The amount that jeans shrink when washed in hot water depends primarily on the temperature of the water and the materials the jeans are made with. Jeans made with a combination of denim and cotton fibers tend to shrink more in hot water than those made with man-made fibers, such as nylon or polyester.
The amount of shrinkage in jeans also depends on the direction in which the fabric is woven and on the tightness of the weave. Tightly woven denim shrinks less than fabric with a loose weave. More shrinkage occurs if the jeans are put into a hot dryer after they are washed with hot water. The higher the temperature, the more the fabric shrinks. The denim also tends to lose its color more rapidly when washed in hot water.
Although jeans may shrink significantly when they are washed in hot water, using this method to ensure a better fit is not always effective. Jeans tend to shrink unevenly when washed in hot water, so they may get smaller, but they won't necessarily fit better. To keep shrinkage down, one should opt for cooler temperatures in the wash and always use a low heat setting when putting jeans in the dryer.