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Calculation Machine

Return Air Grille Size Calculator

Return grille size from return CFM.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Required free area
1 sq ft
Gross grille area
1.429 sq ft
Approx square size
14.3 x 14.3 in
Round duct equivalent
10.2 in at 700 FPM

Quick answer

Return grilles are usually sized for 300-500 FPM face velocity through their free area, lower than supply registers to cut noise.

The formula

Free area = return CFM / face velocity

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Return airflow400 CFM
  • Face velocity400 FPM
  • Free area fraction70 %

Result

  • Required free area1 sq ft
  • Gross grille area1.429 sq ft
  • Approx square size14.3 x 14.3 in
  • Round duct equivalent10.2 in at 700 FPM

How to use the Return Air Grille Size Calculator

  1. 1Enter the total return airflow the grille must handle.
  2. 2Use 300-400 FPM for central returns near living areas, up to 500 FPM elsewhere.
  3. 3Divide by the grille's free-area fraction to get gross size.

Frequently asked questions

Should returns be bigger than supplies?

Yes, returns are usually oversized relative to supply air to keep velocity and noise low.

One return or several?

Central return systems use one large grille; better designs add returns in each closed-door room.

About this calculator