Slack fill
The empty space inside a package. It is a legal problem when a box is made to look fuller than it is for no good reason, such as protecting the product.
Also called: slack-fill, nonfunctional slack-fill, empty space in packaging
The empty space inside a package. It is a legal problem when a box is made to look fuller than it is for no good reason, such as protecting the product.
What it means
Slack fill is the empty space inside a package: the gap between the container's capacity and the amount of product in it. Some slack fill is normal. It becomes a problem when it is "nonfunctional." That means the package is bigger than it needs to be, and shoppers who cannot see inside are misled about how much they are getting.
FDA rules say a food container that does not let you see its contents is misleading if it has nonfunctional slack fill. Slack fill is allowed when it has a real purpose. Examples are protecting the contents, fitting the packing machines, and allowing for settling during shipping. The package may also need the space to do a specific job, or serve as a valuable reusable container. Space is also allowed when the package cannot reasonably be made smaller, such as to fit required labeling or discourage pilfering.
Under federal law, proceedings to enforce the food and drug statute are brought by the United States, not by individual shoppers. Class actions over half-empty boxes therefore usually rely on state consumer protection laws.
What it means for you
A box that is only partly full is not automatically unlawful. The question is whether the empty space served a purpose, and whether the package let you see how full it was. Check the net weight or count printed on the package, which states how much product is inside. Suppose a notice names a product you bought. Check the dates and exact product names in the class definition, and keep receipts or packaging in case proof is requested.
Example
Say a hypothetical candy company sells chocolate pieces in a tall, opaque box that is about half empty. The company says the extra space protects the candy from breaking. A buyer says the box is simply made to look fuller. A court would ask whether the empty space served one of the allowed purposes. If the company settles, buyers during a set period might claim a small payment.
FAQ
Not always. FDA rules treat nonfunctional slack fill in food containers as misleading, but slack fill that serves a real purpose, such as protecting the contents or fitting the packing machines, is allowed. Whether a particular box crosses the line depends on the facts.
Possibly. Only the government can enforce the federal food rules, but shoppers have brought class actions under state consumer protection laws, claiming a package misled them about how much it holds.
Related terms
- 21 CFR 100.100: misleading containers and slack fill (Cornell LII)
- 21 U.S.C. 337: proceedings in the name of the United States (Cornell LII)
Updated Oct 18, 2026. Plain-English definitions for general information — not legal or tax advice.