Visit a tailor and have them measure you. You can actually measure yourself. Just get a tailor’s tape and wrap it around your chest, under your armpits and see what the measurement is. That’s your chest size!
Jackets will be cut 2”-4” larger than that chest measurement and still be your “size”. For instance, I have several 38R jackets that have pit-to-pit measurements of 21”=42” chest size. This is to allow for movement around your actual 38” chest.
The more important thing to look for is how your jacket fits in the shoulders. It shouldn’t hang off, nor be too tight. The seam should rest on top of them perfectly. Your size jacket could actually vary from manufacturer to manufacturer in some cases.