Water Activity Estimate Calculator

Work out water activity estimate instantly with clear inputs, formula shown and shareable results.

About Water Activity Estimate Calculator

Water activity is the ratio of the water vapour pressure above a food to that of pure water, equal to equilibrium relative humidity divided by 100. Raoult's law estimates it as moles of water divided by total moles of water plus dissociated solute. Below 0.6 no microorganism grows; below 0.85 most pathogenic bacteria cannot.

Formulas

Raoult's law

aw = n_water / (n_water + n_solute x dissociation factor)

From ERH

aw = equilibrium relative humidity % / 100

Worked example

With the default values below, Water Activity Estimate Calculator returns Water Activity (Raoult estimate) of 0.93278. Change any input above and every figure here is recalculated in your browser.

Inputs used

Moles Of Dissolved Soluteminimum 0 mol: 2 mol
Water In The Productminimum 0 g: 1,000 g
Dissociation Factoraccepts 1–4: 2
Measured Equilibrium Relative Humidityaccepts 1–100 %: 85 %

Results

Water Activity (Raoult estimate): 0.93278
Water Activity From Measured ERH: 0.85000
Effective Solute Moles: 4.0000
Microbial Stability Band: High moisture - bacterial growth possible

Step by step

1Values used
2Raoult's law
3From ERH
4Water Activity (Raoult estimate)= 0.93278
5Water Activity From Measured ERH= 0.85000
6Effective Solute Moles= 4.0000
7Microbial Stability Band= High moisture - bacterial growth possible

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Water activity is a critical food safety parameter. Estimates must be confirmed by calibrated measurement for any shelf stability or HACCP decision.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Water activity is a critical food safety parameter. Estimates must be confirmed by calibrated measurement for any shelf stability or HACCP decision.

Frequently Asked Questions

Why does water activity matter more than moisture content?

Microorganisms respond to available water, not total water. Two products with the same moisture content but different solute levels have very different water activities and shelf lives.

Which organisms tolerate low water activity?

Staphylococcus aureus grows down to about 0.86, most moulds to 0.80, xerophilic moulds to 0.61 and Zygosaccharomyces rouxii to about 0.62. Below 0.60 nothing grows.