: The shoulder seam of the overcoat should sit slightly past your suit jacket's shoulder (about 1–2 cm). If it drops too far down your arm, the coat is too large; if it pulls, it is too small.

: You should be able to button the coat comfortably without any pulling or tension across the chest.

: If you plan to wear the coat only over thin layers like a t-shirt or a dress shirt for a modern, slim look, consider buying one size smaller than your suit size.

: A classic overcoat should fall at or slightly below the knee. Shorter styles (topcoats) usually end mid-thigh. sizing an overcoat/topcoat - Ask Andy About Clothes

: If you wear a size 42 suit, a size 42 overcoat is intended to fit over it comfortably.

: Reach your arms forward as if hugging someone; if you feel extreme restriction in the back or shoulders, you need a larger size.