Ohm's Law Calculator to Calculate Voltage, Current, and Resistance

What is Ohm's Law?

Ohm's Law is one of the most foundational principles in electrical science. Formulated by German physicist Georg Simon Ohm in 1827, it defines the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. It is used by electrical engineers, students, and technicians worldwide every single day.

⚡ Ohm's Law Calculator

Enter any two electrical values and instantly calculate the third — voltage, current, or resistance.

Calculate Now →

📐 The Three Forms of Ohm's Law

Voltage

V = I × R

Volts (V)

Current

I = V / R

Amperes (A)

Resistance

R = V / I

Ohms (Ω)

⚡ Power — The Fourth Variable

Combining Ohm's Law with the power formula (P = V × I) allows you to calculate electrical power dissipated in a circuit:

P = V × I

P = I² × R

P = V² / R

Energy (J) = P × t

🔌 Real-World Example: LED Circuit

You want to connect an LED (forward voltage 2V) to a 9V battery. The LED needs 20 mA (0.020 A) of current. What resistor do you need?

Voltage across the resistor = 9V − 2V = 7V
R = V / I = 7 / 0.020 = 350 Ω
Power in resistor = V × I = 7 × 0.020 = 0.14 W

Use a standard 360Ω or 330Ω resistor rated for at least 0.25W. ✅

📝 How to Use the Calculator

Select What to Find Choose whether you want to calculate Voltage, Current, or Resistance.

Enter Two Known Values Input the two known electrical quantities with their units.

Click Calculate Instantly get the unknown value plus power dissipation.

Read Results View voltage, current, resistance, and power all at once.

❓ FAQ

Does Ohm's Law work for AC circuits?

Ohm's Law applies to both DC and AC circuits, but for AC circuits with capacitors and inductors, resistance is replaced by impedance (Z), and V = I × Z.

What is a non-Ohmic conductor?

A non-Ohmic conductor is one where V and I are not proportional. Examples include diodes, transistors, and LEDs — they deviate from a straight-line V-I graph.

What is Ohm's Law?