The FDA regulation on Net Weight is found in 21 CFR 101.105. In this regulation FDA makes allowance for reasonable variations caused by loss or gain of moisture during the course of good distribution practice or by unavoidable deviations in good manufacturing practice. FDA states that variations from the stated quantity of contents should not be unreasonably large.
While FDA does not provide a specific allowable tolerance for Net Weight, this matter could come under FTC jurisdiction. FTC has proposed regulations that would unify USDA and FDA Net Contents labeling and incorporate information found in the National Institute of Standards & Technology (NIST) Handbook 133.
NIST Handbook 133 specifies that the average net quantity of contents in a lot must at least equal the net quantity declared on the label. Plus or minus deviation is permitted when caused by unavoidable variation in weighing and measuring that occur in good manufacturing practice. The maximum allowable variance for a package with a net weight declaration of 5 oz is 5/16 oz. Packages under-filled by more than this amount are considered non-compliant.