Tutorial: How To Calculate The Resistance of Coil Builds

Before you start building a coil for your RTA or RDA it is always a good idea to calculate its potential resistance using an online calculator. It helps you avoid any possible shorting and allows you to figure out whether the coil you intend to build will work on your setup. After all, you don’t want to accidentally build a dual 0.2ohm setup only to realize that it does not fire adequately on your iStick 30 (coz of lack of power).

Steam Engine is one of the best vaping calculators around and using it is a very easy task once you understand how to work it. I will divide this tutorial in two parts- 1) Things you have to input for the calculation & 2) learning how to read the output.

The input process

In the image below I have highlighted the fields that you need to put in information for.

calculating coil resistance 1


1. First of all, you need to select the kind of wire you are using. The standard Kanthal is selected by default but you can change that by clicking the drop down menu. So if you are using stainless steel wire for instance, you will select SS316L/elite


2. Then you need to select the gauge (ga) or thickness of the wire. This is usually written up front or at the back of the wire spool. Enter that information in there.
3. What kind of build are you planning? Does your RTA require a dual coil setup or is it like the Serpent which only requires a single coil? Select that option here.


4. Target resistance is the resistance you are aiming for. A general rule of the thumb is that lower the resistance, the bigger the clouds, mostly because you can throw a lot more power at it. This by no means is an exclusive rule (as you can make a cloud chasing build at a higher resistance too) but for the sake of simplicity, we will go by this rule.


5. Lastly, you must enter the inner diameter of the coil you want to build. In general, coils with a bigger diameter allow for more wicking, which allows for more juice to be held within the coil. As you can guess, this helps you avoid dry hits, produces more vapor (mostly) and depending on the RTA, can provide a better vaping experience.

At the same time you need to ensure that the coil diameter will fit your RTA. For example, you’d be hard pressed to fit a 3.5mm coil on the Goblin Mini deck while the massive Sense Herakles deck will take pretty much whatever you install in there. The most common diameters used by sub-ohm vapers are 2mm, 2.5mm & 3mm while MTL vapers prefer smaller diameters. So go ahead and enter your preference in there.


Reading the output

After you have filled in all the info, you will see some numbers in the results panel on the right. So let’s see how to read into them.

calculating coilr esistance 2

Number of wraps is simply the number of times you must wrap the wire to reach your preferred resistance. Below that is the information for full wraps and half wraps, which is important too.

If you are using a velocity style deck, in which both the coil legs point in the same direction, then your coil’s eventual resistance will be closer to the value written next to half wraps. If you are building on a deck where the coil legs face in opposite directions (like the Serpent Mini) then you will use the resistance value next to full wraps. Remember that this information pertains to a single coil. So if you are building a dual one, then divide this number in two.

The number in front of heat capacity tells you how fast your coil will ramp up. The higher that number, the slower your coil is going to heat up and cool down. As is obvious, coils builds with thinner wires will ramp up a lot faster than those with thicker wires. Even a dual coil made with 28 ga wire will fire up quicker than a single coil build with 22 ga wire.

Leg power loss is the power wastage that occurs as a result of heating the coil legs. This is why it is always recommended to keep your coil legs as short as possible. Not only does that save on power, it also avoids that metallic harsh taste that you get with longer coil legs.

Lastly, the infamous heat flux. There have been plenty of scientific articles written on heat flux but in layman’s terms, heat flux simply refers to the heat that our coils generate. The more heat a coil generates, the hotter the vapor produced and vice versa. Also note that the heat flux value in the calculator is not represented in fahrenheit or celsius but in milliwatts per millimeter squared. But for a casual rebuilder there is no need to get into the scientific terms. All you need to remember is this- the higher this value the warmer the vape is going to be.

Now, you can figure out how hot the vapor is going to be by adjusting the wattage in the heat flux section. As you increase the wattage, you will see that the heat flux value increases and so does the color of the icon next to it. If it’s colored blue, then it means that at the wattage that you have selected, the vapor produced will be cool. If it’s green then it will be medium, at yellow it will be warm and at red, it will be hot.

calculating coil resistance 3

This is a great way of figuring out the right wattage range at which you should run a particular coil build for your particular taste. In the picture above you can see that if I run this 0.4 kanthal build at 52 watts then I am going to get a pretty hot vaping experience. In general, you want to keep the heat flux value less than 350 or the vapor produced will be uncomfortably hot.

And that’s about it for this tutorial. If you are a beginner to rebuilding, then this is all the info you need about figuring out coil resistance. It might seem like a lot of numbers at first, but with me, this will come automatically to you. You can access the Steam Engine calculator here. 

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