Ohm's Law Calculator
Solve for Voltage, Current, or Resistance with interactive visualizations and step-by-step solutions.
Ohm's Law Solver
Enter any two values to solve for the third.
Water Pipe Analogy
Circuit Diagram
Ohm's Law Triangle
Power Formulas (Watt's Law)
Understanding Ohm's Law
The Core Formula: V = I × R
Ohm's Law is the fundamental relationship between voltage, current, and resistance. It states that the voltage (V) across a component is directly proportional to the current (I) flowing through it.
The Ohm's Law Triangle:
This triangle is a visual tool to remember the three forms of the equation. Simply cover the variable you want to find:
- To find Voltage (V), cover V:
I × R - To find Current (I), cover I:
V / R - To find Resistance (R), cover R:
V / I
Voltage (V) - The "Push"
Measured in Volts (V), voltage is the electrical potential difference between two points. It's the "pressure" from a power source that pushes charged electrons (current) through a circuit. In the water analogy, it's the height difference creating water pressure.
Current (I) - The "Flow"
Measured in Amperes (A), current is the rate at which electric charge flows. It's the volume of electrons passing a point per second. In the water analogy, this is the flow rate of the water.
Resistance (R) - The "Restriction"
Measured in Ohms (Ω), resistance is the opposition to current flow. It determines how much current will flow for a given voltage. In the water analogy, this is the narrowness of the pipe restricting the flow.
Going Further: Electrical Power & Watt's Law
Electrical power (P), measured in Watts (W), is the rate at which energy is used. It's interconnected with Ohm's Law. The "Power Wheel" shows all the ways to calculate P, V, I, and R if you know any two variables.
To Find Power (P):
- If you know Voltage (V) and Current (I):
P = V × I - If you know Current (I) and Resistance (R):
P = I² × R - If you know Voltage (V) and Resistance (R):
P = V² / R
To Find Voltage (V), Current (I), or Resistance (R):
- Find Voltage (V):
V = P / IorV = √(P × R) - Find Current (I):
I = P / VorI = √(P / R) - Find Resistance (R):
R = V² / PorR = P / I²
Practical Example: Powering an LED
Imagine you have a 9V battery and an LED that requires 2V and can only handle 20mA (0.02A) of current. Connecting it directly would burn it out! Here's how to find the right resistor using Ohm's Law:
-
Find the required voltage drop: The resistor must "use up" the excess voltage.
9V (Battery) - 2V (LED) = 7V (for Resistor) -
Use Ohm's Law to find resistance: You need to limit the current to 0.02A across the resistor with a 7V drop.
R = V / I => 7V / 0.02A = 350Ω
Conclusion: You need a 350Ω resistor to safely power the LED.
The 'Aha!' Moment: An Electronics Hobbyist's Story
Meet Sarah, a hobbyist building her first circuit. She has a 5V power supply and a red LED. She connects it, and... *poof*! The LED burns out. Confused, she finds the Ohm's Law Calculator.
She learns an LED needs a specific current (e.g., 20mA or 0.02A) and has a forward voltage (e.g., 2V). The extra voltage from her power supply (5V - 2V = 3V) was causing too much current to flow.
Using the calculator, she enters V = 3V and I = 0.02A and leaves Resistance blank. The calculator instantly shows she needs a 150Ω resistor. The water pipe analogy makes it click: the resistor is like a narrow pipe that restricts the "flow" (current) to a safe level for the LED. Her next circuit works perfectly.
Frequently Asked Questions
How does the water pipe visualization work?
The visualization represents Voltage as the height difference (pressure) between two tanks of water. Resistance is the narrowness of the connecting pipe, and Current is the speed of the water particles flowing through it. A higher pressure (Voltage) or a wider pipe (less Resistance) results in a faster flow (higher Current).
What are some real-world applications of Ohm's Law?
Ohm's Law is fundamental to electronics. It's used to design circuits in your phone charger to provide the correct voltage, determine the resistance needed for an LED to light up without burning out, and calculate the current drawn by appliances like electric heaters or toasters.
What happens if resistance is zero?
If resistance is zero (a "short circuit"), the formula I = V/R suggests that the current would be infinite. In reality, the power source has its own internal resistance, and the massive current would cause the power source to fail or a fuse to blow.
What is Electrical Power (P)?
Power, measured in Watts (W), is the rate at which electrical energy is transferred or consumed in a circuit. It's calculated as P = V × I. Our calculator automatically computes the power dissipated by the resistor based on your inputs.