What is Ohm's Law?

Ohm's Law describes the fundamental relationship between Voltage (V), Current (I), and Resistance (R) in an electrical circuit. When combined with the power equation, it forms the Power Wheel — a comprehensive set of 12 formulas that enable you to calculate any electrical quantity from just two known values.

Ohm's Law

V = I × R
Voltage equals Current times Resistance

Power Equation

P = V × I
Power equals Voltage times Current

Complete Formula Set: From these two fundamental equations, 10 additional formulas can be derived, covering every possible combination of V, I, R, and P. This calculator handles all 12 formulas automatically — simply enter any two values and instantly get the remaining calculations.

Who Uses This Calculator?

Students

Perfect for those studying electrical engineering or physics courses

Technicians

Essential for troubleshooting circuits and testing components

Hobbyists

Ideal for building DIY electronics and maker projects

Electricians

Useful for calculating wire sizes and load requirements

How to Use

Enter Two Known Values

The calculator provides four input fields: Voltage (V), Current (I), Resistance (R), and Power (P). Simply enter values in any two of these fields to begin your calculation.

Select the Right Unit

Each field includes a unit selector dropdown. Choose the appropriate unit for your measurement:

Voltage: mV (millivolts), V (volts), or kV (kilovolts)
Current: μA (microamps), mA (milliamps), or A (amps)
Resistance: mΩ (milliohms), Ω (ohms), kΩ (kilohms), or MΩ (megohms)
Power: mW (milliwatts), W (watts), or kW (kilowatts)

Read the Results

The remaining two values are calculated automatically and displayed in their respective fields. Input fields are marked with an Input badge, while calculated fields show a Result badge for easy identification.

Using Quick Examples

Click the Examples button to access pre-configured scenarios including LED circuits, USB charging, and household power calculations. Simply click any example to instantly load its values into the calculator.

Adjusting Precision

Use the Decimals selector to customize the number of decimal places displayed in results, ranging from 2 to 6 digits for optimal precision based on your needs.

Features

All 12 Formulas Covered

The calculator supports all six possible input combinations and automatically computes both unknown values using the appropriate formulas:

Known Values	Calculated Results	Formulas Used
V + I	Resistance & Power	`R = V/I`, `P = V×I`
V + R	Current & Power	`I = V/R`, `P = V²/R`
V + P	Current & Resistance	`I = P/V`, `R = V²/P`
I + R	Voltage & Power	`V = I×R`, `P = I²×R`
I + P	Voltage & Resistance	`V = P/I`, `R = P/I²`
R + P	Voltage & Current	`V = √(P×R)`, `I = √(P/R)`

SI Unit Support

Work seamlessly with real-world values using standard SI prefixes. Enter 4.7kΩ directly instead of manually converting to 4700Ω. The calculator handles all unit conversions internally, saving you time and reducing errors.

Interactive Power Wheel

Visual diagram displaying all 12 formulas organized by parameter. Active inputs and results are highlighted in color for easy formula identification.

Formula Display

Shows exactly which formula was used and the computed result with SI-formatted values, helping you verify calculations and understand relationships.

Smart Input Management

Uses only two inputs at a time. If you enter a third value, the oldest input is automatically cleared — keeping your workflow fast and intuitive.

Quick Examples

Six real-world examples are built in, covering common scenarios from LED circuits (3.3V, 20mA) to household power (230V, 100W). Use them to quickly test the calculator or as a practical learning reference.

Learning Tool: The examples demonstrate typical electrical values you'll encounter in everyday electronics, making this calculator both a practical tool and an educational resource.

Frequently Asked Questions

What is Ohm's Law?

Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, with resistance as the constant of proportionality: V = I × R. This fundamental law applies to most electrical circuits and components, forming the foundation of electrical engineering and circuit analysis.

What is the Power Wheel?

The Power Wheel (also called the Ohm's Law Wheel) is a circular diagram that combines Ohm's Law (V = IR) with the power equation (P = VI) to show all 12 formulas for calculating V, I, R, and P. Each quadrant of the wheel contains three formulas for one parameter, making it easy to find the right equation for any calculation.

Why can I only enter two values?

In a simple circuit, any two of the four quantities (V, I, R, P) uniquely determine the other two through mathematical relationships. Entering more than two values would either be redundant or create contradictions in the calculations. The calculator enforces this principle by keeping only the two most recent inputs, ensuring accurate and consistent results.

What units are supported?

The calculator supports standard SI prefixes across all measurements:

Voltage: millivolts (mV) to kilovolts (kV)
Current: microamps (μA) to amps (A)
Resistance: milliohms (mΩ) to megaohms (MΩ)
Power: milliwatts (mW) to kilowatts (kW)

All unit conversions are handled automatically, allowing you to work with values in their most natural form.

Why does entering 0 not produce a result?

A value of zero for voltage, current, resistance, or power leads to division by zero in most formulas, which is mathematically undefined. For example, if resistance is 0Ω, this represents a short circuit with theoretically infinite current. The calculator avoids these undefined results by requiring positive values for all inputs.

Important: Zero values represent special circuit conditions (short circuits, open circuits) that require different analysis methods beyond simple Ohm's Law calculations.

Is my data stored anywhere?

No. All calculations happen entirely in your browser using client-side JavaScript. No data is sent to any server, and nothing is stored after you close the page. Your calculations remain completely private and secure.

Find	Formula
V	V = I × R \| V = P / I \| V = √(P × R)
I	I = V / R \| I = P / V \| I = √(P / R)
R	R = V / I \| R = V² / P \| R = P / I²
P	P = V × I \| P = V² / R \| P = I² × R