# Water activity test

> A water activity test measures how much of a sample's water is free to support microbial growth, reported as aw from 0 to 1, with cured flower targeted around 0.55-0.65.

- Canonical: https://genetixbay.com/knowledge/glossary/water-activity-test.html
- Also written: aw test, water activity testing, aw measurement, water activity meter, a_w test
- Glossary category: Lab testing · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

A water activity test seals a small sample in a chamber and measures the [equilibrium relative humidity](https://genetixbay.com/knowledge/glossary/equilibrium-relative-humidity.md) of the air above it, with a chilled-mirror [dew point](https://genetixbay.com/knowledge/glossary/dew-point.md) or capacitance sensor. [Water activity (aw)](https://genetixbay.com/knowledge/glossary/water-activity.md) is that equilibrium RH divided by 100. It tells you how freely water is available to microbes and chemical reactions, which total moisture alone does not.

## In practice

Regulated markets often set an upper limit around 0.65 aw for flower; check local rules. A [COA](https://genetixbay.com/knowledge/glossary/coa.md) might read aw 0.58 with moisture 10.8%: well-cured and stable. Readings above about 0.65-0.70 raise the risk of mould growth during storage. Home growers approximate the test by sealing buds in a jar with an accurate hygrometer and waiting for it to equilibrate; the jar RH then roughly equals aw × 100. Jar readings before equilibrium are only a proxy.

## Units and formula

aw = equilibrium RH (%) / 100. Dimensionless, 0-1. Pure water = 1.0.

## Don't confuse with

- **Moisture content**: the mass of water (% w/w); two samples with equal moisture can have different aw.
- **Jar RH before equilibrium**: tells you about the air, not yet the bud.

## Myth check

House position: [humidity packs](https://genetixbay.com/knowledge/glossary/humidity-pack.md) rob [terpenes](https://genetixbay.com/knowledge/glossary/terpene.md) and are a marketing gimmick; curing is a slow moisture decline, not holding it constant. See [curing](https://genetixbay.com/knowledge/harvest-cure/curing-burping.md).

## Related terms

[Water activity](https://genetixbay.com/knowledge/glossary/water-activity.md), [Moisture correction](https://genetixbay.com/knowledge/glossary/moisture-correction.md), [Dry-weight basis](https://genetixbay.com/knowledge/glossary/dry-weight-basis.md), [Microbial testing](https://genetixbay.com/knowledge/glossary/microbial-testing.md), [Mycotoxins](https://genetixbay.com/knowledge/glossary/mycotoxins.md)

## Read more

- [Curing and burping: jars, humidity and how long](https://genetixbay.com/knowledge/harvest-cure/curing-burping.md)
- [Long-term storage: keeping cured flower potent and aromatic](https://genetixbay.com/knowledge/harvest-cure/long-term-storage.md)