Weight of wool - why does a 50g ball of yarn not always weigh 50g?

So you have bought some yarn and you suddenly realise that some of the balls or skeins are a bit lighter and some maybe a bit heavier than it says on the label. Is this a mistake? Are you missing out? Or what is going on?

There are reasons why yarn may weigh more or less than stated on the label. Wool and other fibres retain moisture to different degrees. Wool, for example, can retain up to a third of its dry weight in moisture without feeling moist or damp at all. When the same wool is kept in a dry environment, it will release the moisture back into the air. As a result, the same ball of yarn can have a very different weight depending on where it is measured.

There are internationally recognised and legally established measurements for the amount of moisture that can be regained, and these are different for different fibres. 

However, while the weight of the yarn is influenced by the amount of moisture in the air, the length of yarn is not. For that reason, many yarn producers measure the yarn in meters, not in grams.

Packaging laws require that the weight is indicated on the label, but the more important measurement for you as a consumer is actually the length of the yarn, also referred to as meterage or (in American English) as yardage. 

As a general rule of thumb, around 2.5% to 6% difference in weight compared to the stated weight on the label is usually considered acceptable, but that may differ depending on the fibres used in the yarn.