U.S. boot sizes range from width AAA, which is the narrowest, to EEE, which is the widest. Size E refers to a boot width that is one increment larger than a men's medium, which is D, and three increments larger than a women's medium, which is B. Medium is considered a normal size, but the dimensions of a size medium vary according to brand and, in some cases, the country of manufacture.

Boot width sizes are linear. Each size is 3/16 of an inch different from the previous size. Boots with an E width, for example, are 3/16 of an inch wider than the same type of boot in a D width, and 9/16 of an inch wider than the same type of boot in a B width.

The sizes of boots and other shoes often varies by brand or cut. If possible, try on shoes before purchasing them or measure your feet and compare to a brand-specific sizing chart.

Wearing an extra pair of socks or a thicker pair of socks can adjust the fit of boots that are too large. However, boots that are too small usually cannot be stretched. Buying boots slightly too large is therefore better than buying boots that are a little too small, according to Big Black Boots.