Ohm's Law Calculator
Enter any two known values to calculate voltage, current, resistance and power
Leave the values you want to find blank. Enter any two known values — the other two will be calculated.
// All Ohm's Law Formulas
// Resistor Colour Code Chart
| Colour | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | ×1 | — |
| Brown | 1 | ×10 | ±1% |
| Red | 2 | ×100 | ±2% |
| Orange | 3 | ×1k | — |
| Yellow | 4 | ×10k | — |
| Green | 5 | ×100k | ±0.5% |
| Blue | 6 | ×1M | ±0.25% |
| Violet | 7 | ×10M | ±0.1% |
| Grey | 8 | ×100M | ±0.05% |
| White | 9 | ×1G | — |
| Gold | — | ×0.1 | ±5% |
| Silver | — | ×0.01 | ±10% |
Ohm's Law Explained
Ohm's Law states that voltage across a conductor is directly proportional to the current flowing through it, provided temperature remains constant. The constant of proportionality is resistance.
The Four Quantities
Practical Examples
A 12V car battery powering a 60W headlight: I = P/V = 60/12 = 5A. The filament resistance: R = V/I = 12/5 = 2.4Ω. A USB charger supplying 5V at 2A delivers: P = V×I = 5×2 = 10W.
From the Blog
// The VIR Triangle
Cover the value you want to find in the V/I/R triangle. What's left is the formula: V on top, I and R on the bottom (multiply or divide).
// Safe Current
The human body's resistance is roughly 1,000–100,000 Ω. At 230V mains: I = 230/1000 = 230mA — well above the 100mA lethal threshold.
// Series vs Parallel
Resistors in series: Rtotal = R1+R2. In parallel: 1/Rtotal = 1/R1 + 1/R2. Parallel circuits split current; series circuits split voltage.
// Power Dissipation
P = I²R is important for heat calculations. Double the current → four times the heat. This is why fuses blow when current gets too high.