Ohm’s Law Calculator

Please provide any 2 values and click “Calculate” to get the other values in the ohm’s law equations V = I × R and P = V × I.

⚡ Voltage (V)

Volts (V)

Electrical potential difference

🔋 Current (I)

Amperes (A)

Electrical current flow

📏 Resistance (R)

Ohms (Ω)

Electrical resistance

💡 Power (P)

Watts (W)

Electrical power consumption

⚙️ Advanced Options

🔌 Component Database

Select a common electrical component to auto-fill typical values:

⚡ Three-Phase Circuit Calculations

🔋 Line-to-Line Voltage (VLL)

Volts (V)

Three-phase line-to-line voltage (e.g., 480V)

📐 Power Factor (PF)

Decimal (0.0 – 1.0)

Typical industrial motors: 0.8-0.9, Unity power factor: 1.0

Three-Phase Power Formula:
P = √3 × VLL × I × PF
Where VLL is line-to-line voltage, I is line current, and PF is power factor

🌡️ Temperature Effect on Resistance

Calculate resistance changes due to temperature variations:

📏 Reference Resistance (R₀)

Ohms (Ω)

🌡️ Reference Temperature (T₀)

Celsius (°C)

🌡️ Operating Temperature (T)

Celsius (°C)

⚙️ Temperature Coefficient (α)

✏️ Custom α Value

per °C

Temperature Coefficient Formula:
R(T) = R₀ × [1 + α(T – T₀)]
Where R₀ is reference resistance, α is temperature coefficient, T is operating temperature, and T₀ is reference temperature

What is Ohm’s Law?

Ohm’s Law is the fundamental principle that describes the relationship between voltage, current, and resistance in electrical circuits. Discovered by German physicist Georg Simon Ohm in 1827, it states that the current flowing through a conductor is directly proportional to the voltage applied across it, provided the temperature remains constant. For a comprehensive historical and technical overview, see Wikipedia’s Ohm’s Law article.

This relationship is the foundation of electrical engineering and electronics design. Understanding Ohm’s Law is essential for calculating electrical circuit parameters, sizing components, and ensuring safe electrical system operation.

Ohm's Law Circle Diagram showing relationships between voltage, current, resistance, and power

Electrical Circuit Fundamentals

In any electrical circuit, voltage (V) acts as the electrical pressure that pushes current (I) through resistance (R). The relationship is linear and predictable, making Ohm’s Law invaluable for circuit design, analysis, and troubleshooting.

When you know any two of the four electrical quantities (voltage, current, resistance, power), you can calculate the other two using the mathematical relationships derived from Ohm’s Law and the power equation.

Ohm’s Law Formulas

Ohm’s Law – Basic Relationships

Voltage from Current and Resistance:

V = I × R

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

The most fundamental form of Ohm’s Law: voltage equals current multiplied by resistance. This shows that higher resistance requires higher voltage to maintain the same current flow.

Current from Voltage and Resistance:

I = V ÷ R

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

Current equals voltage divided by resistance. This explains why low-resistance circuits draw more current for the same voltage.

Resistance from Voltage and Current:

R = V ÷ I

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

Resistance equals voltage divided by current. This is used to determine the resistance of unknown components or calculate required resistance values for circuit design.

Power Calculations

Power from Voltage and Current:

P = V × I

P = Power in watts (W)
V = Voltage in volts (V)
I = Current in amperes (A)

Electrical power equals voltage multiplied by current. This represents the rate at which electrical energy is converted to other forms (heat, light, mechanical work).

Power from Current and Resistance:

P = I² × R

P = Power in watts (W)
I = Current in amperes (A)
R = Resistance in ohms (Ω)

Power equals current squared multiplied by resistance. This form is useful for calculating heat dissipation in resistors and power losses in conductors.

Power from Voltage and Resistance:

P = V² ÷ R

P = Power in watts (W)
V = Voltage in volts (V)
R = Resistance in ohms (Ω)

Power equals voltage squared divided by resistance. This form is commonly used for calculating maximum power transfer and efficiency in electrical systems.

Practical Examples

💡 Example 1: LED Circuit Design

Scenario: Designing a circuit for a 3.3V LED that draws 20mA (0.02A)

Calculation:
R = V ÷ I = 3.3V ÷ 0.02A = 165Ω
P = V × I = 3.3V × 0.02A = 0.066W

Application: Use a 165Ω resistor (nearest standard value: 160Ω) with a 0.25W power rating. This ensures the LED operates safely within its specifications and prevents overheating.

🔌 Example 2: Household Wiring

Scenario: Determining wire size for a 1,500W heater on a 120V circuit

Calculation:
I = P ÷ V = 1,500W ÷ 120V = 12.5A
V = I × R (where R is wire resistance per foot)

Application: This circuit requires 12-gauge wire (20A capacity) with proper overcurrent protection. Understanding current draw prevents wire overheating and fire hazards.

🏭 Example 3: Industrial Motor Circuit

Scenario: Sizing components for a 5kW motor at 480V three-phase

Calculation:
I = P ÷ (V × √3 × PF) = 5,000W ÷ (480V × 1.732 × 0.85) ≈ 7.1A per phase
R = V ÷ I = 480V ÷ 7.1A ≈ 67.6Ω

Application: This motor requires appropriately sized conductors and a 10A circuit breaker. Proper calculations ensure system efficiency and compliance with electrical codes.

Applications & Use Cases

Circuit Design & Analysis

Electrical engineers use Ohm’s Law for designing circuits, selecting components, and analyzing electrical systems. Understanding the relationships between voltage, current, and resistance ensures circuits operate within safe parameters and component specifications. For advanced circuit analysis, try our Amps to Watts Calculator and Watt-Hour Calculator.

Component Selection

Ohm’s Law helps determine appropriate resistor values, wire gauges, and circuit breaker ratings. Proper component sizing prevents overheating, voltage drop, and electrical failures while ensuring optimal system performance. Use our Watts to Ohms Calculator for power-based resistance calculations.

Troubleshooting Electrical Systems

When electrical problems occur, Ohm’s Law helps diagnose issues by measuring voltage, current, or resistance. This systematic approach quickly identifies faulty components or wiring problems in electrical systems. For voltage-related troubleshooting, check our Volts to Amps Calculator.

Power Management

Understanding power relationships helps optimize energy efficiency and calculate electrical costs. Ohm’s Law is essential for designing energy-efficient systems and managing electrical loads in residential and industrial applications. For comprehensive power analysis, use our Horsepower to Amps Calculator and Torque Calculator.

⚠️ Electrical Safety Disclaimer

This calculator provides theoretical calculations based on standard electrical engineering formulas for educational and planning purposes. Working with electrical circuits can be dangerous and should only be performed by qualified, licensed electricians following local electrical codes and safety regulations.

Table of Contents

Author

Author

Manish Kumar
Manish holds a B.Tech in Electrical and Electronics Engineering (EEE) and an M.Tech in Power Systems, with over 10 years of experience in Metro Rail Systems, specializing in advanced rail infrastructure.
He is also a NASM-certified fitness and nutrition coach with more than a decade of experience in weightlifting and fat loss coaching. With expertise in gym-based training, lifting techniques, and biomechanics, Manish combines his technical mindset with his passion for fitness.

Ohm’s Law Calculator – Voltage, Current, Resistance & Power