In the old days all cotton materials – including denim – would shrink quite a lot after the first couple of washes. They were shrink-to-fit so to speak.
Today, only Levi’s in America, Levi’s Vintage Clothing and other American heritage brands, as well as many of the exclusive Japanese brands, offer shrink-to-fit. Unshrunken and untreated cotton shrinks and the cotton fibres constrict after washing.
Therefore, generally the fabric shrink about 10%, meaning you probably have to go up 2″ in the waist and 3″ in the length.
Source: After the Denim