Describes method in which test skeins of known length are first reeled, under specified conditions, from sample packages that have been adequately conditioned, and then weighed, the linear density in tex units being calculated from the length and mass of each skein. It is applicable to yarns both as-produced and after processing, but not to yarns that elongate more than 5 per cent when tension is increased from 5 to 10 mN/tex, to stiff monofilaments and similar, or to yarns having a linear density greater than 2000 tex.