A perm is a unit of permeance or "water vapor transmission" given a certain differential in partial pressures on either side of a material or membrane.


U.S. perm
The U.S. perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury.
1 U.S. perm  = 0.659045 metric perms
≈ 57.2135 ng·s−1·m−2·Pa−1
Metric perm
The metric perm (not an SI unit) is defined as 1 gram of water vapor per day, per square meter, per millimeter of mercury.
1 metric perm  = 1.51735 US perms
≈ 86.8127 ng·s−1·m−2·Pa−1
Equivalent SI unit
The equivalent SI measure is the nanogram per second per square meter per pascal.
1 ng·s−1·m−2·Pa−1 ≈ 0.0174784 US perms
≈ 0.0115191 metric perms

The base normal SI unit for permeance is the kilogram per second per square meter per pascal.

1 kg·s−1·m−2·Pa−1 ≈ 1.74784x1010 US perms
≈ 1.15191x1010 metric perms

German Institute for Standardization unit

A variant of the metric perm is used in DIN Standard 53122, where permeance is also expressed in grams per square meter per day, but at a fixed, "standard" vapor-pressure difference of 17.918 mmHg. This unit is thus 17.918 times smaller than a metric perm, corresponding to about 0.084683 of a U.S. perm.