Permeability Converter
Convert between water vapor permeance units — the US customary perm and perm-inch, and the SI kilogram per pascal per second per square meter, used for vapor barrier and building-material ratings.
Results from this calculator are estimates provided for general informational purposes only, based on formulas, rates, and standards commonly accepted as of 2026. Figures may differ slightly from other calculators or professional sources due to rounding methods, differing assumptions, or regional regulations, and rules may change over time. Always consult a qualified professional — such as a financial advisor, healthcare provider, or other relevant specialist — before making decisions based on these results.
Result
1 perm (23°C) = 5.74525E-11 kg/(Pa·s·m²)
1 Perm (23°C) = 5.74525E-11 Kilograms per Pascal per Second per Square Meter
1 Kilogram per Pascal per Second per Square Meter = 1.74057E+10 Perms (23°C)
1 perm (23°C) in every supported unit
What is a Permeability Converter?
This converter covers water vapor PERMEANCE — how readily water vapor passes through a building material such as a vapor barrier sheet, an insulation facing, or a roofing membrane, expressed in the traditional US "perm" unit or its SI equivalent, kilogram per pascal per second per square meter. It is not the same quantity as geological or petroleum "permeability" (measured in darcys), which describes how easily a liquid or gas flows through rock or soil pore space — a completely different physical property used in hydrogeology and petroleum engineering. If you are looking for rock or soil fluid-flow permeability, this is the wrong tool; this page is exclusively for building-science water vapor transmission.
The perm rating is central to US building codes, which classify materials into vapor retarder classes based directly on their perm value. Every unit here is defined relative to the SI base unit by a fixed multiplier, letting a manufacturer's perm-inch datasheet figure, an engineer's SI kg/(Pa·s·m²) calculation, and a building code's Class I/II/III threshold all be compared directly.
Conversion chart: Perm (23°C) to Kilograms per Pascal per Second per Square Meter
Conversion table
| Perm (23°C) (perm (23°C)) | Kilogram per Pascal per Second per Square Meter (kg/(Pa·s·m²)) |
|---|---|
| 0.01 perm (23°C) | 5.74525E-13 kg/(Pa·s·m²) |
| 0.1 perm (23°C) | 5.74525E-12 kg/(Pa·s·m²) |
| 1 perm (23°C) | 5.74525E-11 kg/(Pa·s·m²) |
| 2 perm (23°C) | 1.14905E-10 kg/(Pa·s·m²) |
| 3 perm (23°C) | 1.72357E-10 kg/(Pa·s·m²) |
| 5 perm (23°C) | 2.87263E-10 kg/(Pa·s·m²) |
| 10 perm (23°C) | 5.74525E-10 kg/(Pa·s·m²) |
| 20 perm (23°C) | 1.14905E-09 kg/(Pa·s·m²) |
| 50 perm (23°C) | 2.87262E-09 kg/(Pa·s·m²) |
| 100 perm (23°C) | 5.74525E-09 kg/(Pa·s·m²) |
| 1000 perm (23°C) | 5.74525E-08 kg/(Pa·s·m²) |
Supported Units
| Unit | Symbol | In kg/(Pa·s·m²) |
|---|---|---|
| Kilogram per Pascal per Second per Square Meter | kg/(Pa·s·m²) | 1 kg/(Pa·s·m²) |
| Perm (0°C) | perm (0°C) | 5.72135E-11 kg/(Pa·s·m²) |
| Perm (23°C) | perm (23°C) | 5.74525E-11 kg/(Pa·s·m²) |
| Perm-Inch (0°C) | perm·in (0°C) | 1.45322E-12 kg/(Pa·s·m²) |
| Perm-Inch (23°C) | perm·in (23°C) | 1.45929E-12 kg/(Pa·s·m²) |
About These Parameters
- Value
- The permeance value you want to convert, expressed in the "From" unit. Accepts decimals.
- From Unit
- The unit your input value is currently measured in — a manufacturer's perm or perm-inch rating, or an SI kg/(Pa·s·m²) figure from an engineering calculation.
- To Unit
- The unit you want the result converted into. Use the swap button to flip From and To instantly.
How Permeability Conversion Works
The Formula
Every unit here is defined by a fixed multiplier relative to kilogram per pascal per second per square meter. To convert a value from one unit to another:
result = value × (factor of "From" unit ÷ factor of "To" unit)
For Perm (23°C) → Kilogram per Pascal per Second per Square Meter: multiply by 5.74525E-11. For example, 1 perm (23°C) × 5.74525E-11 = 5.74525E-11 kg/(Pa·s·m²).
The Perm Rating System
US building codes classify vapor retarders into three classes based directly on measured perm rating: Class I materials (0.1 perm or less) are near-impermeable vapor barriers such as sheet polyethylene, glass, and rubber membranes; Class II materials (greater than 0.1 up to 1.0 perm) are semi-permeable, including kraft-faced fiberglass batts and some vapor-retarder paints; Class III materials (greater than 1.0 up to 10 perms) are permeable, allowing meaningful vapor movement through them, such as unfaced fiberglass insulation, gypsum board, and most latex paints. Building codes use these thresholds to decide where a vapor retarder is required and how it must be positioned within a wall or roof assembly to avoid trapping moisture.
0°C vs. 23°C Reference Temperatures
The perm and perm-inch units each come in two slightly different SI-equivalent flavors because ASTM E96, the standard test method for water vapor transmission, historically specified test conditions at 0°C for older "Dry Cup" desiccant-method testing and later standardized on a 23°C controlled laboratory reference temperature that better matches typical indoor room conditions. Because permeance depends on temperature, the same physical "1 perm" reading taken under each procedure corresponds to a marginally different SI value — 5.72135×10⁻¹¹ kg/(Pa·s·m²) at 0°C versus 5.74525×10⁻¹¹ kg/(Pa·s·m²) at 23°C. Modern US product datasheets almost always report the 23°C figure, but older or international documentation may still use the 0°C reference, so it is worth checking which procedure a quoted perm value was measured under before comparing products.
Example
A water vapor permeance of 1 perm (23°C) equals 5.74525E-11 kg/(Pa·s·m²). For scale, a typical polyethylene sheet vapor barrier rates around 0.06 perm (well within Class I), while unfaced fiberglass batt insulation can rate anywhere from about 5 to over 30 perms, placing it in Class III or above and making it unsuitable as a standalone vapor barrier.
Frequently Asked Questions
What is a perm rating?
A perm rating measures how easily water vapor passes through a material — specifically, how many grains of water vapor move through one square foot of the material per hour, per inch of mercury of vapor pressure difference across it. Lower perm ratings mean a more effective vapor barrier; higher perm ratings mean the material is more vapor-permeable.
Is this the same as geological permeability (darcy)?
No. Geological/petroleum permeability, measured in darcys, describes how easily a liquid or gas flows through the connected pore spaces of rock or soil — an entirely different physical quantity used in hydrogeology and petroleum reservoir engineering. This converter covers water vapor permeance through building materials (perm, perm-inch, kg/(Pa·s·m²)) only; it has no darcy unit and cannot be used for rock or soil permeability calculations.
How accurate are these conversions?
Every conversion factor used here is the standard, internationally recognized SI equivalent of the corresponding US customary perm or perm-inch unit at its stated reference temperature — results are limited only by floating-point display precision, not by rounded conversion constants.