If your chest measures 40" you likely wear a size 40 (suit) jacket. If you measure the jacket, it is 4” larger than the actual chest measurement, so the real size would be 44".
I was wondering how many inches are added to each kind of garment. 4" seems to be the average for a suit jacket, so you can breath and wear a shirt underneath. What about overcoats, is there an average measure relation too, so you can wear a jacket or cardigan and breath? And sport leather jackets?