Ohm's Law Calculator - Calculate Voltage, Current, Resistance, Power

Free online Ohm's Law calculator. Calculate voltage, current, resistance, or power using V=IR and power formulas. Essential tool for electrical engineering and electronics.

10 min read

Voltage

Current

Resistance

Power

V=IR
P=IV

Ohm's Law Calculator

Free online Ohm's Law calculator. Calculate voltage, current, resistance, or power using V=IR and power formulas. Essential tool for electrical engineering and electronics.

iAbout This Calculator

The Ohm's Law Calculator is an essential tool for anyone working with electrical circuits. Named after German physicist Georg Simon Ohm, Ohm's Law describes the fundamental relationship between voltage (V), current (I), and resistance (R) in an electrical circuit: V = I x R. This calculator extends beyond basic Ohm's Law to include power calculations (P = I x V, P = I^2 x R, P = V^2 / R), making it invaluable for electrical engineers, electronics hobbyists, students, and anyone designing or troubleshooting circuits. Simply enter any two known values, and the calculator instantly computes the remaining electrical parameters.

?How to Use

1
Select which two values you know from the dropdown menu: Voltage & Current, Voltage & Resistance, Voltage & Power, Current & Resistance, Current & Power, or Resistance & Power.
2
Enter the first known value in its field. Use standard units: Volts (V) for voltage, Amperes (A) for current, Ohms for resistance, and Watts (W) for power.
3
Enter the second known value in its corresponding field.
4
Click Calculate to see all four electrical values (V, I, R, P) computed instantly.
5
Review the step-by-step breakdown showing which formulas were used for the calculation.

fFormula

V = I \times R \quad P = I \times V \quad P = I^2 \times R \quad P = \frac{V^2}{R}

Ohm's Law states that voltage equals current times resistance (V = IR). Combined with the power formula (P = IV), we can derive all relationships: V = IR, I = V/R, R = V/I, P = IV = I^2R = V^2/R. These formulas form the foundation of electrical circuit analysis.

V: Voltage in Volts - the electrical potential difference
I: Current in Amperes - the flow of electric charge
R: Resistance in Ohms - opposition to current flow
P: Power in Watts - rate of energy transfer

Examples

LED Resistor Calculation

Inputs: voltage: 12, current: 0.02

R = 600 Ohm, P = 0.24 W

For a 20mA LED on a 12V supply: R = 12V / 0.02A = 600 Ohm. Power dissipated = 0.24W, so a 1/4W resistor is sufficient.

Space Heater Power

Inputs: voltage: 120, resistance: 10

I = 12 A, P = 1440 W

A heater with 10 Ohm resistance on 120V draws 12A current and consumes 1440W (1.44kW) of power.

USB Device Current

Inputs: voltage: 5, power: 2.5

I = 0.5 A, R = 10 Ohm

A 2.5W USB device operating at 5V draws 0.5A (500mA) and has an equivalent resistance of 10 Ohms.

Use Cases

Electronics Design

Calculate component values for circuits, determine resistor values for LEDs, and design voltage dividers.

Electrical Engineering

Size wires, fuses, and circuit breakers based on expected current draw and power consumption.

Solar Panel Systems

Calculate current flow, power output, and wire sizing for solar installations.

Home Electrical Work

Determine power consumption of appliances and ensure circuits are properly rated.

Education & Learning

Verify homework problems and build intuition for electrical relationships in physics classes.

Frequently Asked Questions

What is Ohm's Law and why is it important?

Ohm's Law (V = IR) describes the fundamental relationship between voltage, current, and resistance in electrical circuits. It's essential for designing circuits, troubleshooting electrical problems, and understanding how electrical systems work. Named after Georg Simon Ohm (1789-1854), this law is the foundation of electrical engineering.

Does Ohm's Law work for all electrical components?

Ohm's Law works perfectly for pure resistive components (resistors, heating elements). However, semiconductors (diodes, transistors), capacitors, and inductors don't follow Ohm's Law linearly. For AC circuits with reactive components, you need to consider impedance rather than simple resistance.

How do I calculate the resistor needed for an LED?

Use the formula R = (Vsupply - Vled) / Iled. For example, for a red LED (Vled = 2V, Iled = 20mA) on a 5V supply: R = (5-2)/0.02 = 150 Ohms. Choose the next higher standard value and ensure the resistor's power rating exceeds the calculated power dissipation.

What's the difference between AC and DC for Ohm's Law?

For DC circuits, Ohm's Law applies directly. For AC circuits with resistive loads only, it also works using RMS values. However, AC circuits with capacitors or inductors require considering impedance (Z) instead of resistance, using V = IZ and complex number calculations.

How do I convert between different units?

For current: 1A = 1000mA = 1,000,000uA. For resistance: 1MOhm = 1000kOhm = 1,000,000 Ohm. For voltage: 1kV = 1000V. For power: 1kW = 1000W. This calculator automatically handles large and small values with appropriate unit prefixes.

Conclusion

The Ohm's Law Calculator is an indispensable tool for electrical work, from simple LED circuits to complex power systems. Understanding the relationships between voltage, current, resistance, and power is fundamental to electrical engineering and electronics. Whether you're a student learning circuit basics, a hobbyist building projects, or a professional designing systems, this calculator provides instant, accurate results for any electrical calculation. Bookmark this page for quick access whenever you need to solve electrical problems.