Most cotton fabrics sold are ‘pre-shrunk’, meaning it goes through a wash process to shrink the fabric. If the cotton has been pre-shrunk, it will remain close to its original size with each wash but could still shrink up to 5%. If the cotton has not been pre-shrunk you could get as much as 15% shrinkage on your first wash. Cotton will always shrink but manufacturers usually take this into account when producing garments and other products.
Many people ask why 100% cotton shrinks? The reason lies in the fact that it is a natural fibre and how the cotton is made. Initially, cotton is spun into yarn, and the yarns are then stretched to make a warp and weave. This process involves tension to produce the final fabric, and when you then wash the cotton fabric, the exothermic reaction produces heat which reduces the tension and then leads to the cotton shrinking. You can read more information on the dimensional stability of fabrics in the following link:
To avoid shrinkage of 100% cotton fabric it is important to follow care and wash instructions. Cotton should be washed in cold water and you should also avoid drying cotton in a tumble dryer. Tumble drying reduces the tension in the fabric, which increases the shrinkage. It is best to air dry cotton garments and products for best results