Ohm's Law + Power Calculator

Ohm's Law is the fundamental relationship between voltage, current and resistance in an electrical circuit. This calculator solves for any unknown variable and also computes the electrical power dissipated.

Ohm's Law formulas

The basic relationships are:

• Voltage: V = I × R
• Current: I = V / R
• Resistance: R = V / I
• Power: P = V × I = I² × R = V² / R

Knowing any two values (V, I, R), you can compute all others including power.

Example 1: simple circuit

Problem: Voltage V = 12 V, resistance R = 4 Ω.

1. Current:
   • I = 12 / 4 = 3 A.
2. Power:
   • P = 12 × 3 = 36 W.

Answer: I = 3 A, P = 36 W.

Example 2: light bulb

Problem: A 60 W bulb connected to 120 V.

1. Current:
   • I = P / V = 60 / 120 = 0.5 A.
2. Resistance:
   • R = V / I = 120 / 0.5 = 240 Ω.

Answer: I = 0.5 A, R = 240 Ω.

Common uses of Ohm's Law

• Designing electronic and electrical circuits.
• Sizing resistors for LEDs and components.
• Computing electrical consumption of appliances.
• Diagnosing faults in electrical circuits.
• Selecting cables based on expected current.
• Solving electricity problems in education.

Common mistakes with Ohm's Law

• Confusing voltage (V) with current (I).
• Not using consistent units (volts, amperes, ohms).
• Applying Ohm's Law to non-ohmic components (diodes, transistors).
• Ignoring that resistance can vary with temperature.

Pro tip

Ohm's Law only applies directly to ohmic components (resistors). For diodes, transistors and other semiconductors, the V-I relationship is non-linear and more complex models are needed.