Apparel Manufacturing
A needle too fine shreds the thread; one too coarse leaves needle holes. The window is narrow.
— Nm
Metric needle size, hundredths of a millimetre
Treat the result as the centre of a range and move one size either way for dense or delicate fabric. Always confirm against the needle and thread supplier tables.
Needle Size & Thread Ticket Number Matcher — free, with the formula and a worked example, at Textile School.