needed to maintain the same current
Increasing voltage increases current if resistance remains constant

How to Rearrange Ohm’s Law Formula

The basic formula can be rearranged into two other forms depending on which value you need to find:

To find voltage: V = I × R

To find current: I = V ÷ R

To find resistance: R = V ÷ I

Use whichever form matches the values you know and the value you need to calculate.

What Is the Ohm’s Law Triangle?

The Ohm’s law triangle is a visual memory tool used to recall all three variations of the formula without confusion:


      V
    -----
    I | R

The triangle is divided with voltage (V) at the top and current (I) and resistance (R) at the bottom. To use it:

Cover the variable you want to find
The remaining symbols show the operation needed
If the variables are side-by-side, multiply; if one is above the other, divide

For example, if you cover V, you see I and R side-by-side, reminding you that V = I × R.

Practical Examples of Ohm’s Law

Example 1: Finding Voltage

A circuit has a current of 2 amperes flowing through a 5-ohm resistor. What is the voltage?

V = I × R V = 2 × 5 V = 10 volts

Example 2: Finding Current

A power supply provides 24 volts to a heating element with 8 ohms of resistance. How much current flows through it?

I = V ÷ R I = 24 ÷ 8 I = 3 amperes

Example 3: Finding Resistance

An LED circuit operates at 5 volts with 0.02 amperes of current. What resistor value is needed?

R = V ÷ I R = 5 ÷ 0.02 R = 250 ohms

Power Calculations Using Ohm’s Law

Power (P), measured in watts, describes the rate at which energy is used or dissipated. Ohm’s law can be combined with the power equation P = V × I to create three useful formulas:

P = V × I Power equals voltage multiplied by current.

P = I² × R Power equals current squared multiplied by resistance. Useful when you know current and resistance but not voltage.

P = V² ÷ R Power equals voltage squared divided by resistance. Useful when you know voltage and resistance but not current.

All three formulas are equivalent and produce the same result for the same circuit.

Applications of Ohm’s Law

Ohm’s law is used in virtually every electrical and electronic application:

Circuit design – engineers use it to select appropriate resistor values and ensure safe current levels
Electrical safety – calculating maximum safe voltages and currents to prevent equipment damage or electrical hazards
Troubleshooting – technicians measure voltage and resistance to diagnose faults and predict current flow
Power consumption – determining how much energy a device uses based on its operating voltage and resistance
Heat dissipation – calculating how much heat a resistor or component will generate under specific conditions

When Does Ohm’s Law Not Apply?

Ohm’s law applies only to ohmic materials, where resistance remains constant regardless of voltage or current. Many real-world devices don’t follow this rule:

Diodes – resistance changes dramatically depending on voltage polarity and magnitude
Transistors – operate as voltage-controlled devices with nonlinear resistance
Light bulbs – filament resistance increases as it heats up with current flow
Batteries – internal resistance and chemical properties create nonlinear behavior
Semiconductors – exhibit complex resistance characteristics that vary with temperature and bias

For these nonlinear devices, more sophisticated analysis methods replace or supplement Ohm’s law.

History of Ohm’s Law

German physicist Georg Simon Ohm published his discoveries about electrical resistance in 1827. Through systematic experiments with different wire lengths and thicknesses, he identified the constant relationship between voltage, current, and resistance. His work laid the mathematical foundation for electrical engineering. In recognition of his contribution, the unit of electrical resistance was named the ohm (Ω).

This article provides educational information about electrical principles. Always follow local electrical codes and safety standards when working with electrical systems.

Frequently Asked Questions

What does V = I × R mean?

V is voltage (measured in volts), I is current (measured in amperes), and R is resistance (measured in ohms). The formula shows that voltage equals current multiplied by resistance. If you increase current or resistance while voltage stays the same, one of the others must change.

How do you calculate current using Ohm's law?

Rearrange the formula to I = V ÷ R. Divide the voltage by the resistance to find current. For example, with 12 volts and 4 ohms: I = 12 ÷ 4 = 3 amperes.

What is the Ohm's law triangle used for?

The triangle is a memory tool with V at the top and I and R at the bottom. Cover the value you want to find; the remaining symbols show the operation needed. It helps quickly rearrange the formula without memorizing all three versions.

Does Ohm's law apply to all electrical devices?

No. Ohm’s law applies only to ohmic materials (like most resistors) where resistance stays constant. Nonlinear devices like diodes, transistors, and light bulbs don’t follow Ohm’s law because their resistance changes with voltage or current.

How is power related to Ohm's law?

Power (P) in watts can be calculated using Ohm’s law components: P = V × I, P = I² × R, or P = V² ÷ R. These formulas show how voltage, current, and resistance affect the energy a circuit uses or dissipates as heat.

Who discovered Ohm's law and when?

German physicist Georg Simon Ohm discovered the relationship in 1827 through experiments with electrical conductors. His work became a fundamental principle of electricity and electronics, and the unit of electrical resistance was named the ohm in his honor.