Helical coil calculator

Here you can calculate the inductance for a given size of helical 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.

 Coil diameter mm Number of turns Turns Wire diameter mm Turn spacing mm Wire length meters Coil height mm Inductance uH Optional extra f.ex. tank/topload capacitance size Capacitance nF Resonant frequency kHz

Formulas used

Wire length in meters = ((coil diameter * pi) * number of turns) / 1000

Coil length in mm = number of turns * (wire diameter + turn spacing)

Coil inductance in uH = (number of turns * (((coil diameter / 25.4) / 2)*((coil diameter / 25.4) / 2))) / ((9 * ((coil diameter / 25.4) / 2)) + (10 * (coil length / 25.4)))

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

4 thoughts on “Helical coil calculator”

1. 𝐿= (𝑁^2∗𝑐𝑑^2)/(25.4 ∗(18∗𝑐𝑑+40∗𝑐𝑙)) 𝜇𝐻
𝑓𝑜𝑟 𝐻𝑒𝑙𝑖𝑐𝑎𝑙 𝑐𝑜𝑖𝑙 𝑎𝑖𝑟 𝑐𝑜𝑟𝑒 𝑖𝑛𝑑𝑢𝑐𝑡𝑜𝑟𝑠
where:
N = No of turns
cd = coil dia, mm
cl = coil length2, mm
This is the correct formula

2. Hi Saju
That is actually also the formula used by the script, the formula written underneath is wrong.

Single layer air core solenoid
L (uH) = r^2 * n^2 / (9 * r + 10 * l)

where

r = coil radius in inches
l = coil length in inches
n = number of turns

So results from calculator are correct. Thanks for pointing out the error in the explanation 🙂
Kind regards
Mads

3. What about a Tesla Bifilar flat pancake coil of 20 awg wire (plus jacket = ~18 awg) 39 turns; inner hole diameter is 9 cm and out diameter is 20 cm. Measured inductance is .78 mH. What is length of the 20 gauge speaker wire?

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