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

For DC circuits, Ohm's law applies directly. For AC circuits, resistance is replaced by impedance (Z), which includes resistance plus the effects of capacitance and inductance: V = I × Z.

How does temperature affect resistance?

Most metals increase in resistance as temperature rises (positive temperature coefficient). Semiconductors decrease in resistance with temperature. This is why light bulb filaments have lower resistance when cold.

What is a short circuit?

A short circuit has near-zero resistance, causing extremely high current (I = V/R where R approaches 0). This can melt wires, start fires, and damage equipment. Fuses and circuit breakers protect against shorts.

How are series and parallel resistance calculated?

Series: R_total = R1 + R2 + R3. Parallel: 1/R_total = 1/R1 + 1/R2 + 1/R3. Series adds resistance, parallel reduces it. Two equal resistors in parallel give half the resistance of each.

Ohm's Law Calculator

Calculate voltage, current, or resistance

Ohm's Law

Enter any two values to find the third

Voltage (V)

Current (A)

Resistance (Ohms)

Formula

V = I x R, I = V / R, R = V / I

What Is Ohm's Law?

Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. Formulated by Georg Simon Ohm in 1827, it states that the voltage across a conductor is directly proportional to the current flowing through it, given a constant temperature. This fundamental principle underpins nearly all of modern electronics.

The law is expressed as V = I × R, meaning voltage equals current multiplied by resistance. By rearranging, you can solve for any of the three variables: I = V ÷ R gives current, and R = V ÷ I gives resistance. Engineers, students, and hobbyists use Ohm's Law daily to design circuits, select components, and troubleshoot electrical problems.

How to Use This Calculator

Select which quantity you want to calculate: Voltage (V), Current (I), or Resistance (R).
Enter the two known values in the appropriate fields — include the correct units (volts, amps, ohms).
Click Calculate to instantly see the missing value.
Use the Reset button to clear all fields and start a new calculation.

Formula & Explanation

V = I × R
I = V ÷ R
R = V ÷ I

V = Voltage in volts (V), I = Current in amperes (A), R = Resistance in ohms (Ω). All three forms are equivalent — rearrange as needed for your unknown.

Worked Examples

Example 1 — Find Voltage

A resistor has a resistance of 47 Ω and a current of 0.1 A flowing through it. Voltage = 0.1 × 47 = 4.7 V.

Example 2 — Find Current

A 12 V battery is connected across a 1,000 Ω resistor. Current = 12 ÷ 1,000 = 0.012 A = 12 mA.

Example 3 — Find Resistance

A lamp operates at 120 V and draws 0.5 A. Resistance = 120 ÷ 0.5 = 240 Ω.

Frequently Asked Questions

Does Ohm's Law apply to all materials?▾

Ohm's Law applies to ohmic materials — those with constant resistance regardless of voltage. Non-ohmic devices like diodes and transistors do not follow this linear relationship.

What units are used in Ohm's Law?▾

Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω). Using consistent SI units ensures correct calculations.

What happens to current if resistance doubles?▾

If resistance doubles while voltage stays constant, current is halved. This inverse relationship is a direct consequence of Ohm's Law (I = V ÷ R).

Can I use Ohm's Law for AC circuits?▾

Ohm's Law applies to purely resistive AC circuits. For circuits with capacitors or inductors, you need to work with impedance (Z) instead of pure resistance (R).

What is the power formula related to Ohm's Law?▾

Power (P) can be derived: P = V × I = I² × R = V² ÷ R. These are known as the power equations and are direct extensions of Ohm's Law.