PLENNY LEARN · UNDERSTANDING LAB REPORTS
Two moisture numbers, two different questions
Moisture content describes how much water a sample contains by weight. Water activity describes the water’s availability, expressed as a ratio. A cannabis COA can include both, and the numbers are not interchangeable.
By Plenny · Updated September 18, 2026 · 4 minute read
Water activity vs. moisture content: what is the difference?
Amount of water relative to sample mass. Usually shown as a percentage.
Written as aw or Aw. A dimensionless ratio, usually displayed as a decimal.
A measurement of the air. Its relationship to water activity requires equilibrium.
What does moisture content mean on a cannabis COA?
Read the method and reporting basis alongside the percentage. SC Labs describes its moisture analysis as weight loss on drying, calculated from the difference between the starting and dried sample weights.
For a simplified wet-basis example, a 10-gram sample containing 1 gram of water has 10% moisture content: 1 divided by 10, multiplied by 100. A dry-basis percentage uses dry material in the denominator instead. Confirm which basis the laboratory reports before comparing results.
What does water activity mean?
The FDA defines water activity using the sample’s equilibrium water-vapor pressure relative to pure water at the same temperature. Water bound within a material behaves differently from readily available water, so total moisture alone does not specify water activity.
The relationship between the two depends on the material and temperature. There is no universal conversion from a moisture percentage to Aw.
How to read moisture and water activity together
Illustrative report: Moisture content 10.0% · Water activity 0.60 Aw
These are invented figures for learning how to read units, not a product result, recommended target or pass/fail decision.
What does the 10.0% number describe?
On a wet-weight basis, it represents moisture as a proportion of the sample’s total starting mass. It does not describe the humidity of the air or the amount of THC.
Does 0.60 Aw mean the flower is 60% water?
No. At equilibrium, 0.60 Aw corresponds to 60% equilibrium relative humidity. That relationship does not convert Aw into a percentage of water by weight.
SC Labs explains that water activity testing measures a sample in a sealed chamber, allowing the headspace to equilibrate. A room-humidity reading is a different measurement and cannot simply be substituted for a laboratory result.
Always keep the label attached to the number. “10% moisture,” “0.60 Aw” and “60% relative humidity” answer different questions.
Does low water activity mean cannabis is mold-free?
No. Water activity helps assess conditions relevant to microbial growth; it does not identify every organism or contaminant already present. A result from this test is not a substitute for the microbial section of a COA.
SC Labs lists water activity, microbiological contaminants and mycotoxins in separate test panels. Read each relevant result and its reporting limits. Do not turn a pass in one category into an overall safety guarantee.
What is a good water activity result?
Interpret the number against the laboratory’s stated specification for that product and testing program. Its published panel includes different action limits for different product categories. A limit for one category should not be copied onto another.
If the COA does not clearly identify the applicable limit, ask the testing laboratory what the result means. This guide explains the measurements; it does not establish a universal safe range or certify a particular sample.
A quick COA reading checklist
Use these checks while looking at the actual report. The checklist is a reading aid and does not calculate a safety score.
If a screenshot shows only potency or a summary badge, open the complete report before interpreting it. Our guide to reading a cannabis COA explains how to work through its sections.
Common questions
Can I convert moisture percentage directly to water activity?
No universal formula does that. You need the relationship measured for the particular material at the relevant temperature.
Is water activity the same as humidity?
They are related at equilibrium, but a humidity reading from surrounding room air is not automatically the sample’s water activity.
Does a moisture result tell me the THC percentage?
No. Moisture and cannabinoid concentration are separate reported measurements. Read the cannabinoid panel and its weight basis.
Does an Aw pass mean every other test passed?
No. Check each testing category separately. A pass only answers the question covered by that test and specification.
Keep learning
Sources and scope
This guide uses the FDA’s technical explanation of water activity and SC Labs’ descriptions of moisture analysis, water activity testing and separate laboratory panels. The FDA source explains measurement principles in foods; it is not a cannabis product approval. The laboratory pages describe their own services and methods. Sources checked September 18, 2026.
Chris Garcia, Owner and CEO
September 18, 2026
How this page is prepared: Plenny uses linked research and other relevant sources for educational explanations, and product details and batch COAs where relevant. General strain, flavor, and effect descriptions are not guarantees.
Commercial transparency: Plenny sells products discussed on this website. Educational pages are not medical or legal advice, and content is not medically reviewed unless a named licensed reviewer is shown.