27 CFR § 19.356 - Alcohol content and fill.
(a) General. At representative intervals during bottling operations, a proprietor must examine and test bottled spirits to determine whether the alcohol content and quantity (fill) of those spirits agree with what is stated on the label or the bottle. A proprietor's test procedures must be adequate to ensure accuracy of labels on the bottled product. Proprietors must record the results of all tests of alcohol content and quantity (fill) in the record required by § 19.600.
(b) Variations in fill. Quantity (fill) must be kept as close to 100 percent fill as the equipment and bottles in use will permit. There must be approximately the same number of overfills and underfills for each lot bottled. In no case will the quantity contained in a bottle vary from the quantity stated on the label or bottle by more than plus or minus:
(1) 1.5 percent for bottles 1.0 liter and above;
(2) 2.0 percent for bottles 999 mL through 376 mL;
(3) 3.0 percent for bottles 375 mL through 101 mL; or
(4) 4.5 percent for bottles 100 mL and below.
(c) Variations in alcohol content. Variations in alcohol content may not exceed 0.3 percent alcohol by volume above or below the alcohol content stated on the label.
(d) Example. Under paragraph (c) of this section, a product labeled as containing 40 percent alcohol by volume would be acceptable if the test for alcohol content found that it contained no less than 39.7 percent alcohol by volume and no more than 40.3 percent alcohol by volume.