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Porosity is a volume fraction; permeability is a flow rate. High porosity does not guarantee high flow.
Air Permeability
—CFM
Cubic feet per square foot per minute
Structure & Flow
Volumetric Porosity
—%
Fabric Bulk Density
—g/cm3
Air Velocity through Fabric
—m/s
Flow per Unit Pressure
—L/m²/s/Pa
Always quote the test pressure alongside the reading. A permeability figure without its pressure drop cannot be compared with anything.
Using this calculator
About the Air Permeability & Fabric Porosity Calculator
The formula
This is the expression the tool evaluates. Every term is named underneath, with the unit it must be supplied in.
Face velocity through the fabricairVelocity = airFlow / 1000
1 L/m2/s is exactly 1 mm/s of face velocity — a litre poured over a square metre is a 1 mm layer — so the division by 1000 only carries mm/s to m/s. No fabric property enters here; this is the same measurement in different clothes.
The same reading in the imperial unit buyers quoteairPermeabilityCfm = airVelocity x 3.280840 x 60
3.280840 x 60 = 196.8504 converts m/s to ft/min, and ft/min of face velocity is numerically identical to cubic feet per square foot per minute. The mill shortcut falls out of the same constant: 1 CFM = 5.08 L/m2/s, because 1 ft/min = 0.3048 x 1000 / 60 = 5.08 mm/s. Note that pressureDrop is not in this line — the conversion is pure units, so the test pressure must be quoted separately with the number.
Fabric bulk density, fibre plus the air inside itbulkDensity = gsm / (fabricThickness x 1000)
One square metre of cloth fabricThickness mm thick occupies 1000 x fabricThickness cm3, so g/m2 divided by that volume lands directly in g/cm3. fabricThickness must be the value measured under a stated presser-foot pressure (ISO 5084 uses 1 kPa); a compressible fleece or needlefelt gives a different bulk density at every foot load.
Volumetric porosity, the void fractionporosity = (1 - bulkDensity / fibreDensity) x 100
bulkDensity / fibreDensity is the fraction of the fabric volume that is solid polymer; whatever is left is air. fibreDensity is the density of the fibre substance, not of the yarn or the cloth — cotton 1.52, viscose 1.52, PET 1.38, PA6.6 1.14, PP 0.91 g/cm3. For a blend use the reciprocal rule: 1 / fibreDensity = sum of (mass fraction / component density).
Flow normalised to the test pressurepermeabilityIndex = airFlow / pressureDrop
Useful only for ranking fabrics measured at the same pressureDrop. Flow through cloth is not proportional to pressure — the inertial (Forchheimer) term makes it closer to dP^0.6 to dP^0.8 — so this index must never be used to convert a 100 Pa reading into a 200 Pa reading.
Symbols used above
Symbol
Stands for
Unit
airFlow
Air Flow
L/m²/s
pressureDrop
Test Pressure Drop
Pa
gsm
Fabric Weight
g/m²
fabricThickness
Fabric Thickness
mm
fibreDensity
Fibre Density
g/cm3
airPermeabilityCfm
Air Permeability
CFM
porosity
Volumetric Porosity
%
bulkDensity
Fabric Bulk Density
g/cm3
airVelocity
Air Velocity through Fabric
m/s
permeabilityIndex
Flow per Unit Pressure
L/m²/s/Pa
How the result is derived
Step by step, from the values you type to the figure on screen.
Enter the flow exactly as the instrument reports it, together with the pressure drop it was held at. ISO 9237 defaults to 100 Pa for apparel and 200 Pa for technical fabrics, ASTM D737 to 125 Pa. A permeability figure without its pressure is not a specification and cannot be compared with a supplier's number.
The flow is restated as a face velocity — the speed at which air crosses the plane of the cloth. L/m2/s and mm/s are the same number, which is why an instrument reading of 250 becomes 0.250 m/s with no fabric data involved.
That velocity is converted to CFM, meaning cubic feet of air per square foot of fabric per minute. This is the unit US apparel buyers, down-ticking specifications and filter media datasheets use, and it is the same unit as a baghouse air-to-cloth ratio in ft/min.
Fabric weight and thickness are combined into bulk density: the mass of a unit volume of the fabric as it sits, air included. This is the structural half of the tool and is computed entirely independently of the flow reading.
Bulk density divided by the density of the fibre polymer gives the solid volume fraction; one minus that is porosity. Because the reference is the fibre substance, a fabric of any construction that is 26% solid by volume reads 74% porous — the number says nothing about how those voids are shaped or whether they connect.
Read the two halves together. Porosity says how much void the structure holds; permeability says how easily air crosses the void that is actually connected through the thickness. A dense needlefelt at 80% porosity can flow less than an open leno at 55%.
What each input means
Where to read each value on the floor, the unit it must be in, and the range the tool accepts.
Input
Unit
Accepted range
Default
What it means
Air Flow
L/m²/s
1 to 10000 L/m²/s
250
Test Pressure Drop
Pa
1 to 1000 Pa
100
Fabric Weight
g/m²
5 to 2000 g/m²
180
Fabric Thickness
mm
0.01 to 50 mm
0.45
Fibre Density
g/cm3
0.5 to 3 g/cm3
1.52
What the tool returns
The headline figure and every supporting value it is built from.
A 180 g/m2 cotton woven, 0.45 mm thick, reading 250 L/m2/s at 100 Pa — an ordinary mid-weight apparel cloth. The two halves answer different questions: 73.68% of its volume is air, but it is the 49.21 CFM that a windproof or downproof specification is written against. The porosity number is also the more fragile one — it depends on a thickness measurement, while the CFM figure depends only on the flow reading. Re-run the same fabric with thickness overstated by 10% (0.495 mm) and bulk density falls from 0.4000 to 0.3636 g/cm3 and porosity rises from 73.68% to 76.08%, while air permeability does not move at all.
How to use it
Work through the input groups in order — Permeability Test and Fabric. The defaults are a realistic case, so you can change one value at a time and watch what moves.
There is no calculate button. Every figure recalculates as you type or drag, which is what makes this usable for a what-if sweep rather than a single answer.
Read Air Permeability in the dark results panel — that is the headline figure, expressed in CFM.
Check the supporting rows underneath (Volumetric Porosity, Fabric Bulk Density, Air Velocity through Fabric and Flow per Unit Pressure) before acting on the headline — they are where an implausible input usually shows itself first.
Reset to defaults returns every field to the reference case, which is the quickest way to check whether a surprising result came from the tool or from an input you had changed earlier.
Where this is used
Downproof and windproof shells — down ticking is normally specified in the low single-digit CFM and softshell face fabrics under about 5 CFM. Suppliers outside the US quote L/m2/s, so dividing by 5.08 is the first step in checking whether a claim actually meets the spec.
Filter media selection — a baghouse air-to-cloth ratio is quoted in ft/min, which is numerically the same unit as this tool's CFM output. The design face velocity can therefore be read straight against the media's clean-cloth CFM, and it must sit well below it so that the dust cake, not the cloth, controls the pressure drop.
Coated and laminated technical cloth — airbag base fabric, parachute canopy and inflatable ground sheet are specified at very low permeability and often at elevated test pressure. Porosity computed alongside shows whether a coating has genuinely filled the interstices or is merely sitting on the surface with the yarn structure still open.
Activewear, mesh and lining development — the porosity/permeability pair separates a bulky but closed structure (high porosity, low flow, good insulation) from an open one (moderate porosity, high flow, good ventilation), which weight and thickness alone cannot distinguish.
Finishing and calendering control — if permeability drops across a finishing run while GSM holds steady, thickness has been squeezed out. Tracking bulk density and porosity through the line quantifies how much compaction has occurred and where it happened.
Reading the result
Typical bands and what each one is telling you.
Value
What it indicates
Under 5 CFM (under 25 L/m2/s)
Coated, laminated or downproof-tight. Down ticking, windproof softshell face fabric, airbag base cloth. Below about 1 CFM the reading is governed by the coating or membrane rather than by the weave, and instrument repeatability becomes the limiting factor.
5 to 25 CFM (25 to 127 L/m2/s)
High-density microfibre and tightly woven outerwear shells, closely sett cotton in the Ventile family, dense twill and denim. Wind-resistant without being sealed; still passes enough air to avoid feeling like a plastic bag.
25 to 100 CFM (127 to 508 L/m2/s)
The general apparel band — poplin and shirting wovens, single jersey, most printed cottons and light bottomweights. The worked example at 49.21 CFM sits here. Comfort complaints in this band are usually about moisture transport, not airflow.
100 to 400 CFM (508 to 2032 L/m2/s)
Open knits, mesh linings, voile, gauze, light interlinings and thermal-bonded nonwovens. Chosen for ventilation rather than protection. A filter medium landing here is generally too open to build a stable dust cake.
Above 400 CFM (above 2032 L/m2/s)
Nets, coarse mesh, spacer and scrim. At this flow the pressure loss of the instrument head itself becomes a real share of the reading, so confirm the tester can hold the set pressure at that flow and use the largest available test head before trusting the number.
Assumptions and limits
Always quote the test pressure alongside the reading. A permeability figure without its pressure drop cannot be compared with anything.
Every input is bounded to the range normal practice occupies (Air Flow 1 to 10000 L/m²/s, Test Pressure Drop 1 to 1000 Pa and Fabric Weight 5 to 2000 g/m², and so on for the rest). Those bounds are guard rails against typing errors, not a claim that the formula fails one unit outside them.
The calculation is deterministic: the same inputs always give the same result. It carries no allowance for machine condition, operator skill, ambient conditions or lot-to-lot material variation unless an input above explicitly represents one.
Nothing is sent anywhere. The maths runs in your browser, so the numbers you type never leave the page.
Standards and further reading
ISO 9237 — determination of the permeability of fabrics to air. Defines the test head area and the standard pressure drops: 100 Pa for apparel fabrics, 200 Pa for technical fabrics. Results are reported in mm/s or L/m2/s, the metric input this tool takes.
ASTM D737 — air permeability of textile fabrics. Uses 125 Pa (0.5 inch of water) as its default pressure drop and reports in cubic feet per square foot per minute, which is this tool's primary output. Because the default pressure differs from ISO 9237, results under the two methods are not interchangeable.
ISO 5084 / ASTM D1777 — thickness of textiles under a specified presser-foot pressure. This is the measured input for fabricThickness, and the presser-foot load must be recorded with it or the bulk density and porosity cannot be reproduced.
ISO 3801 / ASTM D3776 — mass per unit area of fabric, the measured input for gsm.
Questions people ask
Can I convert a reading taken at 100 Pa into the 125 Pa figure my customer asks for?
Not by ratio. Flow does rise with pressure but sub-linearly, roughly as dP^0.6 to dP^0.8 for apparel fabrics because of the inertial term alongside the viscous one. Scaling 100 Pa to 125 Pa linearly implies a 25% increase, whereas the real rise is nearer 14% to 20%. Re-test at the pressure the specification names — that is the only defensible answer in a dispute.
My fabric is 74% porous but it barely breathes. Is the calculator wrong?
No — porosity is a volume fraction and permeability is a flow rate, and they are not the same property. A needlepunched nonwoven or a milled wool can be 80% air by volume with pores that are fine, tortuous and poorly connected through the thickness, while a 55% porous leno with a few large open channels flows far more. Flow through a pore scales roughly with the square of its diameter, so a handful of large holes dominates a million small ones. Use porosity for insulation, cover and resin pickup; use permeability for airflow.
What fibre density do I enter for a blend?
Use the reciprocal rule, because volume fractions do not add on mass fractions: 1 / fibreDensity = sum of (mass fraction / component density). For 65/35 polyester/cotton that is 0.65 / 1.38 + 0.35 / 1.52 = 0.7013, giving 1.43 g/cm3. Entering cotton's 1.52 for that blend at 0.4000 g/cm3 bulk density reports 73.68% porosity instead of the correct 71.95% — an overstatement of about 1.7 percentage points.
Why does the CFM output not change when I change the test pressure?
Because the CFM figure is a unit conversion of the flow you typed, not a model of the fabric. The pressure drop is used only in permeabilityIndex, where it normalises the flow so two fabrics tested at the same pressure can be ranked. This is exactly why the reading and its pressure must always travel together on the test report; the number on its own is not comparable to anything.
