Ohm's Law Calculator

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

Enter any two values to calculate the third using Ohm's Law (V = I × R) and power formula (P = V × I). Perfect for electrical engineering and electronics.

Examples

Click on any example to load it into the calculator.

LED Circuit

LED Circuit

A typical LED circuit with voltage and current known, calculating resistance and power.

Voltage: 5 V

Current: 0.02 A

Resistor Power Rating

Resistor Power Rating

Calculating power dissipation in a resistor when voltage and resistance are known.

Voltage: 12 V

Resistance: 100 Ω

Battery Load

Battery Load

A battery powering a load with known voltage and power, calculating current and resistance.

Voltage: 9 V

Power: 15 W

DC Motor

DC Motor

A DC motor with known current and resistance, calculating voltage and power consumption.

Current: 3.5 A

Resistance: 2.4 Ω

Other Titles
Understanding Ohm's Law Calculator: A Comprehensive Guide
Master the fundamental principles of electrical circuits and learn how to calculate voltage, current, resistance, and power using Ohm's Law. This essential tool is perfect for electrical engineering, electronics, and circuit design.

What is Ohm's Law?

  • Fundamental Principles
  • Mathematical Relationship
  • Historical Context
Ohm's Law is one of the most fundamental principles in electrical engineering and physics. It describes the relationship between voltage (V), current (I), and resistance (R) in electrical circuits. Named after German physicist Georg Simon Ohm, who first published his findings in 1827, this law states that the current flowing through a conductor is directly proportional to the voltage applied across it and inversely proportional to its resistance.
The Core Formula: V = I × R
The mathematical expression of Ohm's Law is V = I × R, where V represents voltage in volts (V), I represents current in amperes (A), and R represents resistance in ohms (Ω). This simple equation is the foundation of electrical circuit analysis and design. It allows engineers and technicians to predict how electrical components will behave under different conditions and to design circuits that meet specific requirements.
Power Calculations: P = V × I
While Ohm's Law focuses on the relationship between voltage, current, and resistance, electrical power is calculated using the formula P = V × I, where P represents power in watts (W). Power represents the rate at which electrical energy is converted to other forms of energy, such as heat, light, or mechanical work. Understanding power is crucial for determining component ratings and energy efficiency.
Historical Significance and Discovery
Georg Ohm's discovery was revolutionary because it provided the first quantitative relationship between electrical quantities. Before Ohm's work, electricity was poorly understood and often considered mysterious. His systematic experiments with different materials and circuit configurations led to the formulation of this fundamental law, which remains valid for most practical electrical applications today.

Key Electrical Quantities:

  • Voltage (V): Electrical potential difference, measured in volts. Think of it as electrical 'pressure'.
  • Current (I): Flow of electrical charge, measured in amperes. Represents the rate of electron flow.
  • Resistance (R): Opposition to current flow, measured in ohms. Controls how much current flows.
  • Power (P): Rate of energy conversion, measured in watts. Determines component ratings and efficiency.

Step-by-Step Guide to Using the Calculator

  • Input Requirements
  • Calculation Process
  • Result Interpretation
Using the Ohm's Law calculator is straightforward, but understanding what each value represents and how to interpret the results is crucial for practical applications. This guide will walk you through the process step by step.
1. Determine What You Know
Start by identifying which electrical quantities you already know. You need at least two of the four values: voltage (V), current (I), resistance (R), or power (P). Common scenarios include knowing the voltage and current from a power supply, or having a resistor with known resistance and voltage across it. Be precise with your measurements and units.
2. Enter Your Known Values
Input your known values into the appropriate fields. Leave the fields you want to calculate empty. The calculator will automatically determine which values can be calculated based on your inputs. Ensure you're using consistent units: volts for voltage, amperes for current, ohms for resistance, and watts for power.
3. Review and Apply Results
After clicking 'Calculate', review all the calculated values. Check that the results make sense physically - for example, power should be positive, and resistance should be positive for passive components. Use these results to verify your circuit design, select appropriate components, or troubleshoot electrical issues.

Common Calculation Scenarios:

  • Voltage & Current Known: Calculate resistance and power for component selection
  • Voltage & Resistance Known: Calculate current and power for circuit analysis
  • Current & Resistance Known: Calculate voltage and power for power supply design
  • Voltage & Power Known: Calculate current and resistance for load analysis

Real-World Applications and Circuit Design

  • Component Selection
  • Circuit Analysis
  • Troubleshooting
Ohm's Law is not just a theoretical concept - it's a practical tool used daily by electrical engineers, technicians, and hobbyists. Understanding how to apply these calculations in real-world scenarios is essential for successful circuit design and troubleshooting.
Component Selection and Rating
One of the most common applications is selecting appropriate components for a circuit. For example, when designing an LED circuit, you need to calculate the required series resistance to limit current to a safe level. Using Ohm's Law, you can determine the exact resistor value needed based on the supply voltage, LED forward voltage, and desired current. Similarly, calculating power dissipation helps ensure components are properly rated.
Circuit Analysis and Verification
Ohm's Law is fundamental to analyzing complex circuits. By applying the law to individual components and circuit branches, you can predict voltage drops, current distribution, and power consumption throughout the circuit. This analysis is crucial for ensuring circuits operate within safe limits and meet design specifications.
Troubleshooting Electrical Problems
When electrical systems malfunction, Ohm's Law provides a systematic approach to troubleshooting. By measuring voltage and current at different points in a circuit, you can calculate expected resistance values and compare them to actual measurements. Deviations from expected values often indicate faulty components or connection problems.

Practical Applications:

  • LED Circuits: Calculate series resistance for proper current limiting
  • Power Supplies: Determine output current capability and efficiency
  • Motor Control: Calculate power requirements and heat generation
  • Audio Systems: Match speaker impedance to amplifier output

Common Misconceptions and Limitations

  • Linear vs. Non-Linear
  • Temperature Effects
  • Frequency Considerations
While Ohm's Law is incredibly useful, it's important to understand its limitations and when it doesn't apply. Many electrical components and materials don't follow Ohm's Law under all conditions, and understanding these exceptions is crucial for accurate circuit analysis.
Linear vs. Non-Linear Components
Ohm's Law applies to 'ohmic' or linear components, where the voltage-current relationship is constant. However, many electronic components are non-linear. Diodes, transistors, and LEDs have voltage-current relationships that change with operating conditions. For these components, Ohm's Law can only be applied over limited ranges or with significant approximations.
Temperature Effects on Resistance
Resistance is not always constant. Most materials change resistance with temperature - metals typically increase resistance with temperature (positive temperature coefficient), while semiconductors often decrease resistance with temperature (negative temperature coefficient). This temperature dependence can significantly affect circuit performance in real-world applications.
Frequency and AC Considerations
Ohm's Law in its basic form applies to DC (direct current) circuits. In AC (alternating current) circuits, additional factors come into play, including reactance (capacitive and inductive), impedance, and phase relationships. While the basic principles remain the same, the calculations become more complex and require consideration of frequency-dependent effects.

When Ohm's Law Doesn't Apply:

  • Diodes and LEDs: Non-linear voltage-current relationship
  • Capacitors and Inductors: Reactance depends on frequency
  • Semiconductors: Complex voltage-current characteristics
  • High-Frequency Circuits: Parasitic effects become significant

Mathematical Derivation and Advanced Concepts

  • Derivation from First Principles
  • Power Relationships
  • Series and Parallel Circuits
Understanding the mathematical foundation of Ohm's Law helps develop intuition for electrical circuits and prepares you for more advanced topics in electrical engineering.
Derivation from Electrical Field Theory
Ohm's Law can be derived from fundamental principles of electrical field theory. The electric field (E) in a conductor is related to the voltage gradient, and the current density (J) is related to the electric field through the material's conductivity (σ). This relationship leads directly to V = I × R, where resistance is inversely proportional to conductivity and directly proportional to length and inversely proportional to cross-sectional area.
Power Relationships and Energy Conservation
The power formula P = V × I can be derived from the definition of power as the rate of energy transfer. In electrical circuits, this energy is typically converted to heat, light, or mechanical work. By combining Ohm's Law with the power formula, we can derive additional relationships: P = I² × R and P = V² / R. These alternative forms are often more convenient for specific calculations.
Series and Parallel Circuit Analysis
Ohm's Law is the foundation for analyzing more complex circuits. In series circuits, current is the same through all components, while voltages add up. In parallel circuits, voltage is the same across all components, while currents add up. These principles, combined with Ohm's Law, allow analysis of any combination of series and parallel connections.

Advanced Applications:

  • Voltage Dividers: Calculate output voltage from input voltage and resistor ratios
  • Current Dividers: Determine current distribution in parallel branches
  • Power Matching: Optimize power transfer between source and load
  • Temperature Compensation: Account for resistance changes with temperature