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Ohm's Law Calculator

Calculate voltage, current, resistance, and power in electrical circuits.

Enter values for at least 2 fields and click Calculate or lock a field with the lock icon. The calculator will find the remaining values.

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Understanding Ohm's Law

Ohm's Law is a fundamental principle in electrical engineering that describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. It was named after German physicist Georg Ohm.

The Basic Formula

V = I × R

Where:

  • V = Voltage in volts (V)
  • I = Current in amperes (A)
  • R = Resistance in ohms (Ω)

From this basic equation, we can derive two other forms:

I = V ÷ R

Current equals voltage divided by resistance

R = V ÷ I

Resistance equals voltage divided by current

Power in Electrical Circuits

Power (P) is the rate at which energy is transferred in an electrical circuit. It's measured in watts (W) and can be calculated using several formulas:

P = V × I

Power equals voltage times current

P = I² × R

Power equals current squared times resistance

P = V² ÷ R

Power equals voltage squared divided by resistance

The Ohm's Law Triangle

A common way to remember Ohm's Law is through the "Ohm's Law Triangle," which helps visualize the relationship between variables:

V
I
R

To find the variable you need, cover it with your finger and use the remaining variables in the appropriate relationship.

Practical Examples

Example 1: Finding Current

Problem: What is the current flowing through a 220Ω resistor connected to a 12V battery?

Solution: Using I = V ÷ R
I = 12V ÷ 220Ω = 0.055A = 55mA

Example 2: Finding Resistance

Problem: A 60W light bulb is connected to a 120V supply. What is its resistance?

Solution: First, find the current using P = V × I, so I = P ÷ V
I = 60W ÷ 120V = 0.5A
Then, find resistance using R = V ÷ I
R = 120V ÷ 0.5A = 240Ω

Example 3: Finding Power

Problem: A circuit has a current of 2A flowing through a 5Ω resistor. What is the power dissipated?

Solution: Using P = I² × R
P = (2A)² × 5Ω = 4 × 5 = 20W

Limitations of Ohm's Law

It's important to understand that Ohm's Law is not universal. It applies to:

  • Linear resistors (where current and voltage have a linear relationship)
  • At constant temperature (resistance changes with temperature)
  • DC circuits (direct current) and AC circuits with purely resistive loads

Ohm's Law doesn't apply to non-linear components like diodes, transistors, and thermistors without modification.

Applications of Ohm's Law

  • Circuit design: Determining component values for proper circuit operation
  • Troubleshooting: Identifying faults in electrical systems
  • Power consumption analysis: Calculating energy usage and efficiency
  • Safety calculations: Ensuring circuits operate within safe parameters
  • Component sizing: Selecting appropriate resistors, fuses, and wires

Ohm's Law in Series and Parallel Circuits

Series Circuits

In a series circuit, the total resistance is the sum of all individual resistances:

Rtotal = R₁ + R₂ + R₃ + ...

Parallel Circuits

In a parallel circuit, the total resistance is calculated as:

1/Rtotal = 1/R₁ + 1/R₂ + 1/R₃ + ...

For just two resistors in parallel, a simplified formula is:

Rtotal = (R₁ × R₂) ÷ (R₁ + R₂)