You could also do it mathematically if you know the watts and volts. It'll save you the money from getting an ohm reader if you're really strapped for cash.
The formula is R = E ^ 2 / P.
An Example
Right now, I am running a build at 105 watts and 6.72 volts. To get the resistance, I would square the volts and divide by power because R = E ^ 2 / P.
So 6.72 * 6.72 (this is because we are squaring the voltage) is 45.1584, but we round up to the nearest half for safety's sake, so we get an answer of 45.5. Then divide that by 105, the power, and we get the resistance.
45.5 / 105 = 0.4333333333...
So I am running my build with a resistance of approximately 0.43 ohms.
Ohms law also says that you can figure out how much current you're putting through your batteries, which is also important if your device is unregulated. You have not specified this, therefore I will go through this with you just for safety's sake.
I know the resistance is 0.43, and the wattage is 105.
The formula to calculate current is I = √ P / R
This time, we are dividing the power by the resistance and square rooting the quotient.
So 105 / 0.43 gives us a quotient of approximately 244.18, but again, we're going to round up to the nearest half just for safety's sake. So we get 244.5.
√ 244.5 = 15.6364957.
So I am putting approximately 15.64 amps through my batteries. This build is safe for most batteries (unless you get really cheap ones that can only handle 15 amps. Be careful, and pay attention to the battery ratings - there *are* batteries like that. It will usually say on the wrapping. If it doesn't,
BE SURE TO ASK!! You can seriously hurt yourself if you go beyond a battery's rating. A good vape shop should be able to tell you.) Simply put, good batteries are usually expensive because they can handle more current and they will hold up longer.
If your mod is regulated, you don't have to worry - just practice basic battery safety, and you should be fine. You *really* have to fuck up to get a regulated device to vent a battery, in which case you should just put your mod down and cut your losses. Go here if you have any questions in this regard - the article here will answer your questions.
http://vaping360.com/vaping-battery-safety-part-1/
The difference between unregulated and regulated devices is that regulated devices won't fire coils below a certain resistance. They have a chip built into them that prevents the mod from firing. Unregulated mods will fire any coil you slap in them, so you can go Duke Nukem style, run a 0.1 ohm coil at 150 watts, and it will still fire. Of course, that would be suicidal because that's when batteries and mod go BOOM and people end up with 3rd degree burns and pieces of shrapnel stuck in their hand/face... should they survive the encounter.
This chart is an easy way to remember Ohm's Law for future referrence. I suggest you right click on it and save it to your desktop. It's pretty much our Bible as vapers, or at least it should be.
