Birkenstock Unisex Size Chart:

Size 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Women's US 2-2½ 3-3½ 4-4½ 5-5½ 6-6½ 7-7½ 8-8½ 9-9½ 10-10½ 11-11½ 12-12½
Men's US 4-4½ 5-5½ 6-6½ 7-7½ 8-8½ 9-9½ 10-10½ 11-11½ 12-12½ 13-13½ 14-14½ 15-15½