How is the proof of a spirit calculated?

The proof of a spirit is calculated by multiplying its Alcohol by Volume (ABV) percentage by two. This is because proof is a measurement system that the United States and some other countries use, where the proof value is double that of the ABV. For example, if a spirit has an ABV of 40%, its proof would be 80. This calculation helps consumers understand the strength of the alcohol they are consuming, as a higher proof indicates a higher alcohol content.

The method of multiplying the ABV by two is consistent with standard practices in the beverage industry and is fundamental to accurately conveying the strength of alcoholic beverages. Other formulas provided in the options do not align with this established calculation method.