Regardless of the yarn count system used, it is necessary to measure the weight and length of a yarn in order to determine its count. But most fibres, particularly natural fibres such as wool, absorb moisture from atmosphere. The weight of the yarn will be different at different moisture level. The water content in textiles can be expressed as either moisture content or as regain. Their definitions are:

In commercial transactions, the mass to invoice is worked out on the basis of an agreed conventional regain level, not on the actual regain of the yarns (or other textiles) being traded. This is very important. Because, in the absence of an agreed conventional regain level, smart sellers may take advantage of the moisture absorption property of their textiles and rip the buyers off with large quantity of water in their products. The conventional regain levels, to be used for calculation of the legal commercial mass, have been established by national or international standards. These commercial regain values are purely arbitrary values arrived at for commercial purposes for interested parties, and they often vary from fibre to fibre and from country to country. In Australia, the conventional regain rates for some fibres are given in Table

Conditioning the whole lot of yarns or other textile materials to the conventional regain rates given above is not practical, because of the time required etc. In calculating the commercial mass to invoice for a lot, the following procedures are often followed:

(1) Extract a sample of mass (gw) from the lot (whose total gross weight is GW)

(2) Determine the dry weight (dw) of the sample by oven drying to completely evaporate the moisture contained in it

(3) Calculate the commercial mass to invoice (cw), based on a conventional regain R%, by means of the formula: