Air Permeability Calculator
Air permeability is the air flow through a material divided by the test area: 12 L/min through the 20 cm² ISO 9237 test head is 100 mm/s (19.69 ft³/(ft²·min)).
Area = 20 cm² = 0.002 m² (ISO 9237 / EN ISO 9237 (standard test area))
ISO 9237: 100 Pa (apparel) or 200 Pa; ASTM D737: 125 Pa. Not used in the calculation.
100 mm/s
Air permeability: 100 mm/s (L/m²/s) = 19.68503937 cfm/ft²; flow 12 L/min through 20 cm² at 100 Pa.
| Unit | Value |
|---|---|
| meter per second [m/s] | 0.1 m/s |
| decimeter per second [dm/s] | 1 dm/s |
| centimeter per second [cm/s] | 10 cm/s |
| millimeter per second [mm/s] | 100 mm/s |
| meter per minute [m/min] | 6 m/min |
| foot per minute [ft/min] | 19.68503937 ft/min |
| liter per square meter per second [L/(m²·s)] | 100 L/(m²·s) |
| milliliter per second per square centimeter [mL/(s·cm²)] | 10 mL/(s·cm²) |
| cubic centimeter per second per square centimeter [cm³/(s·cm²)] | 10 cm³/(s·cm²) |
| liter per square decimeter per minute [L/(dm²·min)] | 60 L/(dm²·min) |
| cubic decimeter per square centimeter per minute [dm³/(cm²·min)] | 0.6 dm³/(cm²·min) |
| cubic foot per square foot per minute [ft³/(ft²·min)] | 19.68503937 ft³/(ft²·min) |
How this was calculated (SI units):
- Q = 0.0002 m³/s
- A = 0.002 m²
- v = Q ÷ A = 0.0002 ÷ 0.002 = 0.1 m/s
Air Permeability Formula
R = q / A
R = air permeability (a velocity), q = air flow rate, A = test area
R [mm/s] = q [L/min] ÷ A [cm²] × 1000/6 · R [cm/s] = q [cm³/s] ÷ A [cm²]
Flow divided by area has the dimension of velocity, so air permeability units are velocities in disguise. That is why 1 L/(m²·s) = 1 mm/s, 1 cm³/(s·cm²) = 1 cm/s and 1 ft³/(ft²·min) = 1 ft/min exactly.
Worked Examples
- ISO 9237. 12 L/min through 20 cm²: R = 12 ÷ 20 × 1000/6 = 100 mm/s = 19.69 ft³/(ft²·min).
- ASTM D737. 10 L/min through 38.3 cm²: R = 4.352 cm³/(s·cm²) = 8.566 ft³/(ft²·min) = 43.52 mm/s.
- Flow needed for a target. A fabric specified at 20 cfm/ft² passes 23.35 L/min (389.1 cm³/s) through the 38.3 cm² ASTM D737 head.
Test Areas and Flow per mm/s
| Test area | Flow for 100 mm/s | 1 L/min gives | Specified in |
|---|---|---|---|
| 5 cm² | 3 L/min | 33.333 mm/s | ISO 9237 (alternative test area) |
| 10 cm² | 6 L/min | 16.667 mm/s | Instrument option on many permeability testers |
| 20 cm² | 12 L/min | 8.3333 mm/s | ISO 9237 / EN ISO 9237 (standard test area) |
| 25 cm² | 15 L/min | 6.6667 mm/s | Instrument option; used for nonwovens and small samples |
| 38.3 cm² | 22.98 L/min | 4.3516 mm/s | ASTM D737 (standard test area, ≈ 5.94 in²) |
| 50 cm² | 30 L/min | 3.3333 mm/s | ISO 9237 (alternative test area) |
| 100 cm² | 60 L/min | 1.6667 mm/s | ISO 9237 (alternative); filter media holders (e.g. 32 L/min → 5.33 cm/s) |
ISO 9237 vs ASTM D737
| Parameter | ISO 9237 | ASTM D737 |
|---|---|---|
| Test area | 20 cm² (alternatives 5, 50, 100 cm²) | 38.3 cm² (≈ 5.94 in²) |
| Pressure drop | 100 Pa (apparel), 200 Pa (technical fabrics) | 125 Pa (≈ 0.5 inH₂O) |
| Result unit | mm/s | cm³/(s·cm²) or ft³/(ft²·min) |
Check the current edition of each standard for the full procedure; parameters above are the commonly used defaults.
Frequently asked questions
How do I calculate air permeability?
Divide the measured air flow by the test area. With the flow in L/min and the area in cm², multiply by 1000/6 ≈ 166.7 to get mm/s: 12 L/min through 20 cm² gives 100 mm/s.
Why does ISO 9237 use the factor 167?
It converts L/min per cm² to mm/s. The exact factor is 1000/6 = 166.667; using 167 makes results about 0.2 % higher. This calculator uses the exact value.
Can I compare ISO 9237 and ASTM D737 results?
The units convert exactly (1 cfm/ft² = 5.08 mm/s), but the methods use different test pressures (ISO 9237: usually 100 or 200 Pa; ASTM D737: 125 Pa) and areas. Permeability is not strictly proportional to pressure, so compare only results taken at the same pressure.
What is Frazier air permeability?
It is air permeability in ft³/(ft²·min), named after the Frazier tester. It is measured at 0.5 inH₂O (about 125 Pa), the pressure ASTM D737 adopted. 1 cfm/ft² = 0.508 cm/s = 5.08 L/(m²·s).
Is L/m²/s the same as mm/s?
Yes. One litre per square metre per second is a 1 mm layer of air per second, so L/(m²·s) and mm/s are identical.
Sources
- ISO 9237:1995 Textiles — Determination of the permeability of fabrics to air, International Organization for Standardization .
- ASTM D737 Standard Test Method for Air Permeability of Textile Fabrics, ASTM International .
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology .
Last reviewed: . Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.
Cite this page
APA
Scientific Converters. (2026, October 15). Air Permeability Calculator. Retrieved October 17, 2026, from https://scientificconverters.com/calculators/air-permeability/
IEEE
Scientific Converters, "Air Permeability Calculator," scientificconverters.com. Accessed: Oct. 17, 2026. [Online]. Available: https://scientificconverters.com/calculators/air-permeability/