16 C.F.R. § 500.7
The net quantity of contents shall be expressed in terms of weight or mass, measure, numerical count, or a combination of numerical count and weight or mass, size, or measure so as to give accurate information regarding the net quantity of contents thereof, and thereby facilitate value comparisons by consumers. The net quantity of contents statement shall be in terms of fluid measure if the commodity is liquid, or in terms of weight or mass if the commodity is solid, semi-solid, or viscous, or a mixture of solid and liquid. If there is a firmly established general consumer usage and trade custom of declaring the contents of a liquid by weight or mass, or a solid, semi-solid, or viscous product by fluid measure, numerical count, and/or size, or (as in the case of lawn and plant care products) by cubic measure, it may be used, when such declaration provides sufficient information to facilitate value comparisons by consumers. The declaration may appear in more than one line of print or type.
16 C.F.R. § 500.7