Permeability Converter
Convert water vapor permeability units used for building materials, packaging and membranes: perm and perm-inch at 0 °C and 23 °C, and the SI unit kilogram per pascal per second per square meter.
Select two units to see the conversion factor.
These are water vapor units, not magnetic permeability (H/m) or soil and rock permeability (darcy). A perm and a perm-inch have different dimensions, so they convert only within their own family.
Permeability Formula
Each unit is a fixed multiple of its family reference. Within a family, multiply the value by the size of the source unit and divide by the size of the target unit. Perm units (permeance) are measured against the SI unit kg/(Pa·s·m²): 1 perm (0 °C) = 5.72135 × 10⁻¹¹ and 1 perm (23 °C) = 5.74525 × 10⁻¹¹ kg/(Pa·s·m²). Perm-inch units (permeability) are the perm multiplied by one inch of thickness, 0.0254 m, and are measured in kg/(Pa·s·m).
What Is Water Vapor Permeability?
In building science, packaging and textiles, permeability describes how readily water vapor passes through a material. Two related quantities are in use, and the units on this page cover both. Permeance is the vapor flow through a specific sheet or layer per unit area and per unit of vapor-pressure difference. Permeability is the property of the material itself, with the thickness taken out, so it is permeance multiplied by thickness.
The common US unit of permeance is the perm, defined as one grain of water vapor per hour per square foot per inch of mercury of vapor-pressure difference. In SI it is expressed in kg/(Pa·s·m²), and 1 perm is about 5.72 × 10⁻¹¹ of that unit (about 57 ng per second per square meter per pascal). The matching permeability unit is the perm-inch, which is a perm multiplied by one inch of material thickness.
The temperatures 0 °C and 23 °C in the unit names refer to the reference temperature at which the inch of mercury used in the definition is evaluated. Mercury expands slightly when warm, so a perm at 23 °C is about 0.42 % larger than a perm at 0 °C. Most modern test standards quote the 23 °C value, while older data use 0 °C.
Lower numbers mean a better vapor barrier. In US building codes vapor retarders are grouped by perm rating: Class I is 0.1 perm or less, Class II is above 0.1 up to 1.0 perm and Class III is above 1.0 up to 10 perm.
Do not confuse this with other permeabilities. Magnetic permeability (henry per meter), the permeability of rock or soil to fluids (darcy or m²) and hydraulic conductivity (m/s) are different physical quantities with different units, and this converter does not handle them.
Supported Units
This converter supports the following 5 units, in two families. Permeance: kg/(Pa·s·m²), perm (0 °C) and perm (23 °C). Permeability: perm-inch (0 °C) and perm-inch (23 °C). The last column gives the size of each unit in the SI unit of its family: kg/(Pa·s·m²) for perm units and kg/(Pa·s·m) for perm-inch units. Units from different families cannot be converted without knowing the material thickness.
| Unit | Symbol | Family | Size in SI units of its family |
|---|---|---|---|
| Kilogram per pascal per second per square meter | kg/(Pa·s·m²) | Permeance (water vapor, per square meter) | 1 |
| Permeability (perm, 0 °C) | perm (0 °C) | Permeance (water vapor, per square meter) | 5.72135e-11 |
| Permeability (perm, 23 °C) | perm (23 °C) | Permeance (water vapor, per square meter) | 5.74525e-11 |
| Permeability inch (perm-inch, 0 °C) | perm-in (0 °C) | Permeability (thickness-normalised, perm-inch) | 1.4532229e-12 |
| Permeability inch (perm-inch, 23 °C) | perm-in (23 °C) | Permeability (thickness-normalised, perm-inch) | 1.4592935e-12 |
How to Use the Permeability Converter
- Type the permeability in the value box. Decimals and scientific notation such as 0.0025 or 2.5e-3 are accepted.
- Choose the From unit and the To unit.
- The result appears instantly, or press Convert Now. Very large or very small results are shown in scientific notation.
- Use Swap Units to reverse the conversion, Copy Result to copy the answer, open Show this value in all units for a full list, and Clear to start again.
Step-by-step by hand
- Find the size of each unit in the common base unit (kg/(Pa·s·m²)): 1 perm (0 °C) = 5.72135e-11 kg/(Pa·s·m²) and 1 kg/(Pa·s·m²) = 1 kg/(Pa·s·m²).
- Divide the source size by the target size to get the conversion factor: 5.72135e-11 ÷ 1 = 5.72135e-11, so 1 perm (0 °C) = 5.72135e-11 kg/(Pa·s·m²).
- Multiply by your value: 5 perm (0 °C) × 5.72135e-11 = 2.860675e-10 kg/(Pa·s·m²).
Permeability cannot be negative, so only zero and positive numbers are accepted. If you pick one perm unit and one perm-inch unit the tool explains why they cannot be converted and asks you to choose two units from the same family.
Common Permeability Conversions
- US test report to SI: 5 perm (0 °C) equals 2.860675e-10 kg/(Pa·s·m²).
- SI to perm: 2 × 10⁻¹⁰ kg/(Pa·s·m²) equals 3.481136591 perm (23 °C).
- Temperature basis: 1 perm (23 °C) equals 1.004177336 perm (0 °C), and 10 perm (0 °C) equals 9.958400418 perm (23 °C).
- Class II vapor retarder limit: 1 perm (23 °C) is 5.74525e-11 kg/(Pa·s·m²).
- Perm-inch data: 3 perm-inch (0 °C) equals 2.987520125 perm-inch (23 °C).
- Very small values: 0.05 perm (23 °C) equals 2.872625e-12 kg/(Pa·s·m²).
Perm Conversion – Common Values
Quick reference for frequently used values
| Perm (0 °C) | Perm (23 °C) | kg/(Pa·s·m²) |
|---|---|---|
| 0.01 | 0.009958400418 | 5.72135e-13 |
| 0.05 | 0.04979200209 | 2.860675e-12 |
| 0.1 | 0.09958400418 | 5.72135e-12 |
| 0.5 | 0.4979200209 | 2.860675e-11 |
| 1 | 0.9958400418 | 5.72135e-11 |
| 2 | 1.991680084 | 1.14427e-10 |
| 5 | 4.979200209 | 2.860675e-10 |
| 10 | 9.958400418 | 5.72135e-10 |
| 20 | 19.91680084 | 1.14427e-9 |
| 50 | 49.79200209 | 2.860675e-9 |
Where and Why This Conversion Is Used
- Building envelope design: vapor retarders, house wraps, paints and insulation are specified by perm rating to control moisture in walls and roofs.
- Test reports: water vapor transmission tests are reported in perms in the US and in SI units elsewhere, so values are converted when results from different labs are compared.
- Packaging: barrier films and wrapping materials are rated by vapor permeance.
- Textiles and membranes: breathable fabrics and membranes are compared by vapor permeability.
- Product data sheets: manufacturers list perm and perm-inch values at either temperature.
- Moisture and mold analysis: hygrothermal modelling needs vapor permeability in SI units.
For other building and fluid quantities see the Surface Tension Converter and the Viscosity - Dynamic Converter.