Calculate face velocity in FPM from airflow in CFM and face area, or solve for the airflow or area, with metric units such as m/s, m³/h and m² too.
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Face Velocity Formula
- V = face velocity (fpm)
- Q = volumetric airflow (CFM)
- A = open face area (ft²)
Rearranged for the airflow mode: Q = V × A. Face area depends on shape: A = width × height for rectangular openings, A = π × D² / 4 for round openings. The calculator converts each input to compatible SI units before solving, so you can mix the supplied units. Use free or gross area consistently with the equipment specification; do not substitute filter-media area for face area.
Application-Specific Velocity Checks
The arithmetic result is not a safety or equipment-design verdict. Use equipment performance data, field measurements and the applicable requirements. The table identifies checks; it does not supply universal operating ranges.
| Application | Required Basis | Notes |
|---|---|---|
| HVAC pleated filter | Manufacturer rating | Higher velocity raises pressure drop |
| HEPA filter | Specified filter rating | Per filter rating |
| Supply register / grille | Product performance data | Check sound and pressure data |
| Return grille | Product selection data | Check throw and sound data |
| Lab fume hood | Validated hood specification | Velocity alone does not prove containment |
| Cooling coil | Coil performance data | Check carryover limits for the selected coil |
| Paint spray booth | Applicable booth-specific requirement | Requirements vary by booth and spraying operation |
| Unit Conversion | Equivalent |
|---|---|
| 1 fpm | 0.00508 m/s |
| 1 m/s | 196.85 fpm |
| 1 CFM | 1.699 m³/h = 0.4719 L/s |
| 1 ft² | 0.0929 m² = 144 in² |
Worked Example
Problem: A 20 in × 25 in pleated filter sees 1,000 CFM. What is the face velocity?
- Convert area: (20/12) × (25/12) = 3.472222… ft²
- Apply the formula without intermediate rounding: V = 1,000 / (500/144) = 288 fpm
- Compare 288 fpm with the selected filter’s tested flow and pressure-drop data. This calculation alone does not establish acceptable pressure drop or noise.
FAQ
Is face velocity the same as duct velocity? No. Face velocity is measured at the open face of a filter, coil, hood, grille, or louver. Duct velocity uses the duct cross-section; relative velocities depend on the actual areas and airflow.
Should I use gross area or free area? Use the area convention specified for the equipment and performance data. Free area, gross face area and media area are different, and a free-area velocity cannot be directly compared with a gross-face rating.
Why does my fume hood need a sash-position adjustment? Face velocity is measured across the actual sash opening. As you raise the sash, the area grows and velocity drops unless airflow increases. Variable air volume hoods modulate Q to hold V constant.
