Ohm's law for vapers: the only maths your mod needs

Ohm's law says that voltage equals current multiplied by resistance, written V = I x R. Voltage (V) is the push from the battery, current (I, in amps) is how much electricity flows, and resistance (R, in ohms) is your coil.

For vaping we also care about power, measured in watts, because that is what your mod displays. Power is voltage multiplied by current, P = V x I. Combine the two and you get the formula that explains almost everything about coils: P = V squared divided by R.

You can stop there. Those two lines, plus the amps rule further down, cover every question a vaper realistically asks.

Pop your coil resistance and a voltage into the calculator. A fully charged lithium-ion cell sits at about 4.2V and drifts down towards 3.2V as it empties, while the nominal figure printed on most cells is 3.6 or 3.7V.

Notice what happens when you halve the resistance. At the same voltage the watts double and so do the amps. That is why a 0.15 ohm coil needs a beefy mod and why the battery inside a small pod kit is paired with coils of 0.6 ohm and above.

If you would rather think in watts, which is how most regulated mods work, the calculator also shows the voltage the mod has to deliver to hit that number.

Take a 0.8 ohm pod at 3.7V. Volts squared is 13.69, divided by 0.8 gives about 17W, and the current is 3.7 divided by 0.8, roughly 4.6A. Gentle stuff for any battery.

Now a 0.2 ohm sub-ohm coil set to 50W on a regulated mod. The mod needs about 3.16V across the coil (the square root of 50 x 0.2), and the coil draws roughly 15.8A. The mod pulls even more from the battery when its voltage sags, because the chip has to make up the difference.

The bars show how coil current climbs as resistance falls, all at the same 3.7V.

Every removable vape battery has a continuous discharge rating in amps. That is the current it can supply steadily without overheating. Exceed it and the cell gets hot, its voltage collapses and, in the worst case, it can vent.

On a regulated mod, the chip limits what it will fire and refuses resistances that are too low. On an unregulated mechanical mod nothing stands between your coil and the cell, so the amps from Ohm's law go straight through the battery. Always check your build against the cell rating with a margin to spare.

Buy genuine cells from known brands in our 18650 batteries range and carry spares in a proper case, never loose in a pocket with keys or coins.

A few beliefs keep doing the rounds among vapers, and each one comes from half understanding the formula. Tap each card to see what the maths actually says.

The big one is the idea that a lower ohm coil is automatically hotter. On a regulated mod set to wattage mode, the mod adjusts the voltage to deliver whatever watts you choose, so a 0.4 ohm coil and a 0.2 ohm coil both at 40W receive the same power. What changes is how quickly the metal heats and how much surface area spreads that heat.

Another myth claims voltage is irrelevant now that everything runs in watts. It still matters, because a sagging battery limits how much power a mod can actually deliver to a very low resistance coil.

For most vapers using pre-made coils on a regulated device, Ohm's law is background knowledge. It explains the watt ranges printed on coils and why some tanks need a bigger mod, and that is useful when you shop for replacement coils.

For rebuilders it is essential. When you wrap your own coil you choose its resistance, so you also choose how many amps it will pull. Measure every build on an ohm reader or your mod before firing.

If you are curious about building, our beginner guide to RTAs, RDAs and RDTAs is the next step. Now check yourself.