Fabric Air Permeability Explained: CFM, ASTM D737 vs ISO 9237, and What Buyers Should Specify

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What 'air permeability' actually measures
Units you will see on a test report
The two standards buyers quote
Higher is not always better — match the number to the garment
What changes air permeability in woven fabric
How to read a supplier's air permeability claim
How we handle breathability at Nantong Huayuan

What 'air permeability' actually measures

Air permeability is the volume of air that passes through a fabric per unit area, per unit time, under a fixed pressure difference between the two sides of the cloth. In plain terms: how freely air flows through the fabric when you push on it. It is measured on a dedicated instrument that seals a circular sample and draws air through it.

Important: air permeability is not water-vapour transmission. The two are different physical properties and are tested separately. Together they make up what the market loosely calls 'breathability' — air permeability governs airflow, while moisture-vapour transmission rate (MVTR) and evaporative resistance (Ret) govern how sweat vapour escapes. A fabric can be air-permeable yet poor at moving vapour, and vice versa, depending on coatings and membranes.

Units you will see on a test report

The same fabric can look very different depending on the unit a lab reports. Always read the unit, the test pressure, and the test area before comparing two reports.

Unit	System / standard	Note
CFM (ft³/min/ft²)	ASTM / North America	Cubic feet per minute per square foot. Larger number = more airflow.
cm³/s/cm² (also written cm/s)	ISO 9237 metric	Cubic centimetres per second per square centimetre.
L/m²/s	ISO / Europe, medical	Litres per square metre per second; 1 cm³/s/cm² ≈ 10 L/m²/s.
mm/s	Air velocity	Useful for very open fabrics (mesh, net); pressure-dependent.

Rough conversion: 1 CFM (ft³/min/ft²) ≈ 5.08 cm³/s/cm². Because the conversion is not intuitive, agree on the unit your market expects before requesting a report.

The two standards buyers quote

Two methods dominate international sourcing:

ASTM D737 — the North American method. Commonly runs at a constant pressure of 125 Pa; results reported in CFM.
ISO 9237 — the international method, widely required for EU/UK and Asia exports. Commonly runs at 100 Pa; results reported in cm³/s/cm² or L/m²/s.

GB/T 5453 (China), DIN 53887 (Europe, older) and JIS L1096 (Japan) are regional equivalents. Results between ASTM D737 and ISO 9237 cannot be compared directly unless the pressure, test area and unit are identical — the pressure alone differs (125 Pa vs 100 Pa).

Higher is not always better — match the number to the garment

Air permeability is application-dependent. A high number means open and airy; a low number means dense and barrier-like. Neither is 'good' in isolation:

Activewear, mesh, ventilation panels: buyers target high airflow (open knits and mesh can run well above 100–150 CFM).
Windproof shells, down-proof fabric: buyers often specify a low limit — down-proof shells are frequently specified under ~5 CFM so feathers cannot migrate.
Shirting and suiting wovens: typically denser than knits, so their values sit lower; the right value is the one that meets the garment's comfort and performance target, not a maximum.

When a buyer says 'make it breathable', ask: breathable for what end use, and to what target number? Without a number, the supplier has no testable target.

What changes air permeability in woven fabric

For a woven mill, the main levers are:

Construction: thread density (ends/picks per cm), weave type, and yarn count. A tighter, higher-density weave lets less air through than an open one.
Fibre and yarn: finer, lower-twist, or shorter-fibre yarns and loose structures raise airflow; compacted or filament yarns reduce it.
Finishing: calendering (heat + pressure) flattens and closes the weave, lowering permeability; coatings, lamination and membrane films add a barrier and reduce airflow, sometimes by a large margin.
GSM and state: results differ between greige (grey) and finished/dyed/printed cloth, and can shift after washing. Always ask which state was tested.

As a rule, comparable GSM does not guarantee comparable permeability — weave, density and finish decide it.

How to read a supplier's air permeability claim

A number without conditions is decoration. Before approving a claim, request:

The test method (ISO 9237 or ASTM D737) and the test pressure used.
The test area and number of specimens (an average over several samples beats a single reading).
The reported unit (CFM, L/m²/s, or cm³/s/cm²).
Which face was tested (some constructions differ face to back).
Whether the sample was greige or finished/dyed/printed/coated.
Results before and after washing, if stability matters to the garment.
Third-party test reports (SGS, Intertek, etc.) where the market requires them.

How we handle breathability at Nantong Huayuan

We are an integrated woven mill (yarn dyeing → weaving → fabric dyeing → printing → finishing) founded in 1995 in Nantong, Jiangsu, China. Our in-house woven range includes poplin, twill, canvas, oxford, chambray and yarn-dyed shirting. Because we weave and finish in one chain, construction and finishing can be tuned to a target air-permeability window for the end use.

For a specific style, the achievable air permeability, the standard we test to (ASTM D737 or ISO 9237), and the exact value are confirmed in writing at enquiry — we do not publish generic number ranges that may not match your colour or finish. Certifications held include OEKO-TEX, GRS, GOTS, OCS and BCI (certificate numbers available on request). Sampling is paid and fully deducted against the bulk order; MOQ is 2,000 metres per colour; lead time is confirmed in writing at enquiry. Enquiries: gxy@ntboerli.com.