Ohm's Law Calculator

Enter any two values to calculate the remaining value and power. Leave one field empty.

V
A
Ω

Enter exactly 2 values. The third will be calculated automatically.

Voltage

—

V = I × R

Current

—

I = V / R

Resistance

—

R = V / I

Power

—

P = V × I

Understanding Ohm's Law

The Basic Formula

Ohm's Law defines the relationship between three fundamental electrical quantities:

  • V = I × R — Voltage equals current times resistance
  • I = V / R — Current equals voltage divided by resistance
  • R = V / I — Resistance equals voltage divided by current

Power can also be derived: P = V × I = I²R = V²/R

Series vs Parallel Circuits

Series circuits: Components are connected end-to-end. The total resistance is the sum of all individual resistances (R_total = R1 + R2 + R3). The same current flows through each component, but voltage divides across them.

Parallel circuits: Components share the same two connection points. The total resistance is calculated as 1/R_total = 1/R1 + 1/R2 + 1/R3. Each branch gets the full voltage, but total current divides among the branches.

Practical Applications

  • LED circuits: Calculate the correct resistor value to limit current through an LED
  • Fuse sizing: Determine the expected current draw to select the right fuse
  • Wire sizing: Know the current to choose appropriate wire gauge
  • Battery life: Calculate how long a battery will last given the circuit's current draw

Note: This calculator assumes ideal resistive loads. For AC circuits with reactive components, use impedance calculations instead.

Frequently Asked Questions

What is Ohm's Law?
Ohm's Law is a fundamental principle in electrical engineering that describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. It states that the voltage across a conductor is directly proportional to the current flowing through it, with resistance as the constant of proportionality. The formula is V = I × R.
How do I use this calculator?
Enter any two of the three values — voltage, current, or resistance — and the calculator will determine the missing value along with power. Leave the field you want to calculate empty or set to zero. For example, enter voltage and resistance to find the current, or enter current and resistance to find the voltage.
Does Ohm's Law apply to AC circuits?
Ohm's Law applies directly to DC circuits and to AC circuits with purely resistive loads. For AC circuits with capacitors or inductors, impedance (Z) replaces resistance in the formula, and you must account for phase angles. The generalized form becomes V = I × Z, where Z is a complex number representing both resistance and reactance.
What is the difference between resistance and impedance?
Resistance (measured in ohms) opposes current flow in DC circuits and dissipates energy as heat. Impedance is the AC equivalent and includes both resistance and reactance (from capacitors and inductors). While resistance is a single real number, impedance is a complex number with both magnitude and phase angle, accounting for how components store and release energy.

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