Manufactures have different methods of sizing garments these are the standard chest sizes for the UK

WOMENS FIT

                8 -XS -28″  10 -S -30″  12 -M -32″  14- L-34″   16-XL-36″  18- 2XL-38″

UNIFIT    

                XS-   S    /M  /L/  XL/     2XL/    3XL    /4XL     /5XL

YOUTHS

                 3/4- 26″   5/6      7/8     9/11  12/13