Question: 40 alcohol is what proof?
Answer: (Phew.) 3: In the United States, the system — established around 1848 — is a bit simpler: “Proof” is straight up two times alcohol by volume. So a vodka, say, that is 40 percent ABV is 80 proof and one that is 45 percent ABV is 90 proof. A “proof spirit” is 100 proof (50 percent ABV) or higher.