How do you calculate Power?

Power (measured in Watts) is calculated by multiplying Voltage by Current (P = V × I). Alternatively, you can use P = V² / R or P = I² × R.

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

Q: What is Ohm's Law?

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points. The standard equation is V = I × R.

What Is Ohm's Law?

Ohm's Law is the most fundamental and universally applied principle in all of electrical engineering and physics. Formulated by the German physicist Georg Ohm in 1827, the law defines the mathematical relationship between three core properties of an electrical circuit: Voltage (V), Current (I), and Resistance (R).

In its simplest form, Ohm's Law states that the electrical current flowing through a conductor between two points is directly proportional to the voltage across those two points, and inversely proportional to the resistance between them. If you double the voltage pushing the electrons, the current doubles. If you double the resistance restricting the electrons, the current is cut in half.

A true Ohm's Law Calculator goes beyond the basic V=IR equation by also incorporating Joule's Law, which introduces Power (P) into the equation. This allows you to instantly solve for any unknown variable in an electrical circuit simply by providing any two known variables.

How to Use This Ohm's Law Calculator

This calculator is designed to act as an automatic "Ohm's Law Wheel." Because the four variables (Voltage, Current, Resistance, Power) are mathematically locked together, you only need to input two values to solve the entire circuit.

Step-by-Step Instructions

Identify Your Knowns: Look at your circuit, schematic, or homework problem and identify any two values that you already know.
Enter the First Value: Click on the corresponding field (e.g., Voltage) and type the number.
Enter the Second Value: Click on the second known field (e.g., Resistance) and type the number.

The Magic of Auto-Calculation: As soon as you type the second number, the calculator instantly runs all 12 possible algebraic variations of Ohm's Law in the background and populates the remaining two fields. The calculated values will light up in blue on the dashboard above.

If you need to calculate a different scenario, you can simply type over one of the existing numbers, or click the "Reset All Fields" button to clear the board.

Understanding the 4 Core Electrical Units (The Water Analogy)

To truly understand how this calculator works, it helps to visualize electricity flowing through a wire as water flowing through a pipe.

Voltage (V - Volts): Voltage is the electrical pressure. In our analogy, it is the water pump pushing the water. Higher voltage means more pressure forcing the electrons to move.
Current (I - Amperes/Amps): Current is the flow rate. It measures the volume of electrons passing a single point per second. In our analogy, it is the gallons-per-minute of water flowing through the pipe.
Resistance (R - Ohms/Ω): Resistance is the restriction to the flow. It is the friction inside the wire. In our analogy, it represents a clog in the pipe, or a section of the pipe that suddenly becomes very narrow. High resistance lowers the current.
Power (P - Watts): Power is the total work performed by the circuit. It is a combination of pressure (Voltage) and flow (Current). In our analogy, it is the total force of the water hitting a waterwheel to grind grain. More pressure and more flow equals more total power.

The 12 Ohm's Law Formulas (The Magic Wheel)

While the basic Ohm's Law triangle (V = I × R) only has three formulas, combining it with Watt's Law (P = V × I) yields a complete set of 12 algebraic formulas. Our engine uses these exact equations to solve your inputs.

To Solve for Voltage (V):

V = I × R (Current multiplied by Resistance)
V = P ÷ I (Power divided by Current)
V = √(P × R) (Square root of Power multiplied by Resistance)

To Solve for Current (I):

I = V ÷ R (Voltage divided by Resistance)
I = P ÷ V (Power divided by Voltage)
I = √(P ÷ R) (Square root of Power divided by Resistance)

To Solve for Resistance (R):

R = V ÷ I (Voltage divided by Current)
R = V² ÷ P (Voltage squared, divided by Power)
R = P ÷ I² (Power divided by Current squared)

To Solve for Power (P):

P = V × I (Voltage multiplied by Current)
P = I² × R (Current squared, multiplied by Resistance)
P = V² ÷ R (Voltage squared, divided by Resistance)

Practical Examples of Ohm's Law in Action

Example 1: Sizing a Resistor for an LED

Imagine you have a 9V battery and want to light a small LED. The LED requires exactly 2 Volts and 0.02 Amps (20 milliamps) to operate safely. If you hook it directly to the 9V battery, it will explode. You need a resistor to drop the remaining 7V.

The Math: You know the Voltage you need to drop (V = 7V), and you know the Current flowing through the circuit (I = 0.02A). Enter 7 into the Voltage field and 0.02 into the Current field of the calculator. It will instantly tell you that you need a 350 Ohm (Ω) resistor.

Example 2: Determining the Amp Draw of a Space Heater

You buy a space heater rated for 1500 Watts, and you want to plug it into a standard 120V wall outlet. You want to know if it will trip your 15-Amp circuit breaker.

The Math: You know the Power (P = 1500W) and the Voltage (V = 120V). Enter those two numbers into the calculator. It will instantly tell you the Current is 12.5 Amps. Since 12.5A is less than your 15A breaker limit, you are safe (though you shouldn't plug much else into that same circuit!).

Frequently Asked Questions

What is the formula for Ohm's Law? +

The foundational formula for Ohm's Law is V = I × R (Voltage equals Current multiplied by Resistance). From this single formula, you can derive I = V / R and R = V / I.

How do I calculate power using Ohm's Law? +

Technically, Power calculation falls under Joule's Law (or Watt's Law), but it is universally taught alongside Ohm's Law. The core power formula is P = V × I (Power equals Voltage multiplied by Current). If you don't know the voltage, you can substitute Ohm's Law into the power equation to get P = I² × R.

Can Ohm's Law be used for AC circuits? +

Ohm's Law works perfectly for all Direct Current (DC) circuits and for Alternating Current (AC) circuits that contain only purely resistive loads (like incandescent light bulbs or simple heating elements). However, for complex AC circuits containing motors or capacitors (inductive and capacitive loads), Resistance (R) is replaced by Impedance (Z), which accounts for phase shifts between voltage and current.

Why does resistance increase when wire gets longer? +

Resistance is cumulative. Every inch of wire has a tiny amount of resistance. If you double the length of the wire, you double the total amount of resistance the electrons have to fight through. This is why long cable runs require thicker wire to lower the resistance back down to an acceptable level.

What happens if voltage increases but resistance stays the same? +

According to Ohm's Law (I = V / R), if the Resistance remains constant, the Current is directly proportional to the Voltage. Therefore, if you increase the Voltage, the Current will increase proportionally. If you double the voltage, you will double the current flowing through the circuit.

Ohm's Law Engine.