Bourbon is whiskey that meets a specific set of federal requirements written into U.S. law. There are six core rules, and a spirit must satisfy every one of them to carry the word “bourbon” on its label: it must be made in the United States from a grain mixture that’s at least 51% corn, distilled to no more than 160 proof, put into new charred oak barrels at no more than 125 proof, bottled at 80 proof or above, and free of any added coloring, flavoring, or blending materials.
The 51% Corn Rule
The grain recipe, called a mash bill, is what separates bourbon from rye whiskey, wheat whiskey, and other styles. Federal law requires at least 51% corn. In practice, most distillers use between 60% and 75% corn because higher corn content produces a sweeter, fuller-bodied spirit.
The remaining portion of the mash bill is a mix of secondary grains. Rye is the most common choice, and it adds spice and a sharper bite to the finished product. Some producers swap in wheat instead, which gives a softer, rounder flavor. Nearly every bourbon also includes a small percentage of malted barley, which contains enzymes that help convert starches into fermentable sugars during production.
New Charred Oak Barrels
This is the requirement that surprises most people and the one that has the biggest impact on flavor. Bourbon must be aged in brand-new oak barrels that have been charred on the inside. “New” means exactly what it sounds like: the barrel has never held any spirit before. Other whiskey styles, like Scotch, routinely use barrels that previously aged bourbon or sherry. Bourbon doesn’t get that option.
The barrels are almost always made from American white oak and hold a standard 53 gallons. Coopers char the interior with an open flame before the barrel is filled, creating a layer of caramelized wood sugars. As the bourbon ages, it expands into and contracts out of the wood with seasonal temperature changes, pulling out vanillin, tannins, and caramel compounds. This is where bourbon gets its characteristic amber color, vanilla notes, and oak finish. Because each barrel can only be used once for bourbon, thousands of spent barrels are sold every year to Scotch, rum, and tequila producers around the world.
Proof Limits at Every Stage
Federal standards cap the alcohol content at three different points in production, and each cap serves a purpose.
- Distillation: Bourbon cannot come off the still above 160 proof (80% alcohol by volume). Distilling at a lower proof preserves more of the grain’s original flavor. Spirits distilled closer to the legal ceiling of 190 proof, like vodka, lose most of their character.
- Barrel entry: The spirit must go into the barrel at no more than 125 proof (62.5% ABV). Distillers typically add water after distillation to bring the proof down to this threshold. Lower entry proofs give the spirit more room to interact with the wood over time.
- Bottling: The finished product must be at least 80 proof (40% ABV). There is no upper limit, which is why cask-strength bourbons can reach well above 120 proof.
No Additives Allowed
This is the rule that sets bourbon apart from many other brown spirits around the world. Scotch, Canadian whisky, and several other categories permit the addition of caramel coloring or flavoring agents. Bourbon does not. Federal regulations explicitly prohibit coloring, flavoring, or blending materials of any kind in bourbon whiskey. Every bit of color and flavor in the bottle must come from the grain, the fermentation, the distillation, and the barrel.
If a producer adds anything, the product can no longer be labeled as bourbon. It might be sold as a “whiskey specialty” or a “spirit whiskey,” but the bourbon name is off limits.
Made in the United States
Bourbon must be distilled and aged in the United States. Contrary to a common misconception, it does not have to come from Kentucky. Bourbon is legally produced in states from New York to Texas to Oregon. However, “Kentucky Straight Bourbon Whiskey” is a protected geographic designation. Federal regulations prohibit spirits produced outside Kentucky from using the state’s name on the label, the same way Cognac must come from the Cognac region of France.
Kentucky’s dominance in bourbon production is historical and practical rather than legal. The state has limestone-filtered water, a climate with wide temperature swings that accelerate barrel aging, and centuries of distilling tradition. Roughly 95% of the world’s bourbon is still produced there.
No Minimum Age (With a Catch)
There is no minimum aging period for standard bourbon. Technically, a distiller could fill a new charred barrel, wait a single day, and bottle the contents as bourbon. In reality, almost no one does this because the result would taste harsh and undeveloped.
The catch comes with labeling. To be called “straight bourbon whiskey,” the spirit must age for at least two years. If it’s aged fewer than four years, the label must state the exact age. Once it hits four years, the age statement becomes optional, which is why many familiar bourbons on store shelves don’t list an age at all.
Bottled-in-Bond: A Higher Standard
You may notice some bourbons labeled “Bottled-in-Bond.” This designation dates back to 1897 and imposes four additional requirements on top of the standard bourbon rules. The spirit must be the product of a single distillery in a single distilling season, aged for at least four years in a federally bonded warehouse, and bottled at exactly 100 proof. The label must also identify the distillery where the bourbon was produced and the facility where it was bottled.
Bottled-in-Bond was originally a guarantee of authenticity during an era when adulterated whiskey was common. Today it functions as a quality signal, since meeting all four criteria requires consistency and patience that cheaper products skip.