Face Velocity & Air Permeability Converter

Why flow per area is a velocity

Dividing a volumetric flow (m³/s) by an area (m²) leaves metres per second. That is why air permeability, filter face velocity and the baghouse air-to-cloth ratio are all velocities, and why these identities hold exactly: 1 L/(m²·s) = 1 mm/s; 1 cm³/(s·cm²) = 1 mL/(s·cm²) = 1 cm/s; 1 ft³/(ft²·min) = 1 ft/min = 5.08 mm/s; 1 m³/(m²·min) = 1 m/min.

Comparing ISO 9237 and ASTM D737 results

ISO 9237 reports mm/s (= L/(m²·s)), usually at 100 Pa for apparel or 200 Pa for technical fabrics, on a 20 cm² test area. ASTM D737 reports cm³/(s·cm²) or ft³/(ft²·min) at 125 Pa (0.5 inH₂O) on a 38.3 cm² area. The unit conversion is exact, but results taken at different pressures are not directly comparable, because permeability is not strictly linear with pressure.

Face Velocity & Air Permeability unit definitions

meter per second (m/s)

The metre per second (m/s) is the SI unit of velocity. As a face velocity it equals a volumetric flow of 1 m³/s through 1 m² of area.

decimeter per second (dm/s)

The decimetre per second (dm/s) is one tenth of a metre per second. It is rarely used, but it appears on some instrument scales and older permeability reports.

centimeter per second (cm/s)

The centimetre per second (cm/s) is one hundredth of a metre per second. A flow of 1 cm³/s through 1 cm² gives exactly 1 cm/s.

millimeter per second (mm/s)

The millimetre per second (mm/s) is one thousandth of a metre per second. It is the unit ISO 9237 uses for fabric air permeability, and 1 mm/s is exactly 1 L/(m²·s).

meter per minute (m/min)

The metre per minute (m/min) is a velocity of one metre each minute, 1/60 m/s. For filters it is the same as m³/min of air per m² of media, the "air-to-cloth ratio".

foot per minute (ft/min)

The foot per minute (ft/min, fpm) is a velocity of one foot each minute, exactly 0.00508 m/s. One CFM through one square foot gives exactly 1 fpm.

liter per square meter per second (L/(m²·s))

The litre per square metre per second, L/(m²·s), is the air permeability unit used on European fabric and nonwoven datasheets. Because 1 L = 10⁻³ m³, it is exactly equal to 1 mm/s.

milliliter per second per square centimeter (mL/(s·cm²))

The millilitre per second per square centimetre, mL/(s·cm²), is a permeability unit identical to cm³/(s·cm²) and therefore to 1 cm/s.

cubic centimeter per second per square centimeter (cm³/(s·cm²))

The cubic centimetre per second per square centimetre, cm³/(s·cm²), is the SI result unit of ASTM D737 air permeability. It is exactly 1 cm/s and 0.01 m/s.

liter per square decimeter per minute (L/(dm²·min))

The litre per square decimetre per minute, L/(dm²·min), is an air permeability unit used by some European textile laboratories. It equals 1/600 m/s, about 1.667 mm/s.

cubic decimeter per square centimeter per minute (dm³/(cm²·min))

The cubic decimetre per square centimetre per minute, dm³/(cm²·min), is a large permeability unit equal to 1/6 m/s (≈ 166.7 mm/s). It shows up on some instruments for highly permeable materials.

cubic foot per square foot per minute (ft³/(ft²·min))

The cubic foot per square foot per minute, ft³/(ft²·min) or "CFM per ft²", is the US air permeability unit (often called Frazier permeability). It is exactly 1 ft/min, 0.00508 m/s or 5.08 mm/s.

All face velocity & air permeability conversions

Related converters

• Flow Rate Converter – CFM, m³/h, L/min, L/s, GPM, sccm, SLPM
• Area Converter – m², cm², mm², ft², in², test areas
• Pressure Converter – Pa, kPa, psi, bar, inH₂O, mmHg, torr, atm
• Air Permeability Units Explained (guide)
• Standard Test Areas in Textile and Membrane Testing (guide)
• Printable face velocity & air permeability conversion chart – print, PDF or PNG
• CFM to m³/h for the total airflow
• test area: cm² to in²
• test pressure: Pa to inH₂O
• Face Velocity Calculator – Airflow + filter, duct or hood size → face velocity (fpm, m/s), or solve for flow or area.
• Air Permeability Calculator – Tester flow + test area (ISO 9237, ASTM D737 presets) → permeability in mm/s, L/m²/s, cfm/ft².
• Bubble Point Pore Size Calculator – Bubble-point pressure + wetting liquid → largest pore size (Young–Laplace), or the reverse.
• Bulk Unit Converter – Paste a column or table from Excel, convert every value, copy back or download CSV.

Sources

• The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures.
• ISO 9237:1995 Textiles — Determination of the permeability of fabrics to air, International Organization for Standardization.
• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology.
• ASTM D737 Standard Test Method for Air Permeability of Textile Fabrics, ASTM International.

Last reviewed: 2026-10-15.