# Equilibrium relative humidity

> The relative humidity of the air in a sealed container once it has stopped exchanging moisture with the material inside; ERH / 100 equals the material's water activity.

- Canonical: https://genetixbay.com/knowledge/glossary/equilibrium-relative-humidity.html
- Also written: ERH, equilibrium RH, equilibrated RH, jar RH, sealed-jar RH
- 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"

Equilibrium relative humidity (ERH) is the [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) that the headspace of a closed container settles at when the product inside neither gains nor loses water. At that point, the air's RH reflects the product's [water activity](https://genetixbay.com/knowledge/glossary/water-activity.md): ERH (%) = aw × 100. ERH depends on temperature, so readings are compared at a stable room temperature.

## In practice

ERH is what a hygrometer in a [curing jar](https://genetixbay.com/knowledge/glossary/curing-jar.md) actually tells you, but only after equilibration. Right after you seal or open a jar, the reading is the room's air, not the bud's. Wait several hours, ideally overnight, at stable temperature before trusting it.

House position: jar RH is a proxy. The target is the bud's water activity (about 0.55-0.65) and [moisture content](https://genetixbay.com/knowledge/glossary/moisture-content.md), and equilibrated jar RH is a practical stand-in for aw. A jar that settles above about 65% needs more air; one under about 55% is over-dried.

Calibrate cheap hygrometers before trusting them; errors of 3-5 points are common. A salt test (a sealed bag with damp table salt gives about 75% RH) is the usual home check.

## Units and formula

aw = ERH / 100 (at equilibrium, same temperature).

## Don't confuse with

- **Room RH**: the air in the drying or storage room.
- **Moisture content**: water by weight, not humidity.
- **Humidity pack setting**: what a sachet tries to impose.

## Related terms

[Water activity](https://genetixbay.com/knowledge/glossary/water-activity.md), [Moisture content](https://genetixbay.com/knowledge/glossary/moisture-content.md), [Curing](https://genetixbay.com/knowledge/glossary/curing.md), [Burping](https://genetixbay.com/knowledge/glossary/burping.md)

## Read more

- [Curing and burping: jars, humidity and how long](https://genetixbay.com/knowledge/harvest-cure/curing-burping.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