Water activity (written a_w) measures the free, unbound water in a food on a scale from 0 to 1 — the water that bacteria, yeast, and mold can actually use to grow. It is not the same as total moisture: honey is wet but has very low water activity, which is why it does not spoil.
The number matters legally as much as practically. A water activity at or below 0.85 — the threshold the FDA Food Code (2022 edition) uses for non-potentially-hazardous food, which many states use as a reference baseline for their cottage food rules — is generally the line that keeps cookies, breads, and dry mixes inside cottage food law while pushing cream-cheese frostings and many fillings outside it. Some states set their own threshold or define the category differently, so confirm your own state's rule (Ardent Seller's per-state cottage food guides are a starting point); either way, knowing where a product sits determines whether it can be made and sold from a home kitchen at all.
For a food maker, water activity is a compliance and shelf-life attribute worth recording alongside a product's recipe and batch — part of the same traceability and labeling discipline that lot codes and allergen statements belong to.
Related terms
Traceability
The ability to trace a finished product back to its source ingredients, production run, and batch numbers. Critical for food safety, quality control, and regulatory compliance.
Batch/Lot Number
A unique identifier assigned to a production run or group of items produced together. Enables traceability from raw ingredients to finished products for quality control and compliance.
Cottage Food Law
State laws that allow certain low-risk foods to be made in a home kitchen and sold without a commercial license, within defined limits.
Recipe
A defined set of ingredients, quantities, steps, and equipment needed to produce a finished good. Recipes automatically calculate production costs based on current ingredient prices.