What is counted
Woven fabric has lengthwise threads (the warp, or “ends”) and crosswise threads (the filling, or “picks”). Thread count adds the two together for one square inch. If a sheet has 150 warp threads and 150 filling threads per inch, its thread count is 300.
The industry test method, ASTM D3775, counts each thread once, whether it’s a single strand or plied (several strands twisted together).
Why some labels are so high
Counting each ply as a separate thread multiplies the number. 380 two-ply threads would be labelled 760 that way.
That is a real case. In 2002, FTC staff wrote that they would give the ASTM method “great weight” when judging thread-count claims. They cited a closed investigation of down comforters in which a company changed its package description from “760 White Goose Down” to “finely woven 380 2-ply fabric”. A clear label gives both numbers, such as “300 thread count, 2-ply yarn”.
What’s typical
IDFL, a textile testing lab, says percale sheets are typically 200 to 300 thread count and sateen 500 to 600. It found one sheet labelled 1,500 that actually had 300 threads per square inch. A very high number usually means plies were counted, not that the sheets are better.