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Bobbin Winding Calculation Example

Step-by-step walkthrough: EI-87 core with 20 AWG magnet wire

Given Parameters

1 Turns per Layer

How many turns fit side-by-side across the traverse (winding length)? Divide the available winding length by the wire diameter and round down. 

Turns per layer = floor(winding length ÷ wire diameter)
            = floor(1.188" ÷ 0.0341")
            = floor(34.84)
            = 34 turns

2 Effective Layer Height

Each layer of wire adds to the build height. With perfect winding, this equals the wire diameter. The random winding factor accounts for imperfect layering (1.15 = 15% extra height per layer). 

Effective layer height = wire diameter × random winding factor
                   = 0.0341" × 1.0
                   = 0.0341"

If using a random factor of 1.15, this would be 0.0392" per layer.

3 Available Build Height

We don't fill the bobbin to 100% — that leaves no margin for error and risks the winding not fitting. Apply the build fill limit to determine the usable height. 

Target height = winding height × build fill limit
          = 0.312" × 85%
          = 0.265"

4 Maximum Layers

How many layers fit in the available build height? Divide the target height by the effective layer height and round down. 

Max layers = floor(target height ÷ effective layer height)
       = floor(0.265" ÷ 0.0341")
       = floor(7.77)
       = 7 layers

5 Maximum Turns

Multiply turns per layer by the number of layers to get the maximum winding capacity. 

Max turns = turns per layer × max layers
      = 34 × 7
      = 238 turns
Summary
Parameter Value
Turns per layer 34
Number of layers 7
Maximum turns 238
Actual winding height 0.239" (7 × 0.0341")
Actual build fill 76.6%
Key Insight: The limiting factor here is the build height (0.312"). With 20 gauge wire you only get 7 layers before approaching the fill limit. If you used finer wire like 28 AWG (0.0144" insulated), you'd get approximately 18 layers and significantly more turns — around 1,400 versus 238.
Reference: This calculation follows the methodology used in the Inductance Capacity Calculator — turns per layer from winding length ÷ wire diameter, layers from available height, with build fill limit and random winding factor to account for real-world conditions.