Conversion method
Mass flow per area per vapor-pressure difference, often labeled permeability. A perm at 0 °C differs from a perm at 23 °C. These rounded NIST conventional factors are approximate; neither is intrinsic permeability in darcy.
perm (0 °C) = x × 17478392337.4728
10 kg/(Pa·s·m²) = 174783923374.72803 perm (0 °C).
| kg/(Pa·s·m²) | perm (0 °C) |
|---|---|
| 1 | 17478392337.4728 |
| 5 | 87391961687.36401 |
| 10 | 174783923374.72803 |
| 25 | 436959808436.82 |
| 50 | 873919616873.64 |
| 100 | 1.747839e+12 |
Unit definitions and sources
Mass flow per area per vapor-pressure difference, often labeled permeability. A perm at 0 °C differs from a perm at 23 °C. These rounded NIST conventional factors are approximate; neither is intrinsic permeability in darcy.
| Unit and system | Use | Definition | Relationship |
|---|---|---|---|
| Kilogram per pascal second square meter (kg/(Pa·s·m²)); si | Common units | Base kg/(Pa·s·m²) = value in kg/(Pa·s·m²) × 1. Common use: Water Vapor Permeance. Source checked 2026-09-20 (implementation review). | Exact definition; numerical display may be rounded |
| Perm (0 °C) (perm (0 °C)); si | Common units | Base kg/(Pa·s·m²) = value in perm (0 °C) × 5.72135e-11. Common use: Water Vapor Permeance. Source checked 2026-09-20 (implementation review). | Approximate / conventional |
| Perm (23 °C) (perm (23 °C)); si | Specialist units | Base kg/(Pa·s·m²) = value in perm (23 °C) × 5.74525e-11. Common use: Water Vapor Permeance. Source checked 2026-09-20 (implementation review). | Approximate / conventional |
| Nanogram per pascal second square meter (ng/(Pa·s·m²)); si | Specialist units | Base kg/(Pa·s·m²) = value in ng/(Pa·s·m²) × 1e-12. Common use: Water Vapor Permeance. Source checked 2026-09-20 (implementation review). | Exact definition; numerical display may be rounded |
Sources: NIST: SP 811 Appendix B.8: conversion factors; BIPM: SI prefixes
Precision of water vapor permeance conversions
All 4 water vapor permeance units convert through Kilogram per pascal second square meter (kg/(Pa·s·m²)). 2 have an exact defined relationship to the base unit and 2 are conventional or measured values. Results and the tables on this page come from the same engine, so a table value can be checked against the converter directly.
Standard display shows up to six decimal places, switching to scientific notation for very large or small values. Calculation uses JavaScript binary floating point independently of display rounding, so extra digits are not a claim about measurement accuracy. Each unit lists its own source; a citation records where the definition comes from, not an independent review. See Conversion methodology for the review and correction process.