Alcohol described as “100 proof” indicates a specific concentration of ethanol. The proof number is twice the percentage of alcohol by volume (ABV). Therefore, a spirit labeled as 100 proof contains 50% alcohol. This standard is commonly used for whiskeys, vodkas, gins, and other distilled spirits. For example, if a bottle of bourbon is marked as 100 proof, it signifies that half of its volume is pure alcohol.
The “proof” system has historical roots, originating from a method used to assess the alcohol content of spirits. In the past, spirits were tested by soaking gunpowder with the liquor and attempting to ignite it. If the gunpowder ignited, it was considered to be “proof” that the spirit contained a satisfactory amount of alcohol. While the modern system uses ABV measurements, the term “proof” remains a standard way to communicate alcohol strength to consumers. Spirits at this strength offer a noticeable intensity and are often preferred in cocktails or enjoyed neat by those who appreciate a bolder flavor profile.