Formula & worked example
V=Q/Afree; Afree=Agross·free-area ratio; Agross=Q/(Vtarget·ratio).
800 CFM through a 24×24 in grille at 75% free area gives about 267 fpm net-free-area velocity.
HVAC design calculator · Updated Sep 17, 2026
Grille Free Area Calculator — CFM, Face Velocity & Required Size
Calculate HVAC grille/register net free area and face velocity, or reverse-size required gross face area from airflow, target velocity and free-area ratio.
Inputs
Live result
Gross face area 4 ft²
Net free area 3 ft²
Actual velocity over free area 266.66667 fpm
Required gross area at target 2.6666667 ft² (384 in²)
Equivalent square face 19.595918 in × in
Assumptions & limits
- Use manufacturer published free area when available.
- Noise, throw, pressure drop and NC criteria require grille performance data.
Frequently asked questions
What does this Grille Free Area Calculator calculate?
Calculate HVAC grille/register net free area and face velocity, or reverse-size required gross face area from airflow, target velocity and free-area ratio.
Can I use the result for final HVAC selection?
Use the result as a transparent engineering cross-check. Confirm final selections against measured design conditions, manufacturer data, applicable standards and local code.
Why are the assumptions shown?
HVAC shortcuts can look precise while hiding density, efficiency, free-area or pressure assumptions. MAXScanner keeps the governing inputs visible so the result can be audited.