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Water Activity from Equilibrium RH

Convert a sample’s measured equilibrium relative humidity to water activity. See the input requirements and distinguish aw from moisture content.

Use this result well

Inputs that matter
Equilibrium relative humidity
Output to expect
Water activity from ERH
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Convert the equilibrium reading

Water activity is equilibrium relative humidity divided by 100, under matching equilibrium conditions.

aw = ERH / 100

Source: FDA: Water Activity in Foods

Keep the quantity explicit

The input is a percentage and the result is dimensionless. Values outside 0–100% are rejected; a value is never silently clipped to fit the range.

Use the appropriate measurement method

Room humidity is not the same as sample equilibrium humidity. The conversion does not validate sampling, calibration or temperature control.

Updated: September 2026

Example Scenarios

Enter 65% equilibrium RH.

→ aw = 0.65.

Enter 92.5% equilibrium RH.

→ aw = 0.925. This number does not supply a preservation verdict.

Enter 101% equilibrium RH.

→ A field error replaces the result; no clamped value is returned.

Common Mistakes to Avoid

Entering moisture or sugar percentage

Use measured equilibrium humidity for this conversion. A moisture-content model needs material-specific parameters.

Using room humidity

Use a suitable sample measurement method; ambient room RH is not automatically the sample’s equilibrium RH.

FAQ

Equilibrium relative humidity for the sample, expressed as a percentage.

Water activity is dimensionless. A result of 0.65 corresponds to 65% equilibrium RH.

Not with this conversion. Different materials can have different sorption relationships.

No. Follow the relevant complete process, measurement requirements and storage instructions.

A GAB calculation requires a fitted relationship for the material and conditions. It is a model estimate, distinct from this conversion of a measured equilibrium reading.

About Water Activity from Equilibrium RH

Enter an equilibrium headspace reading for a sample. This simple conversion keeps the measurement basis visible instead of inferring water activity from unrelated ingredient percentages.