Fitting Guide;To Measure Chest Size - Measure chest size just under arms and across shoulder blades, holding tape level (don't hold your breath). Be sure to cross over the shoulder blades and the fullest part of your chest. The St. Micael's College Junior Blazer is a 'standard fit' blazer; If ordering for a slim build child, choose one size down from your chest measurement. For example, if the chest measurement is 34" (inch), choose a size 11 (33").If you are having any trouble with sizes and need assistance you can email us [email protected] *PLEASE NOTE*On smaller sizes, the sleeve length is manufactured slightly longer to allow for alteration if required. SIZE GUIDE (Chest Size) Size 8 9 10 11 12 13 14 15 16 Chest (inches) 30" 31" 32" 33" 34" 35" 36" 37" 38" 17 18 19 20 21 22 23 24 25 39" 40" 41" 42" 43" 44" 45" 46"