Skip to main content
Calcimator

Water Activity Calculator

Estimate water activity (aw) from moisture and solute content using Raoult's Law approximation.

Inputs

Results

Water activity (aw)

0.99

Equilibrium RH (%)99.48
Moisture content (%)90.91
Moles of water5.55
Effective moles of solute0.03
Microbial risk zone4
How to Use This Calculator
  1. Enter water mass and solute mass in grams for your food sample.
  2. Select the solute type (sucrose, NaCl, or glucose) to apply the correct molecular weight and dissociation factor.
  3. Enter storage or measurement temperature in °C.
  4. The calculator shows water activity (aw), equilibrium relative humidity, moisture content, moles of water and solute, and microbial risk zone.
  5. If aw > 0.91, refrigeration or additional preservatives are required to prevent bacterial growth.

How the result changes with Solute mass (g)

Solute mass (g)Water activity (aw)
10,0000.16
35,0000.05
65,0000.03
90,0000.02

What each input means

Water mass (g)
Mass of water in the food sample in grams.
Solute mass (g)
Mass of dissolved solute (sugar, salt, etc.) in grams.
Solute type
Select the solute type
Temperature (°C)
Storage or measurement temperature in Celsius.

What each result means

Water activity (aw)
Predicted aw from 0 (bone dry) to 1 (pure water). Below 0.60 is shelf-stable.
Equilibrium RH (%)
Equilibrium relative humidity = aw × 100.
Moisture content (%)
Wet-basis moisture content of the sample.
Moles of water
Calculated moles of water in the sample.
Effective moles of solute
Moles of solute (adjusted for dissociation if NaCl).
Microbial risk zone
1 = shelf-stable (<0.60), 2 = mold risk (0.60–0.80), 3 = yeast risk (0.80–0.91), 4 = bacteria risk (>0.91).

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Water mass (g) = 100, Solute mass (g) = 10, Solute type = 0, Temperature (°C) = 25 = 4 input(s) provided
  2. Calculate Water activity
    Water activity = min(1, max(0, aw + (temperatureC - 25) * 0.0002))
    0.9948 = 0.9948
  3. Calculate Equilibrium RH
    Equilibrium RH = tempCorrected * 100
    99.48 = 99.48
  4. Calculate Moisture content
    90.91 = 90.91

Related Calculators

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Food Science

pH Adjustment Calculator

Calculate acid dose needed to reach a target pH for food preservation, using buffer capacity and acid type.

Food Science

Thermal Processing Calculator

Calculate F-value, process time, and lethality for canning and pasteurization using D and z values.

Paper Science

Paper Moisture Calculator

Calculate paper moisture content from wet/dry weights. Estimate dimensional stability impact, equilibrium moisture, and strength changes.

Homesteading & Self-Sufficiency

Food Preservation Timeline Calculator

Plan your harvest preservation schedule. Estimate canning days, freezing batches, and dehydrating sessions based on garden size and equipment.

Food Science

Emulsion Stability Calculator

Calculate HLB blend ratios, creaming velocity (Stokes' Law), and emulsion type for food emulsion formulation.

More in Cooking, Food & Beverage.