These will be a little loose. I go between a M and L but for this, I’m getting a large so it can be more of a jacket vs mid layer. If you don’t wear a phone/gun etc… on you hip and normally wear a large, I’d opt for the medium. Nice fall, spring jacket to toss over a t-shirt. I would treat it like a mid layer type jacket. I have one that’s the same style (no side zipper), and it has a hood. If you zip it up, close all the portals (pockets), it’s a very warm jacket. I normally use my eddie bauer 1st ascent “sweater” down (jacket) under my duluth trading company wind proof fleece jacket. it’s a large and it’s a big large. Lost of room to move around. VERY warm. It’s 8 out right now with winds around 20mph gusting to 30+. Only my face was cold. Nice thing is, toss a hard shell jacket over this and you got a outdoor set that isn’t gonna make you cry if you snag it on a fence or something. Snagged my EB jacket. It was 200 bucks and hooked it on a corner of a work bench. Had to supper glue the 2 inch rip.