Spiral coil calculator

Here you can calculate the inductance for a given size of a spiral coil wound in one layer. It is optional to add the capacitance for f.ex. a primary tank capacitor or topload capacitance to find the resonant frequency of the LC circuit.

The formulas used to derive the inductance is simplified and correct to within 1%. Source “Harold A. Wheeler, “Simple Inductance Formulas for Radio Coils,” Proceedings of the I.R.E., October 1928, pp. 1398-1400.”

Switch between the input fields to automatically calculate the values.

Number of turns Turns
Inner diameter mm
Wire diameter mm
Turn spacing mm
Outer diameter mm
Wire length m
Inductance uH
Optional extra f.ex. tank capacitance size
Capacitance nF
Resonant frequency kHz

Formulas used

Outer diameter = inner diameter + ( 2 * number of turns * ( wire diameter + wire spacing))

Wire length = ((Pi * number of turns * (outer diameter + inner diameter)) / 2) / 1000

Width w = ((wire diameter / 25.4) + (wire spacing / 25.4)) * number of turns
Radius r = ((inner diameter / 25.4) + w) / 2
Inductance = (radius^2 * number of turns^2) / (8 * radius + 11 * width)

Resonant frequency = (1 / (2 * pi * sqrt((inductance / 1000000) * (capacitance / 1000000000)))) / 1000

12 Responses to Spiral coil calculator

  1. Miguel says:

    Thank you thank you

  2. rubashka says:

    i have a inductor how can i mak it?

    L: 1.5 / 3mm 0.5CU 2.5 mm

  3. Mads Barnkob says:

    Hi rubashka

    Could you please describe in more details what it is you need.

    Kind regards

  4. David says:

    Hello, I found this page through a Google search. Very informative. I do have one question, how do you arrive at inductance from the equation: (radius^2 * number of turns^2) / (8 * radius + 11 * width) when there is no permeability units in the formula? I get the same answer you do, but the units are in meters, not henries.

    Thank you for your time,

  5. Mads Barnkob says:

    Hi David

    The formula is a simple approximation as described first in the article.

    Permeability is in it, but only for air cored copper coils, this is all a part of Harolds work, he made it simple so you only have to put in some physical dimensions to get a estimated inductance.

    Kind regards

  6. Iyad says:

    Hi there,
    I have 5 turns
    inner diameter is : 18 mm
    wire diameter is :0.2 mm
    turn spacing is : 10 mm
    outer diameter is : 52 mm
    wire length : 0.5 meters
    The capacitance I use is : 1.36 micro farad .
    I could not change the value of outer dimater here so please tell me what is the inductance value.
    Thanks in advance.

  7. Mads Barnkob says:

    Hi Iyad

    The outer diamter is calculated from inner diameter + number of turns (distance), so you should not put that value in, it is calculated.

    Judging from your numbers and the picture, something is not right.

    Kind regards

  8. Wayne Tom says:


    I would have thought that since the inductance goes up as the square of the turns, where as the resistance is linear that I could get a higher Q with copper thin strips, and decreasing the turn spacing, but I don’t get the high Q I’m looking for (not more than 40-50 @ 100khz). Do you have any insight into this problem.

  9. David Whitworth says:


    What formula should I use to calculate the capacitance?

    The formula I am using C=0.29l+0.41r+1.94*(r³/l)^0.5

    I am using magnet wire 0.4mm², with ID of coil 1″ and 44 turns. I get a calculated capacitance of 71pF


  10. Mads Barnkob says:

    Hi David

    I would use the results from JAVATC as I consider this the best and most though-rough calculator available. My own calculator here is only meant for quick estimates where you can get a result from as few factors as possible. Consider it a ideal world with no losses example 🙂


    Kind regards

  11. DHANUSH says:

    How to find circumferential area

  12. Mads Barnkob says:

    In geometry, the area enclosed by a circle of radius r is = π * r * r

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