Milk does not have a chemical formula. It is a mixture of about 100 different things that DO have chemical formulas. The number one ingredient in milk is water (88%) with the formula H2O. After that there is the sugar Lactose (about 4.6%) with formula C12H22O11, the major protein Casein (3.2%) with a very complex formula, and Fat of various types of milk fat (3.4%). Also contains traces of vitamins and minerals with lots of different chemical formulas.