Face Velocity Calculator
Face velocity is airflow divided by face area: 2000 CFM through a 24 × 24 in filter (4 ft²) gives 500 ft/min (2.54 m/s).
Area = 3716.1216 cm² = 0.37161216 m²
500 ft/min
| Unit | Value |
|---|---|
| meter per second [m/s] | 2.54 m/s |
| decimeter per second [dm/s] | 25.4 dm/s |
| centimeter per second [cm/s] | 254 cm/s |
| millimeter per second [mm/s] | 2540 mm/s |
| meter per minute [m/min] | 152.4 m/min |
| foot per minute [ft/min] | 500 ft/min |
| liter per square meter per second [L/(m²·s)] | 2540 L/(m²·s) |
| milliliter per second per square centimeter [mL/(s·cm²)] | 254 mL/(s·cm²) |
| cubic centimeter per second per square centimeter [cm³/(s·cm²)] | 254 cm³/(s·cm²) |
| liter per square decimeter per minute [L/(dm²·min)] | 1524 L/(dm²·min) |
| cubic decimeter per square centimeter per minute [dm³/(cm²·min)] | 15.24 dm³/(cm²·min) |
| cubic foot per square foot per minute [ft³/(ft²·min)] | 500 ft³/(ft²·min) |
How this was calculated (SI units):
- Q = 0.9438948864 m³/s
- A = 0.37161216 m²
- v = Q ÷ A = 0.9438948864 ÷ 0.37161216 = 2.54 m/s
Face Velocity Formula
v = Q ÷ A
v = face velocity, Q = volumetric airflow, A = face area
CFM ÷ ft² = ft/min (fpm) · m³/s ÷ m² = m/s · m³/h ÷ m² ÷ 3600 = m/s
The relation follows from continuity: the volume of air passing the face each second equals the face area times the average speed. Rearranged, Q = v × A gives the airflow for a target velocity, and A = Q ÷ v gives the face area needed.
Worked Examples
- HVAC filter. 2000 CFM through a 24 × 24 in filter: A = 4 ft², v = 2000 ÷ 4 = 500 ft/min = 2.54 m/s.
- ISO 16890 test rig. 0.944 m³/s through a 592 × 592 mm filter face: A = 0.350464 m², v = 2.694 m/s (530.2 ft/min).
- Fume hood. A 60 in wide hood with the sash open 18 in (7.5 ft²) at 100 fpm needs Q = v × A = 750 CFM (1274 m³/h).
- Round duct. 400 CFM in a 10 in round duct: A = 0.5454 ft², v = 733.4 ft/min (3.726 m/s).
Face Velocity of Common Filter Sizes
| Nominal size (in) | 800 CFM | 1200 CFM | 1600 CFM | 2000 CFM |
|---|---|---|---|---|
| 12 × 24 | 400 fpm | 600 fpm | 800 fpm | 1000 fpm |
| 16 × 20 | 360 fpm | 540 fpm | 720 fpm | 900 fpm |
| 16 × 25 | 288 fpm | 432 fpm | 576 fpm | 720 fpm |
| 20 × 20 | 288 fpm | 432 fpm | 576 fpm | 720 fpm |
| 20 × 25 | 230 fpm | 346 fpm | 461 fpm | 576 fpm |
| 24 × 24 | 200 fpm | 300 fpm | 400 fpm | 500 fpm |
Typical Face Velocities
- HVAC panel and pleated filters: commonly rated around 300–500 fpm (1.5–2.5 m/s).
- Cooling coils: often limited to about 500 fpm (2.5 m/s) to avoid condensate carry-over.
- Laboratory fume hoods: an average face velocity of about 80–120 fpm (0.4–0.6 m/s) is a common design target.
- Unidirectional cleanroom and HEPA ceilings: around 90 fpm (0.45 m/s).
- Pulse-jet baghouse filters: air-to-cloth ratios of roughly 3–7 fpm (1–2 m/min).
Ranges are typical design values, not requirements. Check the standard or manufacturer data that applies to your system.
Frequently asked questions
What is face velocity?
Face velocity is the average speed of air approaching or passing through a face such as a filter, coil, hood opening or test area. It equals the volumetric airflow divided by the face area: v = Q / A.
How do I calculate face velocity from CFM?
Divide the airflow in CFM by the face area in square feet; the answer is in feet per minute. For example, 2000 CFM ÷ 4 ft² = 500 fpm (2.54 m/s).
Is fpm the same as CFM per square foot?
Yes. One cubic foot per minute through one square foot is exactly one foot per minute (0.00508 m/s), so CFM/ft² and fpm are numerically identical.
Should I use the nominal or actual filter size?
Filter ratings are usually quoted on the nominal size (e.g. 24 × 24 in). The actual frame is about 0.5 in smaller and the open media area smaller still, so the true velocity is somewhat higher. Use the same convention as the specification you compare with.
What is the difference between face velocity and duct velocity?
Both are flow ÷ area. Face velocity uses the area of a filter, coil or opening; duct velocity uses the internal cross-section of the duct. Filters are often placed in housings larger than the duct to keep face velocity low.
Sources
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology .
- The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures .
- ISO 16890-1:2016 Air filters for general ventilation, International Organization for Standardization .
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). Face Velocity Calculator. Retrieved October 17, 2026, from https://scientificconverters.com/calculators/face-velocity/
IEEE
Scientific Converters, "Face Velocity Calculator," scientificconverters.com. Accessed: Oct. 17, 2026. [Online]. Available: https://scientificconverters.com/calculators/face-velocity/