Enter the permeability coefficient K, the water vapor partial pressures on sides 1 and 2, and the thickness into the calculator to determine the water vapor transmission rate (WVTR).

Water Vapor Transmission Rate Calculator

Pick a mode, enter thickness, get WVTR at standard 38 °C / 90% RH test conditions.

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—g/m²/day
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Water Vapor Transmission Rate Formula

The following equation is used to calculate the Water Vapor Transmission Rate (WVTR) per unit area.

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?

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 following example outlines the steps and information needed to calculate the Water Vapor Transmission Rate.

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