To calculate your size, first measure around your chest at the widest point in inches, and that will indicate which size to go for. If you are slightly over a certain size (for example a 44.5" rather than a 44"), or plan to wear the jacket with a sweater underneath, we would always recommend upsizing. Below is a rough guide to our jacket sizing for each size, which are all based on the same general fit. All sizes below are in inches.