One cycle of washing and drying usually allows for substantial shrinkage, but it should be noted that some fabrics can continue to shrink at each subsequent cleaning. (Ever have a pair of cotton pants get shorter and shorter after each washing?)
Fabric shrinkage can be a major factor in the long-term fit of any garment or interior decorating item. For the maximum protection for your fabric investment, we sincerely recommend that you pre-wash and dry any fabric or garment that is going to be washed during the lifetime of the item PRIOR to having the item altered or constructed. If you are having a garment hemmed, be sure to do this prior to having the hem set. If you are planning interior decoration items, it is crucial that all components of the item are pre-shrunk prior to construction
There are two types of shrinkage, fiber shrinkage and what I call mill shrinkage. Most of us are familiar with fiber shrinkage, which occurs when the fibers of which the threads in the fabric are made change when they are washed and dried. For example, wool can shrink considerably if it is washed, particularly if it is dried without blocking. Did you know that 100% cotton can shrink up to 6" per yard? Rayon, a cellulose fiber, can shrink as well, and often looks terribly deformed immediately after washing, only to return to the original size after line drying and pressing. In these days of blended fiber fabrics, such as the combining of linen and rayon, it is almost impossible to determine whether, or how much, a fabric will change once it is laundered and dried.
Mill shrinkage is the change in a fabric that occurs the first time it is cleaned. When it is loomed, the threads of the fabric are stretched tightly on a huge frame while the actual weaving is being done. When the fabric is removed from the loom, it retains some of this loom tension even after it is rolled onto bolts. There are microscopic spaces between the threads of the fabric, and these re position themselves after the first washing, even if the item is line-dried rather than placed into a drier. The loss of the de facto air spaces creates a change in the appearance of the fabric and can sometimes appear to cause shrinkage, even if the fibers of the fabric do not change. In addition, most new fabrics contain sizing, an additional product added to the fabric to make it more stiff and ‘new’ feeling. Sizing aids in the maintenance of the mill tension, and during the first cleaning, when it is removed, the fabric always changes slightly.
The maximum protection against shrinkage is provided by dry cleaning, but that process is not a guarantee against shrinkage. When having slipcovers constructed from upholstery fabric that you intend to dry clean, it is still recommended that you have the yardage steamed by the dry cleaner before the sewing begins. This process of subjecting the fabric to heat and steam will often achieve ‘mill shrinkage,’ and will increase the likelihood that the slipcover will continue to fit, even after it has been dry cleaned.