Skip to content

GAB Water Activity Calculator

Solve a fitted GAB isotherm for water activity using sample moisture, monolayer moisture, C and K. Convert wet-basis inputs and check the reconstructed result.

Use this result well

Inputs that matter
Sample moisture content, Sample moisture basis, Monolayer moisture Xm, Fitted C, and 1 more
Output to expect
Modeled water activity
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
Was this tool helpful?

Related Tools

Tools you might need next

Recipe Ingredient Scaling Calculator

Enter the original and target serving counts, then one ingredient’s weight. Repeat for the other ingredients using the same serving counts.

Food Science
Food Science Sugar Syrup Calculator

Calculate sugar and water amounts for 1:1 or 2:1 bee feeding syrup. Mix correct ratios for spring stimulation and fall winter feeding.

Food Science
Food Science Waste Calculator

Quantify food waste cost and environmental impact from spoilage and plate waste. Free food waste calculator for restaurants, catering.

Food Science

Reference & details

How it works

Use the GAB equation in its correct direction

The equation predicts dry-basis moisture M from aw and three fitted parameters. This calculator solves the equation for aw rather than substituting moisture in the activity position.

M = Xm C K aw / [(1 − K aw)(1 − K aw + C K aw)]

Source: Research example documenting the GAB equation, section 2.4.1

Reconcile the moisture basis

M and Xm use grams of water per 100 g dry matter. Wet-basis sample moisture is converted before solving.

Dry-basis % = 100 × wet-basis % / (100 − wet-basis %)

Check the modeled solution

The numerical search uses aw from 0 to 1, positive Xm and C, and 0 < K ≤ 1 with K × aw < 1. The returned activity is substituted into the equation to reconstruct the input moisture. A model’s physical validity must still be checked against its fitted range.

Updated: September 2026

Example Scenarios

Enter 10% dry-basis moisture, Xm 5%, C 2 and K 0.8.

→ Modeled aw ≈ 0.772542; reconstructed moisture ≈ 10% dry basis.

Enter 20% wet-basis moisture, Xm 5%, C 2 and K 1.

→ The solver uses 25% dry-basis moisture and gives modeled aw ≈ 0.819804.

Enter 10,000% dry basis, Xm 5%, C 2 and K 0.8.

→ No aw from 0 to 1 matches these inputs; the result is not clipped to 0.99.

Common Mistakes to Avoid

Reusing one fit for every food and temperature

Use parameters fitted for the material, temperature, sorption direction and activity range being studied.

Reading dry-basis percent as percent of total mass

Dry-basis moisture can exceed 100%. Wet-basis moisture includes the water in the denominator and must remain below 100% while dry matter is present.

FAQ

No. They demonstrate the equation and are not a validated fit for a food.

Temperature effects belong to the fitted material model. Use a fit for the actual conditions.

This implementation supports only 0 < K ≤ 1. Fits outside that domain require separate assessment.

It checks whether the modeled activity reproduces the entered moisture in the same equation. It does not validate the fit against physical measurements.

No. A mathematically consistent model result is not a measurement or a preservation approval.

About GAB Water Activity Calculator

Calculate water activity from a supplied material-specific isotherm. The monolayer parameter, energy parameters and moisture basis are all explicit, with a substitution check beside the answer.