Wool can absorb up to approximately 20% of its weight in moisture, and hence it's vital that the weight of wool being traded is certified to an agreed regain. (Moisture content is the weight percentage of moisture calculated on the basis of the 'wet' weight, whereas regain is the weight percentage of moisture calculated on the basis of the oven-dry weight.).

Conditioned mass is determined by sampling the consignment and determining the actual regain of the sample by oven-drying. An agreed factor is then added to this mass to calculate the "conditioned" or invoice mass.  Whilst there are two separate sets of Regulations and test methods for scoured wool and tops, the principles are the same for wool in all forms, including yarns.

In practice, for scoured wool, core samples are taken from every bale and are sealed at the scour. On receipt at the laboratory the samples are dried by prescribed methods (IWTO-33) to determine the oven-dry mass. The conditioned mass is usually calculated for a standard regain of 16% or 17%. The difference between the invoice mass calculated in this manner and the net mass of the consignment is known as the 'gain' or 'loss'.

Tops and sliver are best sampled during production. Although packages can be sampled, this is wasteful and is usually avoided. In this case the samples are tested in accordance with IWTO-34 (which is essentially very similar to IWTO-33)

Wool users normally specify the regain required in the wool since processing performance is affected by regain. In general terms scours aim to produce wool slightly on the dry side of the specified regain because damp wool can deteriorate at the high densities used for shipment. Producing wool that is much drier than specified is wasteful of energy.