| Size (W×H in) | Net Area (ft²) | CFM @ 400 FPM | CFM @ 500 FPM | CFM @ 600 FPM | CFM @ 750 FPM | Your CFM |
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This grille and register sizing calculator turns a design airflow and a target face velocity into a recommended grille size for supply ceiling diffusers, sidewall registers, return grilles, and exhaust or transfer grilles. Enter the CFM and pick the application; the tool shows the recommended ASHRAE face-velocity range, computes the required free area, and matches it against a table of standard grille sizes so you can see the CFM each size carries at common face velocities.
Face velocity is the lever that balances comfort, throw, and noise. Sizing too small drives velocity up and the grille becomes audible; sizing too large wastes wall or ceiling space and may short the throw. Working from a target velocity keeps every outlet inside a sensible acoustic band while still delivering the required air.
| Required free area | A (ft²) = Q (CFM) ÷ Vface (FPM) |
| In square inches | A (in²) = A (ft²) × 144 |
| Net vs. gross | Anet = Agross × 0.70 (typical bar grille) |
| Actual velocity | Vactual = Q (CFM) ÷ Anet (ft²) |
The core relation is simply airflow divided by face velocity, with the 144 factor converting square feet to square inches. The 0.70 constant is the net-free-area fraction for a typical bar grille — only about 70% of the gross face is open after the bars and frame, so the gross size must be larger than the bare area suggests. Recommended face-velocity ranges and the net-area assumption follow standard ASHRAE / manufacturer grille-selection practice; confirm the published Ak factor for ceiling diffusers.