Fluids converters
Permeability Converter
Convert water-vapour permeability units: perms, perm-inches and SI kg/(Pa·s·m²).
Result
5.72135e-11 kg/(Pa·s·m²)
1 perm = 5.72135e-11 kg/(Pa·s·m²)
All units for this value
| Unit | Symbol | Value |
|---|---|---|
| Kilogram/pascal/second/square meter | kg/(Pa·s·m²) | 5.72135e-11 |
| Gram/pascal/second/square meter | g/(Pa·s·m²) | 5.72135e-8 |
| Nanogram/pascal/second/square meter | ng/(Pa·s·m²) | 57.2135 |
| Perm (0 °C) | perm | 1 |
| Perm (23 °C) | perm | 0.99584 |
| Perm-inch (0 °C) | perm·in | 39.3702 |
| Perm-inch (23 °C) | perm·in | 39.2064 |
About this calculator
Water-vapour permeability describes how readily a material lets moisture pass through it, a key figure for choosing vapour barriers, house wraps, and roofing membranes. This converter switches between the SI unit kilogram per pascal per second per square meter and the US perm and perm-inch units printed on most North American building material datasheets.
How it works in plain English
The converter anchors every unit to kilograms per pascal per second per square meter, so an entered value is first rescaled into that base and then rescaled again into whichever unit you select as the target. Grams and nanograms per pascal per second per square meter are simply smaller metric multiples of the same base unit, following factors of 1000 and 1e-9 respectively.
The US perm is historically defined from grain, hour, square foot, and inch-of-mercury measurements, and its exact value depends slightly on the temperature used in the original test method, which is why this converter includes separate figures for perms measured at 0 °C and at 23 °C. The perm-inch adds a material thickness into the definition, describing permeance per inch of material thickness rather than the permeance of a specific sample, which lets datasheets compare materials of different thicknesses on equal footing.
Choose your from and to units, enter a value, and the converted permeability appears immediately, with a table showing that same figure across every supported unit at once. A precision selector controls the number of decimal places shown, and a swap button flips the conversion direction in one click.
The formula
- value_in_target = value_in_source × (source factor to kg/(Pa·s·m²)) ÷ (target factor to kg/(Pa·s·m²))
- 1 perm (0 °C) = 5.72135e-11 kg/(Pa·s·m²)
- 1 perm (23 °C) = 5.74525e-11 kg/(Pa·s·m²)
- 1 perm-inch (23 °C) = 1.45929e-12 kg/(Pa·s·m²)
Worked example
Suppose a housewrap datasheet lists a permeance of 10 perms at the 23 °C test condition, and a builder wants that figure in the SI unit for a specification written in metric terms. Multiplying by the perm (23 °C) factor: 10 × 5.74525e-11 = 5.74525e-10 kg/(Pa·s·m²).
Entering 10 into the perm (23 °C) field with kg/(Pa·s·m²) selected as the target shows about 5.74525e-10 kg/(Pa·s·m²) right away. As an extra step, multiplying that SI figure by 1000 gives roughly 5.74525e-7 g/(Pa·s·m²), a handy cross-check that also appears directly in the converted table.
The unit table also reports this same permeance as roughly 5.74525e-7 g/(Pa·s·m²), letting the builder cross-check the figure in a more familiar order of magnitude without further calculation. Reading across the perm, perm-inch, and SI rows together confirms that the 10-perm housewrap sits well within the vapour-permeable range used in typical wall assembly designs.
Vapour permeance classes used in building construction
| Class | Typical permeance (perms) |
|---|---|
| Vapour barrier (Class I) | ≤0.1 |
| Vapour retarder (Class II) | 0.1-1.0 |
| Vapour retarder (Class III) | 1.0-10 |
| Vapour-permeable housewrap | 5-50 |
| Uncoated building paper | ≥50 |
Common North American building material classifications
Frequently asked questions
Why does the perm value depend on temperature?
The original test methods for the perm involved measuring water vapour moving through a material under controlled humidity and temperature conditions, and the exact rate depends slightly on which reference temperature was used. This converter lists separate 0 °C and 23 °C perm values so that figures from different test standards can be converted using the matching reference condition.
What's the difference between a perm and a perm-inch?
A perm describes the permeance of an actual sample as tested, including its specific thickness, while a perm-inch describes the permeability of the material itself normalized to one inch of thickness. This distinction lets datasheets report a single perm-inch figure that applies across different thicknesses of the same material, then multiply or divide by actual thickness to estimate performance.
Why is the kilogram per pascal per second per square meter such a small number?
This SI unit measures the mass of water vapour, in kilograms, that passes through one square meter of material per second for each pascal of vapour pressure difference, which is an extremely small quantity for any real building material. That's why perm values, which represent a more practically sized unit, are far more common on datasheets, even though the SI unit is used in some international standards.
How do I know if a material counts as a vapour barrier or a vapour retarder?
Building codes commonly classify materials by their perm rating into vapour barrier and vapour retarder classes, with lower perm ratings indicating a material that blocks more moisture movement. This converter doesn't apply those classifications itself, but the reference table lists typical perm ranges associated with each common class for comparison.
Can I convert a metric permeability figure from a European datasheet into US perms?
Yes. Select kg/(Pa·s·m²) or g/(Pa·s·m²) as the from unit and perm (0 °C) or perm (23 °C) as the target, then enter the reported figure. The converter applies the fixed conversion factors automatically, so there's no need to look them up or calculate them manually.
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