Air Cored Inductor Calculator

This calculator is useful when making antenna matching units, low pass filters, crystal sets, antenna traps, resonant circuits or anywhere where an air cored single layer inductor is required.

Diameter and length of inductor are entered in millimetres along with the number of turns. Frequency can be entered to find the Q of the inductor at a specific frequency, but is not required. Inductance values are calculated to 3 decimal places and are given in nH, µH, mH and even to 5 decimal places, H! An example of a T-match inductor for 1.8MHz is already inputted, click calculate, or clear fields to enter your own values.

Input coil diameter, length & number of turns Inductor Symbol
Coil Diameter mm
Coil length mm
Number of turns t
 
Frequency
(Not required, but used for calculating Q)		 MHz

Calculated inductance, Q and wire length
Nano H nH
Micro H µH
Milli H mH
Henry H
Inductor Q Q
Wire Length m
cm
mm

Wire length is calculated by (diameter x π x turns). π is taken to be 3.14159265358979323846264338327950288419716939937510 which is a little over the top I know! As 39 decimal places is sufficient to estimate the circumference of any circle that fits in the observable universe with precision comparable to the radius of a hydrogen atom, the 50 decimal places I have used should be OK for your inductor. Just remember to cut a bit longer for some tails!

13 Comments

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  1. LARRY A BOLES
    LARRY A BOLES

    LOVE YOUR CALULATOR……NOT SURE WHEN I NEED TO USE IT….I AM A HAM VIRGIN….BUT LEARNING…..
    THANKS A BUNCH
    LARRY

    Reply
  2. Arto Salo
    Arto Salo

    Re: Air Cored Inductor Calculator

    Hi, Is the coil diameter measured outside, at the middle or inside of the coil ?

    Rgds/ 73

    Arto Salo
    OH1ID

    Reply
    1. John M0UKD
      John M0UKD

      Hi.
      I never use thick enough wire to worry, but I would use the inside diameter.
      73, John.

      Reply
  3. tymkea
    tymkea

    The coil diameter uses the mean diameter of the copper coil.

    A 1mm wire wound on a 14mm bobbin will have a mean diameter a bit over 15mm.
    The bit over is the uncoiling that naturally occurs when one releases the tensioning force.

    I use >=1.6mm diameter enameled copper wire and prefer to use 2.12mm wire wound onto an AA battery.
    My mean diameter is roughly 16.9mm
    9¾ Turns gives ~1uH

    Reply
  4. tymkea
    tymkea

    Is it possible to add a field for the copper wire diameter?
    And use that to calculate the coil resistance in ohms?

    Reply
    1. John M0UKD
      John M0UKD

      Perhaps that would be better as a separate calculator.

      Reply
  5. cheungchew
    cheungchew

    What is the meaning of Coil Diameter and Coil length?

    Reply
    1. John M0UKD
      John M0UKD

      Diameter is how wide the coil is and length is how long the coil is.

      Reply
  6. amir
    amir

    hi tanx for you helping amateur radio in all of the world

    i used of air core calculator.

    best regards from Tehran-iran
    amirhosein 73

    Reply
  7. Roger Green
    Roger Green

    A good approximate formula (accurate enough to 1%) is:

    L = (n . d)(n . d)/l x 10^{-10) Henrys , where n is number of turns, d is diameter in mm, l is length in cms,

    Reply
  8. bennett wilson
    bennett wilson

    I love the idea that you have Pi to 50 digits. But. It is false precision in the wire length because the length depends on the Turns Per Inch as well. The true length of the line is the length of the hypotenuse of the triangle formed from the true circumference and the turn spacing. It is an error larger than the diameter of an electron, but I think I can live with your formula! ;-))

    Reply
    1. John M0UKD
      John M0UKD

      Very good Bennett! 🙂

      Reply
  9. Robert Spencer
    Robert Spencer

    I just came across this site. I ran some numbers when designing the shunt for my HiQ antenna and found yous to be more accurate than others. My SWR was good without but I wanted to get rid of some unwanted wind noise on digital modes in the especially in the 20 meter band. I used the center of the coil “(ID+OD)/2” which was 37.6mm x 32mm x 10.5 turns. The wire is #8 (3.2mm) dia. My first calculations were less but when I measured it I got 3.1 µH. Yours calculated 3.146. The other calculators I tried online were estimating from 2.2 to 2.8 µH. The wire was some common copper ground wire I picked up at a local hardware store.

    Good job on the calculator.
    Robert N9NQ

    Reply

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