This fits perfectly. I’d wear a Large gym t shirt. The jacket is THIN!! Gossamer thin, but I suppose it helps with its packability, which is very small. Even though it’s so thin it still kept the chill off. It’s got a nice chunky zip that opens from the bottom too, should you require a little more ventilation. I bought this jacket and the Madison Stella gilet together and I’m really pleased with both. (Only worn once, 70 miles) time will tell as far as longevity is concerned.