00 (Chest 18"-20" 2-3yrs), 0 (Chest 20"-22" 4-5yrs), 1 (Chest 22"-24" 6-7yrs), 2 (Chest 24"-26" 8-10yrs), 3a (Chest 26"-28" 11-13yrs), 3 (Chest 29"-32"), 4 (Chest 31"-33"), 5 (Chest 34"-36"), 6 (Chest 36"-38")