# Water activity

> A 0-1 measure of how freely available the water in a material is; for dried cannabis flower the accepted range is about 0.55-0.65 aw.

- Canonical: https://genetixbay.com/knowledge/glossary/water-activity.html
- Also written: aw, a_w, a w, water activity (aw)
- Glossary category: Harvest, drying and curing · 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"

Water activity (aw) is the ratio of the vapour pressure of water in a material to that of pure water at the same temperature. It runs from 0 ([bone dry](https://genetixbay.com/knowledge/glossary/over-drying.md)) to 1.0 (pure water). Unlike [moisture content](https://genetixbay.com/knowledge/glossary/moisture-content.md), it measures how available the water is to microbes and chemical reactions, which is what decides mould risk and shelf stability.

## In practice

ASTM D8197 specifies 0.55-0.65 aw as the acceptable range for dried cannabis flower: below it buds get brittle and harsh, above it mould risk rises. Many moulds can't grow below about 0.70 aw.

House position: what matters during the cure is the bud's moisture content and water activity, not the [jar RH](https://genetixbay.com/knowledge/glossary/equilibrium-relative-humidity.md) as such. Ways to check:
- a calibrated hygrometer in a sealed jar, read only after it has equilibrated (several hours or overnight); then aw ≈ [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) / 100;
- a bench water-activity meter, the lab method (ASTM D8196);
- the [stem snap](https://genetixbay.com/knowledge/glossary/stem-snap-test.md) and weight loss as cross-checks.

## Units and formula

aw = p / p₀ (vapour pressure over sample / over pure water, same temperature). At equilibrium, aw = ERH / 100. So 0.62 aw ≈ 62% equilibrated jar RH.

## Don't confuse with

- **Moisture content**: how much water there is, in % by weight.
- **Jar RH**: only equals aw × 100 once equilibrated.
- **Relative humidity of the room**: air, not bud.

## Related terms

[Equilibrium relative humidity](https://genetixbay.com/knowledge/glossary/equilibrium-relative-humidity.md), [Moisture content](https://genetixbay.com/knowledge/glossary/moisture-content.md), [Curing](https://genetixbay.com/knowledge/glossary/curing.md), [Stem snap test](https://genetixbay.com/knowledge/glossary/stem-snap-test.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)

## Sources

1. ASTM D8197. Standard Specification for Maintaining Acceptable Water Activity (aw) Range (0.55 to 0.65) for Dry Cannabis Flower. ASTM International. https://store.astm.org/d8197-22.html
2. ASTM International, Standardization News. Two cannabis standards you should know about (D8196 water activity measurement; D8197 range). https://sn.astm.org/node/2445/printable/print