Ohm's Law Calculator

Calculate voltage, current, resistance, and power using Ohm's Law (V = IR). Enter any two values to solve for the others instantly.

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.

Frequently Asked Questions

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 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.

Related Tools