27 CFR 31.161: Conversion between metric and U.S. units.

When liters are converted to wine gallons, the quantity in liters must be multiplied by 0.264172 to determine the equivalent quantity in wine gallons. Once converted to wine gallons, the proof gallons of spirits in cases must be determined as provided in § 30.52 of this chapter. Cases containing the same quantity of spirits of the same proof in metric bottles may be converted to U.S. units by multiplying the liters in one case by the number of cases to be converted, as follows:

(a) If the conversion from liters to U.S. units is made before multiplying by the number of cases, the quantity in U.S. units must be rounded to the sixth decimal; and

(b) If the conversion is made after multiplying by the number of cases, the quantity in U.S. units must be rounded to the nearest hundredth.