Enter any two values. The other two are calculated automatically.
The Four Ohm's Law Relationships
| Find | Formula | Alternates |
|---|
| Voltage (V) | V = I × R | V = P / I, V = √(P × R) |
| Current (I) | I = V / R | I = P / V, I = √(P / R) |
| Resistance (R) | R = V / I | R = V² / P, R = P / I² |
| Power (P) | P = V × I | P = V² / R, P = I² × R |
Common Household Values
| Appliance | Voltage | Typical Watts | Amps (@ voltage) |
|---|
| LED bulb | 120V | 10W | 0.08A |
| Laptop charger | 120V | 65W | 0.54A |
| Microwave | 120V | 1,000W | 8.3A |
| Electric stove | 240V | 3,000W | 12.5A |
| EV charger (Level 2) | 240V | 7,200W | 30A |
Wire Gauge Selection
Wire must handle the load current safely. 14 AWG handles up to 15A (120V circuits, max 1,800W). 12 AWG handles 20A (2,400W). 10 AWG handles 30A (for dryers, ACs). Using wire too thin for the current creates heat, melts insulation, and causes fires. Always follow local electrical codes — they exist because of physics, not bureaucracy.
How to Use This Ohm's Law Calculator
Enter any two of: voltage (V), current (I), or resistance (R). The calculator solves for the missing value and shows power (P) as well.
Formula & How It Works
V = I × R (Ohm's Law). Power: P = V × I = I²R = V²/R. Current I = V/R. Resistance R = V/I.
Calculation Example
A circuit with 12V and 4Ω resistance: Current = 12/4 = 3 Amps. Power = 12 × 3 = 36 Watts.
Expert Tips
Remember: V=IR (Ohm's Law). For series circuits, resistances add. For parallel, reciprocals add: 1/Rtotal = 1/R1 + 1/R2. Always check that your units are consistent (V, A, Ω).