The permeability number is illustrated by the image below.

To compute for the permeability number, five essential parameters are needed and these parameters are **Volume of Air (V), Height of the Specimen (H), Cross-sectional Area (A), Air Pressure (P)** and** Time (t).**

The formula for calculating permeability number:

P_{N} = ^{VH}/_{APt}

Where:

P_{N} = Permeability Number

V = Volume of Air

H = Height of the Specimen

A = Cross-sectional Area

P = Air Pressure

t = Time

Let’s solve an example;

Find the permeability number when the volume of air is 2, the height of the specimen is 7, the cross-sectional area is 3, the air pressure is 9 and the time is 4.

This implies that;

V = Volume of Air = 2

H = Height of the Specimen = 7

A = Cross-sectional Area = 3

P = Air Pressure = 9

t = Time = 4

P_{N} = ^{VH}/_{APt}

P_{N} = ^{(2)(7)}/_{(3)(9)(4)}

P_{N} = ^{(14)}/_{(108)}

P_{N} = 0.129

Therefore, the **permeability number **is **0.129.**