How do you calculate copper fill factor?

Divide the net copper conductor area in the slot by the total slot area and multiply by 100. With 100 mm² of copper in a 250 mm² slot, fill is 40%.

What is a good copper fill factor?

Random round-wire windings typically land around 35-45% net copper fill, hairpin and form-wound designs can exceed 55-70%. The 40% in the example is normal for a random-wound stator but short of a 42% target.

Why is my fill below target?

In the example the achieved 40% trails the 42% target by 2 points. Common causes are loose winding, oversized slot liners, too much wedge area, or a wire gauge with thicker insulation than assumed.

Should I use bare or insulated wire area?

For copper fill factor use bare copper area, since the metric is about conductor content. Using insulated area gives a slot-fill-with-insulation figure that runs higher and is not comparable to copper fill benchmarks.

How does fill factor affect motor efficiency?

More copper in the same slot lowers winding resistance and I²R losses, so higher fill generally raises efficiency and continuous torque. Closing a 2-point gap can measurably cut copper losses and winding temperature rise.

Copper fill factor vs slot fill factor — are they the same?

Roughly, but be precise: copper fill uses bare conductor area, while some shops quote slot fill including insulation and varnish. Always state which you mean so a 40% copper fill is not confused with a higher gross slot fill.

Motors, Generators & Electrification Equipment calculator

Copper fill factor Calculator

Copper fill factor, also called slot fill, is the share of a stator slot's cross-section actually occupied by copper conductor rather than insulation, wedges, and air. Motor design engineers and winding process specialists track it because higher fill lowers winding resistance, cuts I²R losses, and raises efficiency and torque density. It is a core lever in meeting IE3/IE4 efficiency classes and in deciding whether a slot can hold the required turns. This calculator returns the achieved fill and the gap to your design target so you can see how much room is left.

What this calculator does

Calculate stator slot copper fill factor so winding and design engineers can see how much of the slot the conductors occupy and compare it to the design target.
Use it when you need a clean copper fill percentage and gap to target for a stator slot design or winding review.
It computes the percentage of total slot area filled by copper conductor and reports the point gap between that achieved fill and your target.

Formula used

Copper fill factor = copper conductor area in slot ÷ total slot area × 100
Fill factor gap to target = copper fill factor - target copper fill factor

Inputs explained

Net copper conductor area in the slot:
Total stator slot cross-section area:
Target slot copper fill factor:

How to use the result

Use it during winding design, when validating a new wire gauge or insulation system, or when troubleshooting why a stator runs hotter or less efficient than expected.
It uses net conductor (bare copper) area; if you instead enter the insulated wire area you will overstate fill, and the formula says nothing about whether the slot is physically windable at that fill.

Current U.S. benchmarks

The producer price index for copper and brass mill shapes stands at 559.593 (BLS, May 2026), up 76.8% from a year earlier. Quotes priced off last quarter's material cost miss this move. Global copper trades at $13,484 per tonne (IMF via FRED, May 2026).
Steel mill PPI stands at 348.53 (BLS, May 2026), up 6.7% from a year earlier. New factory orders are up 2.3% year over year (Census).
The U.S. has 5,397 electrical equipment and appliances establishments employing about 369,437 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate copper fill factor? Divide the net copper conductor area in the slot by the total slot area and multiply by 100. With 100 mm² of copper in a 250 mm² slot, fill is 40%.
What is a good copper fill factor? Random round-wire windings typically land around 35-45% net copper fill, hairpin and form-wound designs can exceed 55-70%. The 40% in the example is normal for a random-wound stator but short of a 42% target.
Why is my fill below target? In the example the achieved 40% trails the 42% target by 2 points. Common causes are loose winding, oversized slot liners, too much wedge area, or a wire gauge with thicker insulation than assumed.
Should I use bare or insulated wire area? For copper fill factor use bare copper area, since the metric is about conductor content. Using insulated area gives a slot-fill-with-insulation figure that runs higher and is not comparable to copper fill benchmarks.
How does fill factor affect motor efficiency? More copper in the same slot lowers winding resistance and I²R losses, so higher fill generally raises efficiency and continuous torque. Closing a 2-point gap can measurably cut copper losses and winding temperature rise.

Last reviewed 2026-05-12.