Most females will fit into the S/M band and most males will fit into the M/L band.
95% of people will fit in S/M or M/L, but if they have very large hands, get them an XL band. There is some overlap between band sizes, so if you are unsure, go with the larger size.
Apple Watch bands correspond to the size of the Apple Watch face that you have.
View the Size Guide for more details.
Please note that US orders will incur a $35 shipping fee and all other orders will have a $65 fee, which covers both outbound and return shipping.