In the UK a size 10 dress or skirt has a 28 inch waist and sizes go up in increments of 2″.

Size 12 would therefore have a 30″ waist, size 14, a 32″ inch waist, size 16, a 34″ waist, size 18, a 36″ waist and so on. For a waist 33″ you would either have to settle for a size 14, with a 32 inch waist, or go one larger and get a size 16, which would have a 34″ waist.