Ohm's Law Calculator
Calculate voltage, current, and resistance
Ohm's Law Calculator
Calculate voltage, current, resistance, and power
Ohm's Law Formulas
Privacy & Security
All calculations happen entirely in your browser using JavaScript. No circuit data, values, or calculations are ever uploaded to our servers, stored in databases, logged, or shared with anyone. Your work remains completely private and secure.
About Ohm's Law Calculator
Master electrical calculations with our free Ohm's Law calculator designed for electronics engineers, students, electricians, and DIY enthusiasts. Whether you're designing circuits, troubleshooting electrical systems, selecting resistors, or learning electrical fundamentals, our tool instantly calculates voltage (V), current (I), resistance (R), and power (P) using the fundamental relationships: V=IR, P=VI, P=I²R, and P=V²/R. Simply enter any two values and the calculator determines the remaining parameters with detailed formulas showing exactly how each result is derived.
Key Features
Calculate Any Parameter
Find voltage, current, resistance, or power by entering any two known values
Multiple Unit Support
Work with volts (V, mV, kV), amperes (A, mA, μA), ohms (Ω, kΩ, MΩ), and watts (W, mW, kW)
Power Calculations
Automatically calculates power dissipation using P=VI, P=I²R, and P=V²/R formulas
Formula Display
Shows the exact formulas used for each calculation to help you learn
Instant Results
Real-time calculations as you type with no delays
Circuit Diagram
Visual representation of the circuit with labeled voltage, current, and resistance
Engineering Notation
Results displayed in standard engineering notation for easy reading
Copy Values
One-click copy for quick use in documentation or datasheets
Error Validation
Prevents invalid inputs and ensures physically possible results
Completely Free
No registration, no limits, unlimited calculations for all your projects
How to Use the Ohm's Law Calculator
Select Calculation Mode
Choose what you want to calculate: voltage, current, resistance, or power
Enter Known Values
Input any two parameters from your circuit (e.g., voltage and resistance)
View Results Instantly
See all calculated parameters including formulas and power dissipation
Copy or Use Values
Copy results to use in circuit design, component selection, or homework
Frequently Asked Questions
What is Ohm's Law and why is it important?
Ohm's Law is the fundamental relationship in electrical circuits discovered by Georg Ohm in 1827. It states that voltage (V) equals current (I) times resistance (R): V = I × R. This law is essential for all electrical and electronics work because it allows you to calculate unknown circuit parameters, design safe circuits, select proper components, and understand how electricity behaves in conductors.
How do I calculate power dissipation in a resistor?
Power dissipation can be calculated three ways depending on known values: P = V × I (power equals voltage times current), P = I² × R (power equals current squared times resistance), or P = V² / R (power equals voltage squared divided by resistance). Our calculator automatically uses the appropriate formula based on your inputs and displays power in watts (W). This is critical for selecting resistors with adequate power ratings to prevent overheating and component failure.
What units does the calculator support?
The calculator supports multiple units for flexibility: voltage in volts (V), millivolts (mV), and kilovolts (kV); current in amperes (A), milliamperes (mA), and microamperes (μA); resistance in ohms (Ω), kilohms (kΩ), and megohms (MΩ); power in watts (W), milliwatts (mW), and kilowatts (kW). Results are automatically displayed in the most appropriate engineering notation.
Can I use this for AC circuits?
This calculator is designed for DC (direct current) circuits and resistive loads. For AC circuits with capacitors and inductors, you need to consider impedance (Z) instead of resistance (R), and calculations become more complex involving phase angles and reactive power. For simple resistive AC loads like heaters and incandescent bulbs, Ohm's Law still applies using RMS (root mean square) values.
Why is knowing power dissipation important?
Power dissipation determines how much heat a component generates. Resistors are rated for maximum power (typically 1/4W, 1/2W, 1W, etc.). If actual power dissipation exceeds the rating, the component overheats, changes value, or fails completely. Always select components with power ratings at least 2× higher than calculated dissipation for safety and reliability.
Is this calculator accurate for real circuits?
Yes, the calculations are mathematically precise for ideal resistive circuits. However, real-world circuits have additional factors like component tolerances (±5%, ±10%), temperature coefficients, contact resistance, and wire resistance. For critical applications, always measure actual values and include safety margins in your designs.
Is this tool free to use?
Absolutely! Our Ohm's Law calculator is completely free with unlimited calculations, no registration required, and no hidden costs. All processing happens in your browser for instant results.
Does it work on mobile devices?
Yes! The calculator is fully responsive and works perfectly on smartphones, tablets, and desktop computers. Great for on-the-go calculations in the lab, workshop, or classroom.
Common Use Cases
- Circuit Design: Calculate required resistor values to achieve desired current in LED circuits, voltage dividers, and sensor interfaces
- Component Selection: Determine power ratings needed for resistors, transistors, and other components to prevent overheating
- Troubleshooting: Verify voltage drops, current flows, and resistance values when debugging faulty circuits
- Electronics Education: Learn and practice Ohm's Law calculations for physics, electronics, and electrical engineering courses
- Battery Calculations: Calculate current draw and power consumption to estimate battery life and select appropriate batteries
- Arduino & Raspberry Pi: Design safe circuits with correct resistor values for LEDs, sensors, and GPIO protection
- Power Supply Design: Calculate load current and power requirements for power supply and voltage regulator selection
- Home Electrical Work: Determine wire gauge, circuit breaker ratings, and verify safe electrical loads for household wiring
Why Use Our Ohm's Law Calculator?
Manual electrical calculations are time-consuming and error-prone, especially when working with different units or calculating power dissipation. Our free Ohm's Law calculator eliminates calculation errors by instantly computing all four electrical parameters from any two known values, automatically handling unit conversions, and showing you the exact formulas used. Whether you're a student learning circuit theory, an engineer designing electronics, a hobbyist building Arduino projects, or a technician troubleshooting equipment, you need accurate calculations fast. Stop reaching for your scientific calculator and manual formulas - just enter your values and get instant, reliable results with proper engineering notation and power ratings for safe component selection.