How many proof and volume of bourbon?

Bourbon must be distilled to no more than 160 proof (80% alcohol by volume) and entered into the barrel for aging at no more than 125 proof (62.5% ABV). The minimum aging requirement for bourbon is two years, but to be labeled as "straight bourbon," it must be aged for at least four years. There is no minimum volume specification for production, but it must be made in the U.S. and contain at least 51% corn.