Clothing sizes are the sizes with which garments sold off-the-shelf are labeled. Sizing systems vary based on the country and the type of garment, such as dresses, tops, skirts, and trousers. There are three approaches:

• Body dimensions: The label states the range of body measurements for which the product was designed. (For example: bike helmet label stating "head girth: 56–60 cm".)
• Product dimensions: The label states characteristic dimensions of the product. (For example: jeans label stating inner leg length of the jeans in centimetres or inches (not inner leg measurement of the intended wearer).)
• Ad hoc sizes: The label states a size number or code with no obvious relationship to any measurement. (For example: Size 12, XL.) Children's clothes sizes are sometimes described by the age of the child, or, for infants, the weight.

Traditionally, clothes have been labelled using many different ad hoc size systems, which has resulted in varying sizing methods between different manufacturers made for different countries due to changing demographics and increasing rates of obesity, a phenomenon known as vanity sizing. This results in country-specific and vendor-specific labels incurring additional costs, and can make internet or mail order difficult. Some new standards for clothing sizes being developed are therefore based on body-dimensions, such as the EN 13402 "Size designation of clothes".