Ohms Law Calculator

About the Ohm's Law Calculator

Ohm's Law is the fundamental principle of electrical circuits. Discovered by Georg Simon Ohm in 1827, it describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. Whether you're designing electronics, troubleshooting car electrical issues, or wiring a home theater, Ohm's Law is your starting point.

Our calculator solves for any variable when you know the other two:

• Voltage (V): Electrical pressure, measured in volts
• Current (I): Flow of electrons, measured in amps
• Resistance (R): Opposition to flow, measured in ohms (Ω)
• Power (P): Energy consumption, measured in watts

Ohm's Law Formulas

The core equation: V = I × R

From this, you can derive:

• Current: I = V ÷ R
• Resistance: R = V ÷ I
• Power: P = V × I (also P = I² × R, or P = V² ÷ R)

The Ohm's Law Triangle

    V
   / \
  /   \
 I ×   R

Cover the variable you want to solve for:

• Cover V → I × R
• Cover I → V ÷ R
• Cover R → V ÷ I

Example Calculations

Finding Current

Scenario: A 12V car battery connected to a 4Ω speaker I = V ÷ R = 12 ÷ 4 = 3 amps

Finding Resistance

Scenario: A 120V outlet providing 5 amps to a space heater R = V ÷ I = 120 ÷ 5 = 24 ohms

Finding Voltage

Scenario: A circuit with 2 amps flowing through 60 ohms resistance V = I × R = 2 × 60 = 120 volts

Finding Power

Scenario: A laptop charger outputs 19.5V at 4.62 amps P = V × I = 19.5 × 4.62 = 90.09 watts

Common Electrical Scenarios

Series and Parallel Circuits

Series Circuits

In series, components are connected end-to-end. The same current flows through all components.

Total Resistance: R_total = R₁ + R₂ + R₃ + ...

Example: Three resistors in series: 10Ω, 20Ω, 30Ω R_total = 10 + 20 + 30 = 60Ω

If connected to 12V: I = 12 ÷ 60 = 0.2 amps

Parallel Circuits

In parallel, components share the same two connection points. The same voltage is across all components.

Total Resistance: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + ...

Example: Three resistors in parallel: 10Ω, 20Ω, 30Ω 1/R_total = 1/10 + 1/20 + 1/30 = 0.1 + 0.05 + 0.033 = 0.183 R_total = 1 ÷ 0.183 = 5.45Ω

If connected to 12V: I_total = 12 ÷ 5.45 = 2.2 amps

Wire Gauge and Current Capacity

Safety Considerations

Circuit Breakers

Circuit breakers protect wires from overheating by limiting current:

Never use a larger breaker than the wire can safely handle.

Voltage Drop

Long wire runs cause voltage drop. For critical circuits, keep drop under 3%:

Example: 100-foot run of 12 AWG wire carrying 15 amps Voltage drop ≈ 3.84 volts On a 120V circuit: 120 - 3.84 = 116.16V (3.2% drop — acceptable)

Frequently Asked Questions

What is Ohm's Law used for?

Ohm's Law is used to design circuits, select components, troubleshoot electrical problems, and ensure safety in any electrical system.

Can Ohm's Law be used for AC circuits?

Yes, but with modifications. In AC circuits, you use impedance (Z) instead of resistance (R), which includes reactance from capacitors and inductors.

Why does a short circuit draw so much current?

A short circuit has near-zero resistance. By Ohm's Law (I = V ÷ R), as R approaches zero, current approaches infinity—causing wires to overheat.

What happens if I exceed a resistor's power rating?

The resistor overheats and may burn out, change value, or catch fire. Always choose resistors with power ratings at least 2× the expected power dissipation.

How do I measure resistance in a live circuit?

You don't. Always disconnect power before measuring resistance with a multimeter. Measuring resistance in a live circuit can damage your meter and is dangerous.