Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points. The formula is V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. It also relates to power: P = V × I. It's one of the most fundamental principles in electrical engineering and electronics.

How do I calculate voltage using Ohm's Law?

Voltage is calculated as V = I × R (current × resistance). For example, if a circuit has 2 amps of current flowing through an 8-ohm resistor, the voltage across it is 16 volts. You can also calculate voltage from power and current using V = P / I, or from power and resistance using V = √(P × R).

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

Ohm's Law applies directly to DC (direct current) circuits. For AC (alternating current) circuits, the simple V = IR formula applies to resistive loads, but inductive and capacitive loads introduce reactance, requiring impedance (Z) instead of pure resistance. The AC version is V = I × Z, where Z is the vector sum of resistance and reactance.

What happens if I exceed a component's power rating?

Exceeding a component's power rating causes it to overheat. A resistor rated for 1/4W that dissipates 1/2W will become extremely hot, potentially burning out, desoldering itself, or damaging nearby components. Always choose components with a power rating at least twice your calculated dissipation for a safety margin.

How do I calculate the right resistor for an LED?

Use R = (V_supply − V_LED) / I_LED. With a 5V supply, a 2V LED, and a desired current of 20mA: R = (5 − 2) / 0.02 = 150 Ω. Choose the nearest standard resistor value. Power dissipation is small, so a 1/4W resistor works fine.

Ohm's Law Calculator — Voltage, Current, Resistance, Power

What Is Ohm's Law?

Ohm's Law describes the relationship between voltage, current, and resistance in electrical circuits. It states that current (I) equals voltage (V) divided by resistance (R): I = V / R. This is one of the most fundamental equations in electronics and electrical engineering. The law also extends to power calculations: P = V × I. By entering any two known values, you can calculate the other two.

Common Applications

Circuit design: Determine resistor values needed to achieve target current
Troubleshooting: Verify if components are operating within specifications
Power supply selection: Calculate required voltage and current for loads
Battery life: Estimate current draw and power consumption
LED circuits: Calculate current-limiting resistor values

Ohm's Law Triangle

The Ohm's Law triangle helps remember the three formulas: V = I × R, I = V / R, R = V / I. Cover the variable you want to find, and the remaining two show the operation. Add the power formula P = V × I for complete circuit analysis.

Practical Applications

Ohm's Law is essential for choosing the right resistor for an LED. If you have a 5V supply and a 2V LED that needs 20mA, the resistor must drop 3V at 20mA, giving R = 3 / 0.02 = 150 Ω. Using a resistor with too low a value can burn out the LED, while too high a value will make it too dim. This same principle applies to any component that needs current limiting.

In larger circuits, Ohm's Law helps calculate wire gauge requirements. A device drawing 10A over a wire with 0.1 Ω resistance will drop 1V (V = 10 × 0.1) and dissipate 10W of heat (P = 10² × 0.1). Undersized wires can overheat and become fire hazards. Always verify that your wiring can handle both the current and the power dissipation.

Ohm's Law Applications in Practice

Ohm's Law is used daily in circuit design, troubleshooting, and component selection. Engineers use it to determine whether a power supply can handle a given load, to calculate voltage drops across long cable runs, and to select appropriate fuse ratings. In automotive electrical systems, Ohm's Law helps diagnose issues like dim headlights (excessive resistance in wiring) or blown fuses (excessive current draw).

For component selection, Ohm's Law ensures that resistors, capacitors, and other parts operate within their rated specifications. A resistor used in a voltage divider must be chosen so that the current through it stays within its power rating. Similarly, when designing a PCB trace, the width must be sufficient to carry the expected current without overheating — a calculation that directly uses the relationship between current, resistance, and power dissipation.

Understanding Voltage, Current, and Resistance Intuitively

The water pipe analogy is the most common way to understand Ohm's Law intuitively. Imagine a water pipe system: voltage is like water pressure (pushing water through the pipe), current is like the flow rate (how much water passes per second), and resistance is like the pipe's narrowness (how much it restricts flow). Increasing the pressure (voltage) increases the flow (current), while making the pipe narrower (increasing resistance) decreases the flow.

This analogy helps make sense of real circuits. A battery's voltage is the "pressure" pushing electrons through the wire. A resistor is like a narrow section of pipe that limits how many electrons can flow. The power dissipated as heat is like the energy lost when water rushes through a constriction — the heat generated in a resistor is exactly that lost energy.

Power Formula Relationship (P = V × I)

Power in electrical circuits follows a straightforward relationship: P = V × I, where P is power in watts, V is voltage in volts, and I is current in amperes. This formula can be combined with Ohm's Law to derive other useful forms: P = I² × R (useful when you know current and resistance) and P = V² / R (useful when you know voltage and resistance).

Understanding power is critical for component selection. A 100 Ω resistor with 10V across it dissipates 1W (P = 10² / 100 = 1W). If that resistor is rated for only 1/4W, it will overheat and fail. Always check power ratings when designing circuits, and as a rule of thumb, select components rated for at least twice the expected power dissipation to provide a safety margin.

Practical Example: Calculating Resistor Values for LED Circuits

LEDs are one of the most common applications of Ohm's Law for hobbyists. An LED needs a current-limiting resistor to prevent it from drawing too much current and burning out. The formula is R = (V_supply - V_LED) / I_LED. For example, with a 5V supply, a red LED with a 2V forward voltage drop, and a desired current of 20mA (0.02A): R = (5 - 2) / 0.02 = 150 Ω.

Different LED colors have different forward voltage drops: red LEDs are typically around 1.8-2.2V, green LEDs around 2.0-3.0V, and blue or white LEDs around 3.0-3.6V. Always check the LED datasheet for exact values. The power dissipated by the resistor is small in LED circuits — in this example, P = I² × R = 0.02² × 150 = 0.06W — so a standard 1/4W resistor works perfectly fine. If you connect multiple LEDs in series, the voltage drops add up, so ensure your supply voltage is high enough to overcome the combined forward voltages.

Ohm's Law Calculator