Moisture and Water Activity: Why Both Matter in Cannabis Testing

Moisture is more than a post-harvest quality measure in cannabis. It can affect reported potency, product weight, storage stability and the risk of microbial growth. For that reason, laboratories and producers generally distinguish between moisture content—the amount of water in a sample—and water activity, which indicates how available that water is to support chemical or biological activity.

Cannabinoid concentrations are commonly reported as a percentage by weight. If a 10-gram flower sample contains 2 grams of THC, its THC concentration is 20% by weight. But the result depends on what is included in the sample’s total weight and whether the laboratory reports potency on an “as received” or dry-weight basis.

Two samples containing the same mass of THC can therefore produce different percentages if one contains more water. Removing water lowers the sample’s total mass without removing the THC, increasing the calculated concentration on that basis. This does not mean the flower has gained THC; it means the cannabinoid represents a larger share of the remaining weight. Laboratories should clearly define their reporting basis and use a consistent sample-preparation procedure.

Moisture measurement is also important during drying, curing, packaging and storage. Testing material near its intended retail condition can help producers evaluate whether a batch is sufficiently dry and whether moisture is migrating into or out of the package. It can also help laboratories reduce variability caused by samples arriving at different moisture levels.

For regulatory testing, the ASTM practice for determining water activity in cannabis flower provides a standardized framework for measuring a property associated with product stability and microbiological risk. The method is not a substitute for cannabinoid analysis, but it complements potency and contaminant testing.

Several approaches are used to estimate or determine moisture content:

• Loss on drying: A weighed portion of flower is heated under controlled conditions and weighed again. The mass lost is treated as moisture, although volatile compounds other than water can also contribute to the loss. For that reason, temperature, time and sample preparation matter.
• Bench-top moisture analyzers: These instruments combine a balance with a controlled heat source and automatically track mass loss. They are faster and more repeatable than a basic oven-and-balance setup when properly calibrated. An application note from Shimadzu describes this loss-on-drying approach for cannabis and hemp flower.
• Moisture meters: Consumer wood meters can provide rapid, approximate readings by detecting electrical changes in a material. Some cultivators use a softwood setting as a practical guide during drying and curing. However, such meters are not automatically validated for cannabis, and their readings can vary with sample density, temperature, contact, calibration and the instrument’s assumptions about the material. They should not be treated as compliance-grade results without appropriate validation.

One technical correction is important when using an oven method: the moisture percentage is not simply the oven-dried weight divided by the original weight. That calculation represents the proportion of dry matter remaining. Moisture on a wet basis is generally calculated as the mass lost during drying divided by the original sample mass, multiplied by 100. Laboratories may use different conventions, so the basis should be documented.

Water activity answers a different question. Moisture content measures how much water is present; water activity measures how energetically available that water is. It is expressed on a scale from 0 to 1 and is commonly determined from the humidity of the air in equilibrium with the sample. Water-activity instruments may use capacitive or resistive humidity sensors, while chilled-mirror instruments determine the dew point with a direct thermodynamic measurement.

A sample can contain measurable moisture without having the same microbial risk as another sample with an identical moisture percentage. The way water is bound within the plant matrix, along with temperature, packaging and storage conditions, affects its availability. This is why water activity is often more useful than moisture content alone when the goal is to assess shelf stability and mold risk.

California illustrates how the two measurements can be used together. Under the Department of Cannabis Control’s testing rules, laboratories analyze dried flower—including pre-rolls—for both moisture content and water activity. The current rule sets a water-activity limit of 0.65 for dried flower and requires the result to be reported on the certificate of analysis. Batches that fail the water-activity requirement cannot be released for retail sale. The California cannabis regulations contain the detailed requirements, while the Department of Cannabis Control’s laboratory guidance summarizes the broader testing program.

Moisture content and water activity should therefore be viewed as complementary measurements. Moisture testing helps track drying performance, weight and analytical consistency. Water-activity testing provides a more direct indication of whether the remaining water may support microbial growth or compromise storage stability. For laboratories, producers and processors, validated methods, representative sampling and clear reporting practices are essential to making those results meaningful.

The importance of consistent procedures extends beyond one batch. The National Institute of Standards and Technology’s cannabis laboratory quality-assurance work has examined how laboratories measure moisture in cannabis and hemp materials, underscoring the value of proficiency testing and method consistency across facilities.