Ohm's Law Calculator

Calculate Voltage (V), Current (I), Resistance (R), and Power (P) using Ohm's Law. Automatic cross-calculation for students and engineers.

Volts (V)
Amperes (A)
Ohms (Ω)
Watts (W)

Formulas Used:

V = I × ROhm's Law
P = V × IPower Formula
P = I² × RPower (Current)
P = V² / RPower (Voltage)

Understanding Ohm's Law

Ohm's Law is one of the most fundamental principles in electrical engineering and electronics. It describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. Named after German physicist Georg Ohm, this law states that the current through a conductor between two points is directly proportional to the voltage across the two points.

The Ohm's Law Formula

The basic Ohm's Law equation is V = I × R, where:

  • V (Voltage): Measured in volts (V) - the electrical potential difference
  • I (Current): Measured in amperes (A) - the flow of electrical charge
  • R (Resistance): Measured in ohms (Ω) - opposition to current flow

Power Calculations Using Ohm's Law

In addition to the basic relationship, Ohm's Law extends to power calculations:

  • 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

How to Use This Ohm's Law Calculator

  1. Enter any two known values (voltage, current, resistance, or power)
  2. The calculator automatically computes all remaining parameters
  3. Results are displayed instantly with proper units
  4. Switch between different unit prefixes (mV, V, kV) as needed

Practical Applications

Ohm's Law is essential for:

  • Circuit Design: Selecting appropriate resistor values for LED circuits, voltage dividers, and current limiting
  • Troubleshooting: Diagnosing electrical faults and verifying circuit performance
  • Power Management: Calculating heat dissipation and power consumption in electronic devices
  • Component Selection: Choosing resistors, power supplies, and wire sizes based on load requirements

Example Calculations

Example 1: LED Current Limiting Resistor
Given: 5V supply, LED forward voltage = 2V, desired current = 20mA
Voltage across resistor:
V=5V−2V=3VV = 5V - 2V = 3V

Required resistance:
R=V/I=3V/0.02A=150ΩR = V / I = 3V / 0.02A = 150Ω
Example 2: Power Dissipation
Given: 100Ω resistor with 12V across it
Current:
I=V/R=12V/100Ω=0.12A(120mA)I = V / R = 12V / 100Ω = 0.12A (120mA)

Power:
P=V2/R=144/100=1.44WP = V² / R = 144 / 100 = 1.44W

Common Mistakes to Avoid

  • Confusing resistance with impedance in AC circuits
  • Forgetting to account for component tolerances
  • Using the wrong power formula for the given parameters
  • Not considering power ratings when selecting resistors

Frequently Asked Questions

Q: Does Ohm's Law apply to AC circuits?
A: Yes, but you must use impedance (Z) instead of resistance (R) and consider RMS values for voltage and current in AC circuits.

Q: What happens if I exceed the power rating of a resistor?
A: The resistor will overheat and may be damaged or cause a fire hazard. Always select resistors with power ratings at least 2x higher than calculated.

Q: Can Ohm's Law be used for non-linear components?
A: No, Ohm's Law only applies to ohmic (linear) materials. Diodes, transistors, and other non-linear components don't follow this relationship.

Q: How do I calculate current if I know voltage and power?
A: Use I = P / V. For example, a 60W light bulb at 120V draws I = 60W / 120V = 0.5A.