Grille/Register Face Velocity and Free-Area Sizing
Face velocity V = CFM/(gross area x free-area ratio), or the required gross size from a target velocity.
Example
You enter
- Solve for size
- Airflow (cfm) 400
- Effective-area ratio Ak / gross (0-1, default 0.75; from the maker's Ak) 0.75
- Target face velocity (fpm) 500
You get
- A gross req (ft²) 1.067
- A gross req (in²) 153.6
Checked against published worked examples from: Hart & Cooley.
Details, formula, and sources
Scope400 CFM to a 500 fpm supply -> 154 in^2 (a 12x14); returns run slower so they size larger. The manufacturer free-area and throw data govern.
Formula or tableA_free = A_gross x ratio; V_face = CFM / A_free; A_gross_req = CFM / (V_target x ratio).
Source and editionThe grille/register free-area face-velocity relation from ASHRAE / SMACNA air-distribution practice, by name.
Free accessFace velocity = airflow / free area is a standard air-distribution relation; the free-area ratio and throw/NC data come from the manufacturer's published grille catalog. The manufacturer's selection data govern.
What governsEstimate. AHJ and licensed professional govern.
API field namesmode, cfm, ratio, V_target, A_gross_req_ft2, A_gross_req_in2
- Free-area velocity V_face = CFM / (A_gross x ratio); A_gross_req = CFM / (V_target x ratio)ASHRAE / SMACNA
- Velocity bands supply ~500-800 fpm (700 common), returns 400-600 fpm maxHart & Cooley Engineering Data (2026)
- No throw/NC throw, drop, and noise come from the manufacturer's catalog, not this tilescope of this tile