A unit used by petroleum engineers to express the permeability of a porous subsurface reservoir. (Sometimes cubic feet are used instead of barrels.)

flow in barrels × distance in feet × viscosity in centipoises days × square feet × pressure in pounds square inches MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbIrP5wySbcrYfMz 0bqee0evGueE0jxyaibaieYlf9irVeeu0dXdh9vqqj=hEeeu0xXdbb a9frFj0=OqFfea0dXdd9vqaq=JfrVkFHe9pgea0dXdar=Jb9hs0dXd bPYxe9vr0=vr0=vqpWqaaeaabiGaciaacaqabeaadaqacqaaaOqaam aalaaabaGaaeOzaiaabYgacaqGVbGaae4DaiaabccacaqGPbGaaeOB aiaabccacaqGIbGaaeyyaiaabkhacaqGYbGaaeyzaiaabYgacaqGZb GaaeiiaiaabEnacaqGGaGaaeizaiaabMgacaqGZbGaaeiDaiaabgga caqGUbGaae4yaiaabwgacaqGGaGaaeyAaiaab6gacaqGGaGaaeOzai aabwgacaqGLbGaaeiDaiaabccacaqGxdGaaeiiaiaabAhacaqGPbGa ae4CaiaabogacaqGVbGaae4CaiaabMgacaqG0bGaaeyEaiaabccaca qGPbGaaeOBaiaabccacaqGJbGaaeyzaiaab6gacaqG0bGaaeyAaiaa bchacaqGVbGaaeyAaiaabohacaqGLbGaae4CaaqaaiaabsgacaqGHb GaaeyEaiaabohacaqGGaGaae41aiaabccacaqGZbGaaeyCaiaabwha caqGHbGaaeOCaiaabwgacaqGGaGaaeOzaiaabwgacaqGLbGaaeiDai aabccacaqGxdGaaeiiaiaabchacaqGYbGaaeyzaiaabohacaqGZbGa aeyDaiaabkhacaqGLbGaaeiiaiaabMgacaqGUbWaaSaaaeaacaqGWb Gaae4BaiaabwhacaqGUbGaaeizaiaabohaaeaacaqGZbGaaeyCaiaa bwhacaqGHbGaaeOCaiaabwgacaqGGaGaaeyAaiaab6gacaqGJbGaae iAaiaabwgacaqGZbaaaaaaaaa@9E39@

At 0°C, 1 perm is approximately 5.72135 × 10⁻¹¹ kilograms per newton-second; at 23°C, approximately 5.74525 × 10⁻¹¹ kilograms per newton-second.

Frederick J. Ticknell, O. E. Mechem and R. C. McCurdy.
Some studies on the porosity and permeability of rocks.
Transactions of the AIME, vol. 103, no. 1, December 1933.


A unit used in the construction industry to express permeance: how easily water vapor can flow through a barrier. The smaller the number, the harder it is for water vapor to get through the barrier. Architects, building officials and contractors use it as a figure of merit, and it is often encountered in regulations and building codes. To be meaningful, a measurement in perms must be accompanied by a statement of the thickness of the barrier.

Several, differing perms exist.

The US perm is defined as

USperms= massingrains timeinhours × areainsquarefeet × pressure difference in inches of mercury MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbIrP5wySbcrYfMz 0bqee0evGueE0jxyaibaieYlf9irVeeu0dXdh9vqqj=hEeeu0xXdbb a9frFj0=OqFfea0dXdd9vqaq=JfrVkFHe9pgea0dXdar=Jb9hs0dXd bPYxe9vr0=vr0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaai aabwfacaqGtbGaaGjbVlaabchacaqGLbGaaeOCaiaab2gacaqGZbGa aGjbVlaab2dacaaMe8+aaSaaaeaacaqGTbGaaeyyaiaabohacaqGZb GaaGjbVlaabMgacaqGUbGaaGjbVlaajEgacaqIYbGaaKyyaiaajMga caqIUbGaaK4CaaqaaiaabshacaqGPbGaaeyBaiaabwgacaaMe8Uaae yAaiaab6gacaaMe8UaaKiAaiaaj+gacaqI1bGaaKOCaiaajohacaqG GaGaae41aiaabccacaqGHbGaaeOCaiaabwgacaqGHbGaaGjbVlaabM gacaqGUbGaaGjbVlaajohacaqIXbGaaKyDaiaajggacaqIYbGaaKyz aiaaysW7caqIMbGaaKyzaiaajwgacaqI0bGaaeiiaiaabEnacaqGGa GaaeiCaiaabkhacaqGLbGaae4CaiaabohacaqG1bGaaeOCaiaabwga caqGGaGaaeizaiaabMgacaqGMbGaaeOzaiaabwgacaqGYbGaaeyzai aab6gacaqGJbGaaeyzaiaabccacaqGPbGaaeOBaiaabccacaqIPbGa aKOBaiaajogacaqIObGaaKyzaiaajohacaqIGaGaaK4BaiaajAgaca qIGaGaaKyBaiaajwgacaqIYbGaaK4yaiaajwhacaqIYbGaaKyEaaaa aaa@9E16@

and the mass is the loss of mass of the container of water.

The metric perm (an unfortunate name, since it is not an SI unit) is defined as

metricperms= massingrams time in days (=24hours) × areainsquaremeters × pressure difference in millimeters of mercury MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbIrP5wySbcrYfMz 0bqee0evGueE0jxyaibaieYlf9irVeeu0dXdh9vqqj=hEeeu0xXdbb a9frFj0=OqFfea0dXdd9vqaq=JfrVkFHe9pgea0dXdar=Jb9hs0dXd bPYxe9vr0=vr0=vqpWqaaeaabiGaciaacaqabeaadaqacqaaaOqaai aab2gacaqGLbGaaeiDaiaabkhacaqGPbGaae4yaiaaysW7caqGWbGa aeyzaiaabkhacaqGTbGaae4CaiaaysW7caqG9aGaaGjbVpaalaaaba GaaeyBaiaabggacaqGZbGaae4CaiaaysW7caqGPbGaaeOBaiaaysW7 caqINbGaaKOCaiaajggacaqITbGaaK4CaaqaaiaabshacaqGPbGaae yBaiaabwgacaqGGaGaaeyAaiaab6gacaqGGaGaaeizaiaabggacaqG 5bGaae4CaiaabccacaqGOaGaaeypaiaaysW7caaIYaGaaGinaiaays W7caqGObGaae4BaiaabwhacaqGYbGaae4CaiaajMcacaqGGaGaey41 aqRaaeiiaiaabggacaqGYbGaaeyzaiaabggacaaMe8UaaeyAaiaab6 gacaaMe8UaaK4CaiaajghacaqI1bGaaKyyaiaajkhacaqILbGaaGjb Vlaaj2gacaqILbGaaKiDaiaajwgacaqIYbGaaK4CaiaabccacaqGxd GaaeiiaiaabchacaqGYbGaaeyzaiaabohacaqGZbGaaeyDaiaabkha caqGLbGaaeiiaiaabsgacaqGPbGaaeOzaiaabAgacaqGLbGaaeOCai aabwgacaqGUbGaae4yaiaabwgacaqGGaGaaeyAaiaab6gacaqGGaGa aKyBaiaajMgacaqISbGaaKiBaiaajMgacaqITbGaaKyzaiaajshaca qILbGaaKOCaiaajohacaqIGaGaaK4BaiaajAgacaqIGaGaaKyBaiaa jwgacaqIYbGaaK4yaiaajwhacaqIYbGaaKyEaaaaaaa@B1C5@

To convert metric perms to US perms, multiply by 1.51735.

Getting rid of the non-SI units, such as days and millimeters of mercury, gives us the so-called SI perm:

SIperms= massinnanograms time in seconds × areainsquaremeters × pressure difference in pascals MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbIrP5wySbcrYfMz 0bqee0evGueE0jxyaibaieYlf9irVeeu0dXdh9vqqj=hEeeu0xXdbb a9frFj0=OqFfea0dXdd9vqaq=JfrVkFHe9pgea0dXdar=Jb9hs0dXd bPYxe9vr0=vr0=vqpWqaaeaabiGaciaacaqabeaadaqacqaaaOqaai aabofacaqGjbGaaGjbVlaabchacaqGLbGaaeOCaiaab2gacaqGZbGa aGjbVlaab2dacaaMe8+aaSaaaeaacaqGTbGaaeyyaiaabohacaqGZb GaaGjbVlaabMgacaqGUbGaaGjbVlaab6gacaqGHbGaaeOBaiaab+ga caqGNbGaaeOCaiaabggacaqGTbGaae4CaaqaaiaabshacaqGPbGaae yBaiaabwgacaqGGaGaaeyAaiaab6gacaqGGaGaae4CaiaabwgacaqG JbGaae4Baiaab6gacaqGKbGaae4CaiaabccacqGHxdaTcaqGGaGaae yyaiaabkhacaqGLbGaaeyyaiaaysW7caqGPbGaaeOBaiaaysW7caqG ZbGaaeyCaiaabwhacaqGHbGaaeOCaiaabwgacaaMe8UaaeyBaiaabw gacaqG0bGaaeyzaiaabkhacaqGZbGaaeiiaiaabEnacaqGGaGaaeiC aiaabkhacaqGLbGaae4CaiaabohacaqG1bGaaeOCaiaabwgacaqGGa GaaeizaiaabMgacaqGMbGaaeOzaiaabwgacaqGYbGaaeyzaiaab6ga caqGJbGaaeyzaiaabccacaqGPbGaaeOBaiaabccacaqGWbGaaeyyai aabohacaqGJbGaaeyyaiaabYgacaqGZbaaaaaa@9979@

The actual SI measure of permeance is the kilogram per second per square meter per pascal.


A list of perm values for common building materials is provided at

An interesting PowerPoint presentation at a 2013 ABAA conference:


ASTM E96/E96M - 16. Standard Test Methods for Water Vapor Transmisson of Materials.


Vapour permeability is a material property, expressed independently of material thickness, in units of ng/Pa s m, and given the symbol, μ. Vapour permeance is a measure of the ease of vapour flow through a material layer, in units of perms (equal to 1 ng/Pa s m2 or 1 grain/(hr·in Hg· ft2)) and given the symbol M. Permeability and permeance are analogous to thermal conductivity and thermal conductance respectively. Imperial US perms can be converted to metric perms by multiplying by 57.1.

Many codes define a vapor barrier as any material or system that has a permeance of less than 1 US perm. This is an arbitrary value based on a limited and questionable study conducted in the 1940’s. Vapor diffusion flow through a wall may need to be controlled with vapor resistant layer in some special cases, but plastered strawbale walls usually don’t need them, and often appear to perform much better without them.

John Straube.
Moisture Properties of Plaster and Stucco for Strawbale Buildings.
Available on the internet at the Canada Mortgage and Housing Corporation
Page 5. Reprinted here with the author's permission.


A unit of electric permeance suggested by Vladimar Karapetoff in 1911, but not adopted.


When permittance is measured in farads, elastance is measured in darafs (see the chapter on the Electrostatic Circuit in the author's Electric Circuit).

Analogously, when two or more magnetic paths are in parallel it is convenient to use the reciprocals of the reluctances. The reciprocal of the reluctance of a magnetic path is called its permeance; eq. (1) becomes then

Φ = M, (2)


= 1/. (3)

A script is used for permeance in order to avoid confusing it with power. For the unit of permeance corresponding to the rel, the author proposes the name perm. A magnetic path has a permeance of one perm when one maxwell of flux is produced for each ampere-turn of magnetomotive force applied along the path. The unit “perm” has been in use among electrical designers for some time, although no name has been given to it. Notably Mr. H. M. Hobart has used it extensively in his writings, in the calculation of the inductance of windings. He speaks of “magnetic lines per ampere-turn per unit length” (of the embedded part of a coil). This is equivalent to perms per unit length.

V. Karapetoff.
The Magnetic Circuit.
New York: McGraw-Hill 1911.
Page 9.


Printed sources refer to a unit called the “AFS perm”, which apparently measures a property of the sand from which molds are made in foundries (“AFS” is from American Foundrymen's Society, later renamed the American Foundry Society). So far we have not been able to find a definition of this unit in the Society's publications. If you have any knowledge of this topic, please contact us.

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