Calculate water vapor transmission rate (WVTR) in g/m²/day from mass gain, specimen area and test time, or estimate package moisture ingress.

Enter the magnitude of net, blank-corrected mass change over a linear steady-state interval.

Vapor permeance
Blank omits permeance. Enter the vapor pressure difference across the specimen.

Water Vapor Transmission Rate Formula

The interface calculates WVTR from a steady-state gravimetric interval: WVTR = net water-mass change ÷ exposed specimen area ÷ elapsed days. For example,0.12 g over0.05 m² in24 hours gives2.4 g/(m²·day). Use the magnitude of a linear, blank-corrected mass change and report the test temperature, humidity and method. The calculator does not fit a time series or verify a test method.

WVTR = K × (P1 - P2) / d
  • Where WVTR is the Water Vapor Transmission Rate (mass of water vapor transmitted per unit area per unit time; e.g., g/m²/day or g/100 in²/day)
  • K is the water vapor permeability coefficient of the membrane/material (units depend on the unit system used)
  • P1 is the water vapor partial pressure on side 1 (upstream/high humidity side)
  • P2 is the water vapor partial pressure on side 2 (downstream/low humidity side)
  • d is the thickness of the membrane/material.

What is a Water Vapor Transmission Rate?

Package mode uses moisture ingress = WVTR × total barrier area × storage days. For0.5 g/(m²·day),0.4 m² and180 days, ingress is36 g. The adjustable1.5× planning default gives54 g water capacity; it does not specify desiccant mass or guarantee shelf life. The model assumes unchanged service conditions and excludes seams, openings, initial moisture and product sorption.

Conversions use1 in² =0.00064516 m²,100 in² =0.064516 m² and1 day =86400 s. Optional permeance divides the SI rate by a positive vapor-pressure difference and converts grams/day to nanograms/second. Blank or zero pressure omits permeance. The annualized output uses365 days.

Definition:

A water vapor transmission rate (WVTR), sometimes referred to as moisture vapor transmission rate (MVTR), is the rate at which water vapor passes through a material per unit area per unit time, typically reported under specified temperature and humidity conditions.

How to Calculate Water Vapor Transmission Rate?

Example Problem:

The example below is a separate homogeneous-material permeability reference: WVTR = permeability × pressure difference ÷ thickness. Those permeability and thickness inputs are not part of this interface. See the ASHRAE vapor-permeability discussion for the distinction between permeability and permeance.

First, determine the permeability coefficient K for the material (for example, from a datasheet or an experimental measurement). In this example, assume K = 31.5 (g·in)/(100 in²·day·mmHg).

Next, determine the water vapor partial pressure on side 1. In this example, P1 = 10 mmHg.

Next, determine the water vapor partial pressure on side 2. In this example, P2 = 5 mmHg.

Next, determine the thickness of the membrane/material. In this example, d = 0.50 in.

Finally, calculate the water vapor transmission rate using the formula above:

WVTR = K*(P1-P2) / d

WVTR = 31.5*(10-5) / 0.50

WVTR = 315 g/100 in²/day