Air Permeability Units Explained
By Scientific Converters · Updated · Factors from NIST SP 811 and the BIPM SI Brochure
Short answer: Air permeability is a flow divided by an area, so every air permeability unit is a velocity: 1 L/(m²·s) = 1 mm/s, 1 cm³/(s·cm²) = 1 cm/s = 10 mm/s, and 1 cfm/ft² = 1 ft/min = 5.08 mm/s exactly.
Why permeability is a velocity
An air permeability test pushes air through a known area of fabric, paper or membrane at a fixed pressure drop and measures the flow. The result is reported per unit area: flow ÷ area. Because volume per time divided by area leaves length per time, every permeability unit is a velocity. The value is the average speed at which air approaches the sample face, which is why the same units are used for filter face velocity and baghouse air-to-cloth ratio.
Exact equivalences
These identities follow directly from the definitions of the litre, centimetre and foot, so they are exact:
- 1 L/(m²·s) = 1 mm/s
- 1 cm³/(s·cm²) = 1 mL/(s·cm²) = 1 cm/s = 10 mm/s = 10 L/(m²·s)
- 1 ft³/(ft²·min) (cfm/ft²) = 1 ft/min = 0.00508 m/s = 5.08 mm/s
- 1 L/(dm²·min) = 1/600 m/s ≈ 1.66667 mm/s
- 1 m³/(m²·min) = 1 m/min ≈ 16.6667 mm/s
| Unit | Value |
|---|---|
| L/(m²·s) | 100 L/(m²·s) |
| cm³/(s·cm²) | 10 cm³/(s·cm²) |
| cfm/ft² | 19.685 ft³/(ft²·min) |
| L/(dm²·min) | 60 L/(dm²·min) |
| m/min | 6 m/min |
ISO 9237 vs ASTM D737
ISO 9237 reports results in mm/s, normally on a 20 cm² test area at 100 Pa for apparel or 200 Pa for technical fabrics. ASTM D737 reports cm³/(s·cm²) or ft³/(ft²·min) on a 38.3 cm² area at 125 Pa (0.5 inH₂O, the pressure of the original Frazier tester). The unit conversion between the two is exact, but the test pressures differ. Converting a 125 Pa result to mm/s does not make it a 100 Pa result.
From a flow reading to permeability
Many testers display flow, not permeability. Divide by the test area in consistent units. With the flow in L/min and the area in cm², multiply by 1000/6 to get mm/s. For example, 12 L/min through 20 cm² is 100 mm/s. ISO 9237 prints this factor as 167; the exact value is 166.67, so results calculated with 167 read about 0.2 % high.
| Test area | 1 L/min gives |
|---|---|
| 5 cm² | 33.333 mm/s |
| 10 cm² | 16.667 mm/s |
| 20 cm² | 8.3333 mm/s |
| 25 cm² | 6.6667 mm/s |
| 38.3 cm² | 4.3516 mm/s |
| 50 cm² | 3.3333 mm/s |
| 100 cm² | 1.6667 mm/s |
Common mistakes
- Treating cfm/ft² as a flow. It is a velocity; the flow in CFM depends on the area tested.
- Mixing test pressures. Always state the pressure next to the result, e.g. "102 mm/s at 100 Pa".
- Using standard flow units. Permeability uses the actual air volume. If your flow meter reads sccm or SLPM, correct it to actual conditions first.