What does copper fill factor mean?

It is the share of a slot cross-section filled by bare copper. A slot with 100 mm² of copper in 250 mm² of total area fills to 40%, and the rest carries insulation, air and clearance.

Why does the page block copper area above slot area?

Because copper cannot occupy more cross-section than the slot holds. A fill over 100% means the two areas disagree on units or on the slot they describe, so the page stops instead of printing an impossible result.

What is a typical slot fill target?

Targets vary by machine class and winding process, so enter the target your design or manufacturing standard sets. Hand-wound prototype slots usually run lower than machine-wound production slots.

How do I find net copper area per slot?

Count the conductors in one slot, multiply by the bare copper area of the wire gauge, and add parallel strands. Use the copper cross-section, not the insulated outside diameter.

Motors, Generators & Electrification Equipment calculator

Copper Fill Factor Calculator

Check how full a stator slot is from the net copper area and the total slot area, then compare the fill with a target. The page reports the gap and the copper area a target needs.

What this calculator does

Copper fill percent and its gap to a target, with the copper area a target requires.

Formula used

Copper fill factor = net copper area ÷ total slot area × 100
Gap to target = target fill − copper fill factor
Copper area at target = target fill ÷ 100 × total slot area
Copper area shortfall = MAX(target copper area − net copper area, 0)

Inputs explained

Net Copper Area per Slot: Bare copper cross-section in the slot, from the winding design.
Total Slot Cross-Section: Full slot cross-section before the winding is inserted.
Target Slot Fill: Fill percentage the design or process standard sets for this slot.

How to use the result

Best suited to checking a slot before winding, choosing wire gauge for a fill target, reviewing a traction motor redesign.
Slot liners, wedges and phase insulation take space that the bare copper figure ignores. A fill target alone does not prove the winding is producible; wire gauge and turns also decide that.

Current U.S. benchmarks

The producer price index for primary nonferrous metals (a broad metals benchmark, not copper alone) stands at 544.731 (BLS, Aug 2026), up 49.3% from a year earlier. Quotes priced off last quarter's material cost miss this move. Global copper trades at $13,543 per tonne (IMF via FRED, Jul 2026).
Steel mill PPI stands at 381.162 (BLS, Aug 2026), up 23.4% from a year earlier. New factory orders are up 8.5% 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

What does copper fill factor mean? It is the share of a slot cross-section filled by bare copper. A slot with 100 mm² of copper in 250 mm² of total area fills to 40%, and the rest carries insulation, air and clearance.
Why does the page block copper area above slot area? Because copper cannot occupy more cross-section than the slot holds. A fill over 100% means the two areas disagree on units or on the slot they describe, so the page stops instead of printing an impossible result.
What is a typical slot fill target? Targets vary by machine class and winding process, so enter the target your design or manufacturing standard sets. Hand-wound prototype slots usually run lower than machine-wound production slots.
How do I find net copper area per slot? Count the conductors in one slot, multiply by the bare copper area of the wire gauge, and add parallel strands. Use the copper cross-section, not the insulated outside diameter.

Last reviewed 2026-10-01.