Q and Answer: What Size Overcoat Should I Wear?
William asks: If I wear a 38 regular blazer, what size overcoat should I be looking for? 
You should look for your suit and blazer size when buying an overcoat. Manufacturers size with the expectation that you’ll be wearing an overcoat that you’ll be wearing an overcoat over a suit jacket or sport coat. If you wear a size 38 jacket, you’ll want a size 38 overcoat.
If you don’t expect to wear the overcoat on top of another jacket, you may want to consider sizing down one size. And of course you’ll always want to try on or check measurements, as they vary from manufacturer to manufacturer. While the vast majority of brands size overcoats by coat size, it’s not universal.
Generally speaking, you’ll want the chest of the overcoat to measure 2” larger than your actual chest size for a snug fit, and 4” or so larger than your actual chest to fit over a suit jacket.

Q and Answer: What Size Overcoat Should I Wear?

William asks: If I wear a 38 regular blazer, what size overcoat should I be looking for?

You should look for your suit and blazer size when buying an overcoat. Manufacturers size with the expectation that you’ll be wearing an overcoat that you’ll be wearing an overcoat over a suit jacket or sport coat. If you wear a size 38 jacket, you’ll want a size 38 overcoat.

If you don’t expect to wear the overcoat on top of another jacket, you may want to consider sizing down one size. And of course you’ll always want to try on or check measurements, as they vary from manufacturer to manufacturer. While the vast majority of brands size overcoats by coat size, it’s not universal.

Generally speaking, you’ll want the chest of the overcoat to measure 2” larger than your actual chest size for a snug fit, and 4” or so larger than your actual chest to fit over a suit jacket.