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Mean Length of Turn (MLT)

Understanding and calculating the average wire path around a bobbin

What is MLT?

The Mean Length of Turn (MLT) is the average length of wire needed for one complete turn around the bobbin. It's essential for calculating:

Core (Tongue × Stack) T (Tongue) D (Stack Height) t h_build Bobbin Winding

Cross-section looking down the core axis, so the core reads tongue × stack. The coil builds outward across the window width, T/2 each side — 0.375″ on an EI-75 — while the window height of 3T/2 is the traverse, running into the page.

MLT = 2(T + D + 2t) + π(hbuild/2)
Where:
T = Tongue (center leg width)
D = Stack height
t = Bobbin wall thickness
hbuild = Winding build height (layers × wire diameter)
Understanding the Formula

1 The Rectangular Path: 2(T + D + 2t)

The wire path starts as a rectangle around the core and bobbin. Each turn must go:

This gives the perimeter: 2 × (T + D + 2t)

2 The Corner Radius: π(hbuild/2)

As layers build up, the wire path gets longer. The average turn sits at the midpoint of the winding build. The corners effectively add a quarter-circle at each of the four corners, using the average build height as the radius.

Four quarter-circles = one full circle = 2π × (hbuild/2) = π × hbuild/2

Why This Matters: A simplified formula like MLT = 2(T + D) underestimates wire length significantly. For a typical choke with 8-10 layers of wire, the actual MLT can be 10-20% longer than the simplified estimate, leading to errors in resistance and loss calculations.
Worked Example

Given Parameters

1 Calculate Turns per Layer

Effective winding width = Window Height - 2 × Bobbin Wall 

Effective width = 0.438" - 2 × 0.0625"
            = 0.438" - 0.125"
            = 0.313"

Turns per layer = floor(0.313" / 0.0341")
            = floor(9.18)
            = 9 turns

2 Calculate Number of Layers

Number of layers = ceil(180 / 9)
             = ceil(20)
             = 20 layers

3 Calculate Winding Build Height

Build height (hbuild) = 20 layers × 0.0341"
                  = 0.682"

4 Calculate MLT

MLT = 2(T + D + 2t) + π(hbuild/2)
= 2(0.875 + 1.0 + 2 × 0.0625) + π(0.682/2)
= 2(0.875 + 1.0 + 0.125) + π(0.341)
= 2(2.0) + 1.071
= 4.0 + 1.071
= 5.07"

5 Calculate Total Wire Length

Wire length = MLT × Turns
        = 5.07" × 180
        = 912.6"
        = 76.1 feet
Comparison: Simplified vs. Accurate MLT
Method Formula Result
Simplified (no bobbin, no build) 2(T + D) 3.75"
With bobbin walls 2(T + D + 2t) 4.00"
Full formula (with build) 2(T + D + 2t) + π(h/2) 5.07"
Impact: Using the simplified formula would underestimate wire length by 26% (56 ft vs 76 ft), leading to significant errors in resistance calculations and potentially running short on wire during winding.
Practical Considerations

Bobbin Wall Thickness: Typical values range from 0.0625" (1/16") for small bobbins to 0.125" (1/8") for larger ones. Thicker walls provide better insulation but reduce winding space.

Build Height Estimation: For random-wound coils, multiply the calculated build by a factor of 1.1-1.2 to account for imperfect layering. This increases the effective MLT.

Layer Insulation: If using layer insulation tape between winding layers, add the tape thickness to each layer height when calculating build.

First Layer Offset: The first layer sits directly on the bobbin, not at the average build height. For very few layers (1-3), a more detailed calculation may be warranted, but for typical multi-layer windings, the average approach is sufficiently accurate.

Reference: This calculation is implemented in the Class A Choke Calculator, which automatically computes MLT based on core dimensions, bobbin wall thickness, and the calculated winding build.