Advanced Reset
:
mmin
The distance between each "ridge" on the twisted wire. Use 0 for non-twisted wire (parallel strands).
Max 4 for twisted wire.
AWG r mmin
mmin × mmin
The width should normally be larger than thickness. Swap them
In a multi coil setup, all coils must be identical. Two coils in parallel. Total resistance is halved. (12 R) Two coils in serial. Total resistance is doubled. (2 R) All three coils in parallel. Total resistance is divided by three. (13 R) All three coils in serial. Total resistance is tripled. (3 R) All four coils in parallel. Total resistance is quartered. (14 R) All four coils in serial. Total resistance is quadrupled. (4 R) Two coils in serial, connected in parallel to a second pair of serially connected coils. Or the other way around: Two coils in parallel, connected in serial to a second pair of parallelly connected coils. Total resistance is the same as for a single coil. (1 R)
Ra Ω
Set the desired total coil resistance for your build. The calculator will tell you the length of theeach resistance wire, and the resistance of each coil.
This calculator does not predict shorts, so always measure your coil before you fire it!
R
l
Ω
mm
Ω
in
Choosing a new material, AWG or wire diameter will update this value.
lr mmin
Choosing a new Material, Setup, Target resistance, or Wire resistance. will update this value.
c mmin
ll mmin
Total length of the resistance wires between coil and contact points.
mmin
The space between each coil loop. Keep close to 0 with touching coils (mini/micro/nano).

Results – how to wrap, and how it will perform

Resistance wire length
Wrap count
"full wraps"
"half wraps"
Coil Ω Ω
Heat flux
@ W
mW/mm² mW/in²
Heat capacity mJ K-1
Leg loss %
to ( dec)

Coil dimensions (each coil)

Wire length
Outer ⌀
Neutral axis ⌀
Circumference
Helix angle °
Loop length
Width
Surface area mm² in²

Wire dimensions (each coil)

Volume mm³ in³
Density g/cm³ oz/in³
Mass mg gr
Surface area mm² in²
Cross section area mm² in²
How it works

How the coil calculator works – what it does, and what it doesn't do

Platform and precision of engine parts

All calculations are done in JavaScript, which uses 64 bit floating point. This yields a precision of 15–17 significant decimal digits, which is more than sufficient for the purpose of modeling a coil build.

Internally, all variables are stored and calculated in metric units. Unneccessary unit conversions are avoided in order to prevent accumulation of rounding errors when using imperial units.

Three values are written to the input fields during use (advanced mode): Wire diameter, wire resistance per mm, and resistance wire length. These numbers are rounded in the input fields, but still preserved with full precision in memory. If you manually override a value, you can enter your own number with any precision you want. When you save, and subsequently load the settings, rounded values will be displayed, but the number will still exist with the full precision in memory.

Inner workings – a peek inside the engine room

Resistance wire length

AWG is converted to diameter by using the formula that defines AWG. This should make the AWG conversion more precise than the numbers stated by many resistance wire vendors.

Wire resistance per length is determined by the specific resistivity of the wire material, and the cross section area of the wire. The specific resistivity for each material is looked up in a small table of constants.

The resistance wire length is your set target resistance divided by the wire resistivity per mm. Leg length is subtracted before calculating the number of wraps.

Material Specific resistivity (Ω mm²/m)
Kanthal A1/APM 1.45
Kanthal A/AE/AF 1.39
Kanthal D 1.35
Nichrome N20 0.95
Nichrome N40 1.04
Nichrome N60 1.11
Nichrome N70 1.18
Nichrome N80 1.09

Wraps

When you input the inner diameter of the coil, the outer diameter is simply the inner diameter plus twice the wire thickness. The circumference of your coil is then by multiplying the outer diameter with π, and we have length of a single wrap. The wrap does not go in a straight circle around the mandrel, but rather in a helix, making it slightly longer than the coil circumference. For twisted coils, the 2–4 strands are combined into one diameter using the diameter of an outer circle encompassing the 2 4 tangent circles of each strand.

Heat

The heat flux is more or less evenly distributed over the resistance wire. Hot legs are undesirable, so the power used to heat the legs can be regarded as "lost".

The density of the coil material is used to calculate the wire mass and heat capacity. Because of lacking data on the density of different Nichrome alloys (except N80), the density of the Nichrome qualities are interpolated from the densities of the main alloy elements.

The heat capacity of the wire materials do not vary much between the alloys used. Therefore 0.46 kJ kg-1 K-1 is used for all kanthal, and 0.447 kJ kg-1 K-1 is used for all nichrome.

Possible error sources – or spherical cows in a vacuum

This coil calculator is a pretty simple and straightforward digital model of the geometry and electrical properties of an atomizer coil, and can be expected to be concistent with at least itself. Real life, on the other hand, involves a myriad of ways to introduce error to your numbers:

  • Depending on the quality, the resistance wire might be slightly thicker or thinner than specified, or the alloy might be slightly different, which would affect resistivity.
  • When you wrap a coil, the wire is also being stretched, increasing resistivity. This is seldom very significant, but that depends on how small the inner diameter of your coil is, and how much tension you put on the wire while you wrap it. Thinner wire stretches more easily, but it also bends more easily, requiring less tension on a small mandrel.
  • In a coil with touching loops (e.g. a micro coil), a little current will flow between the loops. Even though the oxidation of kanthal creates an thin insulating layer of alumina around the wire, no insulator is perfect. The amount of current that will "leak" depends on the thickness of the alumina layer, which in turn depends on the alloy used, and how much you torched it. It also depends on the area of the loops actually touching, how hard they are touching, the voltage potential between each loop, etc.
  • E-juice does not conduct electricity very well, but like everything else, it does conduct a little. Burnt juice leads to carbon buildup on the coil, and carbon conducts electricity fairly well.

These are some of the factors that can impact real life accuracy. Another possible error source is the inner diameter of the coil. If the mandrel is off spec by only 0.1 mm, the length of a single wrap will be off by roughly 0.314 mm. Multiplied by ten wraps, this small error has grown more than thirtyfold. The output from a calculator can never be better than the input.

All these error sources can cancel each other out to some degree, but they can also add up. This is one of the reasons why you should always have a decent multimeter handy, and measure your coil after you build it. A model is great for getting you into the ballpark, but getting the final build right still requires your skills, and some measuring equipment. The Steam Engine is not intended to replace a multimeter.