The Ohm’s Law
Vaping Calculator
Know your numbers before you fire.
Enter any two known values and calculate the third. Built for vapers who build coils, tune wattage, and need to verify their amp draw before firing. Three modes: solve for Amps, Watts, or Resistance.
How to Use This Calculator
- Choose your mode using the tabs below — find Amps, Watts, or Resistance.
- Enter your known values. Use your ohm meter reading for resistance. Use 4.2V for a fully charged 18650, or your mod’s displayed output voltage.
- Hit Calculate. Your result appears instantly in the yellow readout.
- Compare amp draw to your battery’s CDR rating. If your draw exceeds the CDR, rebuild with more wraps or thicker wire to raise resistance.
Calculate Amp Draw — Enter Voltage and Resistance
Calculate Wattage — Enter Voltage and Resistance
Calculate Resistance — Enter Voltage and Wattage
Safety & Ohm’s Law — The Non-Negotiables
If you take only one thing from this page: check your resistance on a dedicated ohm meter before firing any build. Every single time. No exceptions.
As coil resistance decreases, current demand increases rapidly. A coil at 0.5 ohms on a fully charged cell at 4.2V draws 8.4 amps. A coil at 0.2 ohms at the same voltage draws 21 amps. Battery cells have rated continuous discharge limits (CDR) that must not be exceeded.
Battery CDR Quick Reference
| Resistance | Voltage | Amp Draw | 20A Battery | 30A Battery |
|---|---|---|---|---|
| 1.0 Ω | 4.2V | 4.2A | ✓ Safe | ✓ Safe |
| 0.5 Ω | 4.2V | 8.4A | ✓ Safe | ✓ Safe |
| 0.3 Ω | 4.2V | 14A | ⚠ Caution | ✓ Safe |
| 0.2 Ω | 4.2V | 21A | ✗ Exceeds CDR | ⚠ Caution |
| 0.1 Ω | 4.2V | 42A | ✗ Dangerous | ✗ Dangerous |
For sub-ohm building below 0.5 ohms, use high-drain cells from reputable manufacturers: Sony, Samsung, Murata, Molicel, and LG are the standard references. Avoid rewrapped cells — their advertised specs are frequently fraudulent.
Always Verify with a Dedicated Ohm Meter
Do not rely on your mod’s resistance reading alone. Always verify coil resistance on a dedicated ohm meter before the first fire. Mod displays can be off, especially on fresh builds. A $15 ohm meter is the best safety investment you can make.