Convert Perm-Inches (0°C) to Perms (0°C)
Perm-Inch (0°C) (perm·in (0°C)) to Perm (0°C) (perm (0°C)) water vapor permeance conversion — enter any value below to get an instant result, or use the table for common values.
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·in (0°C) = 0.025399949 perm (0°C)
1 Perm-Inch (0°C) = 0.025399949 Perms (0°C)
1 Perm (0°C) = 39.370157 Perm-Inches (0°C)
1 perm·in (0°C) in every supported unit
Conversion chart: Perm-Inch (0°C) to Perms (0°C)
Conversion table
| Perm-Inch (0°C) (perm·in (0°C)) | Perm (0°C) (perm (0°C)) |
|---|---|
| 0.01 perm·in (0°C) | 0.00025399949 perm (0°C) |
| 0.1 perm·in (0°C) | 0.0025399949 perm (0°C) |
| 1 perm·in (0°C) | 0.025399949 perm (0°C) |
| 2 perm·in (0°C) | 0.050799899 perm (0°C) |
| 3 perm·in (0°C) | 0.076199848 perm (0°C) |
| 5 perm·in (0°C) | 0.12699975 perm (0°C) |
| 10 perm·in (0°C) | 0.25399949 perm (0°C) |
| 20 perm·in (0°C) | 0.50799899 perm (0°C) |
| 50 perm·in (0°C) | 1.2699975 perm (0°C) |
| 100 perm·in (0°C) | 2.5399949 perm (0°C) |
| 1000 perm·in (0°C) | 25.399949 perm (0°C) |
Perm-Inch (0°C) (perm·in (0°C))
Definition: A US customary unit of water vapor PERMEABILITY (as opposed to permeance) — a perm rating normalized to a material one inch thick, measured under the 0°C test procedure, allowing materials of different thicknesses to be compared on equal footing.
History: Developed alongside the perm unit so that permeance figures (which depend on a sample's actual thickness) could be converted into a thickness-independent material property for comparing different products.
Current use: Used to compare the intrinsic vapor resistance of different material types (e.g. polyethylene sheeting vs. foam board) independent of how thick the tested sample happened to be.
Perm (0°C) (perm (0°C))
Definition: The traditional US customary unit of water vapor permeance, historically defined as 1 grain of water vapor passing through 1 square foot of material per hour, per inch of mercury of vapor pressure difference, measured under the 0°C (32°F) test procedure.
History: Originated in early 20th-century US building-materials testing, when vapor transmission was measured with grain, square foot, hour, and inch-of-mercury units already standard in American engineering.
Current use: The 0°C procedure corresponds to ASTM E96's older "Dry Cup" desiccant method reference condition and still appears on some legacy building-material data sheets.
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-Inch (0°C) → Perm (0°C): multiply by 0.025399949. For example, 1 perm·in (0°C) × 0.025399949 = 0.025399949 perm (0°C).
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·in (0°C) equals 0.025399949 perm (0°C). 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
How many Perms (0°C) are in 1 Perm-Inch (0°C)?
1 Perm-Inch (0°C) (perm·in (0°C)) equals exactly 0.025399949 Perms (0°C) (perm (0°C)).
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.