27 CFR 30.36: General requirements.

The quantity determination of distilled spirits that are withdrawn from bonded premises in bulk upon tax determination or payment shall be by weight. The quantity of other distilled spirits or denatured spirits may be determined by weight or by volume. When the quantity of distilled spirits or denatured distilled spirits is determined by volume, such determination may be by meter as provided in 27 CFR part 19, or when approved by the appropriate TTB officer, another method or device.