Return Air Sizing Calculator

Calculate the size of return air grilles and ducts needed to avoid excessive pressure drop in an HVAC system.

CFM
ft/min
%

Gross Grille Area Needed

3.33 sq ft

Grille Area480 sq in
Return Duct Diameter21.4 in

Gross Grille Area Needed vs System Airflow

सूत्र देखें →

सूत्र

## Return Air Sizing **Grille Area = CFM / (Velocity x Free Area %)** Return grilles must be large enough to avoid creating excessive negative pressure. The free area percentage accounts for the grille bars or louvers that block part of the opening. Keep return grille velocity below 400 ft/min for quiet operation.

हल किया गया उदाहरण

1,000 CFM system, 400 ft/min max grille velocity, 75% free area.

  1. 01Effective velocity = 400 x 0.75 = 300 ft/min
  2. 02Grille area = 1000 / 300 = 3.33 sq ft = 480 sq in
  3. 03Could use two 16x16 in grilles (512 sq in total)
  4. 04Return duct = 2 x sqrt(1000/400/pi) x 12 = 21.4 in