Bourbon being poured into a glass.
The Difference Between Full Proof, Barrel Proof, And Cask Strength Bourbon
A bourbon's type of proof, or strength of alcohol content, can be determined by one of three terms: full proof, barrel proof, and cask strength. Each one means something specific.
Bourbon's proof before aging is called the entry proof. As it ages, its proof fluctuates depending on several factors, and its resulting proof is known as the exit proof.
Full proof is the entry proof. To make it full proof, a distiller will add water to the barrel until the exit proof returns to the entry proof, making for a consistent process.
Barrel proof spirits are the least adulterated, as there's no water added. It's the exit proof that matters, as the proof that comes out is the proof that goes into the bottle.
Cask strength means the same thing as barrel proof; the proof of a whiskey is the same thing as its strength. However, not all cask strength whiskeys are single barrel whiskeys.