Introduction

This page contains three inductor calculators. With all of these calculators, the main limitation is that they be Single-Layer Air-Core inductors.

The first one, Calculator Ⅰ, uses formula published in the ARRL Handbook For Radio Amateurs, and many others. It's nice because it requires only a few inputs to get a estimate of the inductor.

The second calculator, Calculator Ⅱ, provides more information, like Reactance, Q, and Self Resonant Frequency. Note, however, that Calculator Ⅱ requires the wire size and plating material. There is a slight difference in Conductivity, Permeability, and intended Frequency. Both calculators come up with inductance estimates that are very close to each other.

The third calculator, Calculator Ⅲ, provides you with a per-turn estimate of a coil that is pre-wound or one that you intend to wind.

Coil Inductance Estimating Calculator Ⅰ
Input Data
Coil Diameter
Coil Length
Number of Turns
Output Data
Inductance (uH)
Coil Turns Spacing

This is a very simple calculator for estimating the inductance of a Single-Layer Air-Core inductor. The program uses formula published in the ARRL Handbook For Radio Amateurs and other radio publications.

In the text areas provided, enter the Coil Diameter, Coil Length, and Number Of Turns. You can use either inches or millimeters and you can mix and match. The estimated Inductance, in uh, and Coil Turns Spacing will be calculated. Recalculation is automatic. Just click outside the entry area after changing any input data.

The formula is correct to 1%, provided that L > 0.8 × D/2, i.e. the coil is not too short.

If you need a more accurate reading, or the coil does not meet the Single-Layer Air-Core coil criteria, use an Inductance Bridge or Grid-Dip Meter to determine the inductance.

Coil Inductance Estimating Calculator Ⅱ

This calculator, takes into account the wire size and wire composition. It was addressed by Serge Stroobandt, ON4AA on http://hamwaves.com/antennas/inductance.html. Please refer to that site for detailed information. I only reconfigured it a bit to make it easier to use.

Input Data
Mean Diameter (D)
of the air core coil.

Number of Turns (N)
Length of the coil (L).
Wire Diameter (d)
Plating Material
Plating Conductivity ρ = x nΩm
Plating Permeability μr =
Design Frequency MHz

The table below lists some of the results from intermediate calculations and the final output. Of particular interest might be the Effective Wire Length. That value would be useful when you are creating a shortened dipole or a multiband dipole. For a shortened dipole, the Effective Wire Length indicates the reduction in length, for the wire past the inductor.

Intermediate Results
Winding pitch. p = mm
Proximity factor. Φ =
Effective coil diameter. Deff = mm
Correction factors
Field non-uniformity
correction factor.
kL =
Self-Inductance, Round
wire correction factor
ks =
Mutual-Inductance, Round
wire correction factor
km =
Wire
Physical wire length. wire =
Effective wire length. wire, eff =
Skin depth at
design frequency.
δi = µm
Sheath helix waveguide mode
Effective pitch angle. ψ = °
Axial propagation factor. β = rad/m
Characteristic impedance. Zc = Ω
Final Output Data
Results (at design frequency)
Effective series inductance. LEff,s = µH
Effective series reactance
of round wire coil.
XEff,s = Ω
Effective series AC resistance
of round wire coil.
REff,s = Ω
Effective unloaded quality
factor of round wire coil.
QEff,ul =
Lumped circuit equivalent
Frequency-independent
series inductance.
Ls = µH
Series reactance of
round wire coil.
XL,s = Ω
Series AC resistance
of round wire coil.
RL,s = Ω
Unloaded quality factor
of round wire coil.
QL,ul =
Parallel stray capacitance. CL,p = pF
Self-resonant frequency
Self-Resonant Frequency,
λ/4 (parallel)
fres,L = MHz

Pre-Wound Inductor Estimating Calculator Ⅲ
Input Data Intermediate Calculations
Coil Length (ℓ)
Total Turns
N = ℓ × TPI =
Coil Diameter (D)
L(µH)  =     D2 × N2    = 
18×D + 40×ℓ
   = 
Wire Diameter
Turns Per Inch (TPI)
Start Turn  End Turn 
Output Data
Turn Ind. µH Length Turn Ind. µH Length

This calculator is for situations when you have a section of pre-wound inductor, or you are intending to wind an inductor with specific dimensions, and want to know what the inductance is for each turn. Here, you can specify the Coil Diameter, Turns Per Inch, and the Wire Diameter and the output would be a table starting and ending turn number.

An example might be that, you have a 43 Foot Vertical antenna and want to make it easier to tune on 80 and 160 Meters. For that you would might need a 11 µH inductor for 80 Meters, and a 33 µH inductor for 160 Meters. You have a pre-wound inductor that is 12" long and 2" in diameter. It is wound with #10 AWG wire at 8 Turns-Per-Inch. How many turns of this coil would you need to get 11 µH or 33 µH?