OOFOS come in whole sizes only and are sized unisex, so a size is listed as its men's and women's equivalent together (M5/W7). The letters indicate gender equivalence, not width.

The half-size rule runs in opposite directions: if you wear a women's half size, order UP to the next whole size (W8.5 orders W9); if you wear a men's half size, order DOWN (M10.5 orders M10). OOfoam doesn't stretch or break in, so if a size already feels tight, go up rather than forcing it.

• Whole sizes only across the range.
• Women in half sizes round up; men in half sizes round down.
• Your foot should sit securely in the footbed, with its arch under your own arch.
• The strap should feel comfortable rather than tight, and your foot should slide in easily.
• Size up on the OOcloog if you plan to wear thick socks with it.
• OOFOS need little breaking in, but the footbed feels different from normal footwear while you adjust — that's expected.