Standard shoe sizes in the United States follow number designations that indicate length and letter designations that indicate width. The size indicators all correspond to more precise measurements expressed in inches.

For width, the letters range from A to E, and measure width in proportion to length. A indicates a narrow foot; E indicates a wide foot. A less-common "Mondopoint" system dispenses with the letters and simply expresses length and width in millimeters.

The precise measurement varies with the basic length indicator. For example, a 6C is 3.3 inches wide; a 6E is 3.7 inches wide. The increment from one letter designation to the next is 1/16th of an inch.

On the Brannock device used in shoe stores to measure length and width, the width desginations run from AAA (the narrowest), to D (the standard width, used in 95% of all shoes), up to E (wide), EE (double-wide), and finally EEE, also known as 3E.

For women’s sizes, the width designations are narrow, average, wide, and extra wide; another system uses the letters A (narrow), B (average), and D (wide).

Shoe sizes are not standardized and exact measurements corresponding to length and width measures can vary from one shoe manufacturer to the next. Some companies even have their own sizing systems.