Ohm's Law states that voltage equals current multiplied by resistance: V = I × R. It describes the relationship between these three fundamental electrical quantities in a resistive circuit.

How do I calculate resistance with Ohm's Law?

Rearrange the formula to R = V / I. Divide voltage (volts) by current (amperes) to get resistance in ohms (Ω).

What is the unit of resistance?

The unit of resistance is the ohm, written as Ω (omega). It is named after physicist Georg Simon Ohm.

What happens if resistance is zero?

Zero resistance means a short circuit. In theory, current becomes infinite. In practice, wire resistance and the power source limit the current, but short circuits are dangerous and can damage components.

Ohm's Law Calculator

Enter any two values to instantly solve for the third — voltage, current, or resistance.

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Solve for:

Voltage (V)

Volts (V)

Current (I)

Amperes (A) — e.g. 0.02 for 20mA

Resistance (R)

Ohms (Ω)

Voltage

—V

⚠ Please enter valid numbers in both input fields.

What Is Ohm's Law?

Ohm's Law is the most fundamental relationship in electronics. It was formulated by German physicist Georg Simon Ohm in 1827 and states that the electric current through a conductor between two points is directly proportional to the voltage across those two points.

In simple terms: if you increase the voltage, more current flows. If you increase the resistance, less current flows. This relationship is the foundation of all circuit design.

The Formula

V = I × R

Where:

V = Voltage in Volts (V)
I = Current in Amperes (A)
R = Resistance in Ohms (Ω)

The formula can be rearranged to solve for any of the three variables:

V = I × R

Find Voltage

I = V ÷ R

Find Current

R = V ÷ I

Find Resistance

Worked Examples

Example 1 — Find the current through a resistor

Problem

A 9V battery is connected to a 470Ω resistor. How much current flows?

Solution: I = V ÷ R = 9 ÷ 470 = 0.0191 A = 19.1 mA

Example 2 — Find the required resistance

Problem

You want exactly 20 mA to flow from a 5V source. What resistance do you need?

Solution: R = V ÷ I = 5 ÷ 0.020 = 250 Ω (use nearest standard value: 270Ω)

Example 3 — Find the voltage drop

Problem

A current of 50 mA flows through a 100Ω resistor. What is the voltage across it?

Solution: V = I × R = 0.050 × 100 = 5 V

Frequently Asked Questions

What is Ohm's Law? ▼

Ohm's Law states that V = I × R — voltage equals current multiplied by resistance. It describes how these three quantities relate in any resistive electrical circuit.

How do I calculate resistance? ▼

Use R = V ÷ I. Divide the voltage (volts) by the current (amperes). The result is in ohms (Ω). Example: 12V ÷ 0.5A = 24Ω.

What unit is resistance measured in? ▼

Resistance is measured in ohms, written as Ω (omega). Larger values are expressed in kilohms (kΩ = 1,000Ω) or megohms (MΩ = 1,000,000Ω).

Does Ohm's Law apply to AC circuits? ▼

Ohm's Law applies directly to resistors in AC circuits. For capacitors and inductors, impedance (Z) replaces resistance. The formula becomes V = I × Z.

What is a short circuit? ▼

A short circuit has near-zero resistance. According to Ohm's Law, this causes very high current (I = V ÷ ~0), which can damage components or cause fires. Always protect circuits with fuses.

Ohm's Law Calculator

Calculate

What Is Ohm's Law?

The Formula

Worked Examples

Example 1 — Find the current through a resistor

Example 2 — Find the required resistance

Example 3 — Find the voltage drop

Frequently Asked Questions

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