Read the system as well as the number
Sewing threads can be labelled wt, a numbered count, Tex or denier. They do not all increase in the same direction.
| Label | General direction |
|---|---|
| Common wt labels | Higher number usually means finer thread within a comparable product line |
| Tex | Grams per 1,000 m; higher is heavier for the same length |
| Denier | Grams per 9,000 m; higher is heavier for the same length |
Why an exact conversion can mislead
Superior Threads explains that common wt labels may omit ply information and do not form one consistent universal scale. A spool labelled 40 wt is therefore not guaranteed to be exactly Tex 25. Compare the manufacturer’s specifications, fiber, construction and actual thread before substituting.
Tex and denier are defined by mass per length: a Tex 25 thread is 225 denier because 9 × 25 = 225. This arithmetic does not establish an equivalent commercial wt label.