Duncan Suit,

How to Measure for Men’s Blazers:

With arms relaxed at sides, measure directly under your arms around the fullest part of your chest. Blazer sizes should correspond to your chest size. (i.e., if you are a man with a 95″ chest, you should probably fit best in a size 48 blazer).

Measure the area where you want your trousers to sit, keeping the tape comfortably loose.

With arms at your sides, have someone measure around the broadest part of your shoulders and upper arms.