Grille net free area versus gross size

Louver bars block about a quarter of a grille face, so only the ~75% net free area passes air. Size the grille up so its open area — not the nominal face — meets the velocity limit.

A 20 by 20 grille face drawn with horizontal louver slats, where only the open gaps (about 75 percent of the face) pass air and the bars block the rest. On the right, the math for 800 CFM at 400 fpm: about 288 square inches of open area is needed, divided by 0.75 free area gives about 384 square inches gross, which rounds to a 20 by 20 grille. — Source: Grille manufacturer free-area ratings; ACCA Manual D

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A 20 by 20 grille face drawn with horizontal louver slats, where only the open gaps (about 75 percent of the face) pass air and the bars block the rest. On the right, the math for 800 CFM at 400 fpm: about 288 square inches of open area is needed, divided by 0.75 free area gives about 384 square inches gross, which rounds to a 20 by 20 grille.

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  <a href="https://remodelcalculators.com/construction-diagrams/return-grille-free-area" target="_blank" rel="noopener">
    <img src="https://remodelcalculators.com/api/figure-embed/return-grille-free-area" alt="A 20 by 20 grille face drawn with horizontal louver slats, where only the open gaps (about 75 percent of the face) pass air and the bars block the rest. On the right, the math for 800 CFM at 400 fpm: about 288 square inches of open area is needed, divided by 0.75 free area gives about 384 square inches gross, which rounds to a 20 by 20 grille." width="800" height="440" style="width:100%;height:auto;border:0" loading="lazy">
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    Diagram: <a href="https://remodelcalculators.com/construction-diagrams/return-grille-free-area" target="_blank" rel="noopener">Grille net free area versus gross size</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
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