16 C.F.R. § 500.9 – Units of weight or mass, how expressed.

(a) The term net weight or net mass may be used in stating the net quantity of contents in terms of weight or mass. However, where the term “net weight” or “net mass” is not used, the quantity of contents shall always disclose the net quantity of contents. For example: “453 g (1 lb)” or “Net Wt 1 lb (453 g)” or “Net Mass 453 g (1 lb)”.
(b) With the exception of random packages, the statement of net quantity of contents in terms of avoirdupois weight shall be expressed as follows:
1. (1) If less than 1 pound, in terms of ounces. (Examples: “Net Weight 12 oz. (340 g)” or “Net Mass 340 g (12 oz)”.)
2. (2) If at least 1 pound but less than 4 pounds, in whole pounds, with any remainder in ounces or common or decimal fractions of the pound, except that it shall be optional to include an immediately adjacent additional expression of net quantity in terms of ounces.
  (Examples:“Net Wt. 1 lb. 8 oz. (680 g)” or “Net Wt. 1.5 lb./24 oz. (680 g)” or “24 oz. (1 1/2 lb.) 680 g”.)
3. (3) If 4 pounds or more, in terms of whole pounds, with any remainder in terms of ounces or common or decimal fractions of the pound, except that it shall be optional to include an immediately adjacent additional expression of net quantity in terms of ounces. (Examples: “Net Weight 5 pounds 4 ounces (2.38 kg)” or “Net Mass 2.38 kg (5 lbs 4 oz)” or “Net Wt. 5 1/4 lbs. (2.38 kg)” or “Net Mass 2.38 kg (5 1/4 lbs.)” or “Net Wt. 5.25 lbs. (2.38 kg) or “Net Mass 2.38 kg (5.25 lbs)”.)
(c) If the net quantity of contents declaration appears on a random package it may, when the net weight exceeds 1 pound, be expressed in terms of pounds and decimal fractions of the pound carried out to not more than three decimal places. When the net weight does not exceed 1 pound, the declaration on the random package may be in terms of decimal fractions of the pound in lieu of ounces. (Examples: “Net Wt. 0.75 lb.” and “Net Weight 1.05 pounds.”) Such decimal declaration shall be exempt from the type size and placement requirements of section 4(a) of the Act if the accurate statement of net weight is presented prominently and conspicuously on the principal display panel of the package. The net quantity of contents declaration on a random package is not required to, but may include a statement in terms of the SI metric system carried out to not more than 3 decimal places.
(d) It is sufficient to distinguish avoirdupois ounce from fluid ounce through association of terms. (Examples: “Net Wt. 6 oz.” vs. “6 fl. oz.” or “Net Contents 6 fl. oz.”)