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Foil-wound coil calculator

When you need this

A multilayer coil wound from thin metal strip with an insulating interleaf. Typically secondaries and work coils, where low self-capacitance and a large conductor perimeter matter.

Foil coil: former diameter D, strip width w, thickness t, gap g
D — former diameter, w — strip width, t — thickness, g — gap between turns
Required inductance L (µH)
Former diameter D (mm)
Foil width w (mm)
Foil thickness t (mm)
Gap between turns g (mm)
Result
Number of turns N
Outer winding diameter Do
Foil length Lth
Resistance of the copper foil

📐 Formulas​

Strip cannot be treated as round wire of equivalent cross-section: the inductance depends not on the area but on the geometric mean distance of the cross-section, and the GMD of a long thin rectangle differs from the GMD of a circle of the same area by a large factor.

Self-GMD of a strip:

ln(GMD) = ln(w) − 3/2      →      g_self = e^(−3/2)·w ≈ 0.2231·w

The mutual GMD of two strips separated radially by s:

ln(GMD₁₂) = (s²/w²)·ln(s) + ½·(1 − s²/w²)·ln(w² + s²)
          + 2·(s/w)·arctg(w/s) − 3/2

The pair of strips is then replaced by an equivalent pair of filaments whose radial separation equals the mutual GMD and whose product of radii is preserved:

R₁ − R₂ = GMD₁₂,   R₁ · R₂ = r₁ · r₂

For that pair the ordinary Maxwell formula applies.

Notation:​

  • L — inductance [µH]
  • D — former diameter [mm]
  • w — strip width [mm]
  • t — strip thickness [mm]
  • g — gap between turns [mm]
  • Do — outer diameter of the winding [mm]
Limits of the calculator:
  • The strip must be at least 100 times wider than it is thick — otherwise the cross-section is no longer a «strip» and the GMD formula does not apply
  • The strip width cannot exceed the former diameter
  • Skin and proximity effects are not modelled: on alternating current the real resistance will be higher
  • The stated accuracy is no worse than ±10 % as long as the strip is narrower than the former diameter

🎯 Where it is used​

  • Work coils in induction heaters.
  • Tesla coils with strip windings.
  • High-current chokes, where round wire would suffer too much skin effect.
Practical notes:
  1. The resistance shown is for copper strip. For aluminium multiply by 1.6.
  2. For the same cross-section a strip has a larger perimeter than round wire — which is exactly why it wins on losses at high frequencies.
  3. The gap between turns sets not only the size but also the self-capacitance. A thinner interleaf means a more compact coil and a lower resonant frequency.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «Foil-wound coil calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/coils/foil_winding (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

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