Suit sizing format: chest size + length code. Example: 40R = 40" chest, Regular length.
Length codes: S = Short (5'4"–5'7"), R = Regular (5'8"–6'0"), L = Long (6'1"–6'4"), XL = Extra Long (6'5"+).
Pants come 6" smaller than jacket by default (40R jacket = 34W pants). This is called the 'drop'.
Buy for the shoulders. Everything else can be tailored — shoulders cannot. If the shoulder seam sits beyond your natural shoulder, size down.
EU equivalents: US 38 ≈ EU 48, US 40 ≈ EU 50, US 42 ≈ EU 52, US 44 ≈ EU 54.