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Return Grille Sizer
Quick answer: Enter your system airflow in CFM, a target face velocity — 300 feet per minute is a common 1-inch rating point — and the number of filter grilles. The sizer divides airflow by velocity to get face area per grille and suggests the smallest common nominal 1-inch filter that meets it. It is a planning estimate.
The math
Face area (ft²) = CFM ÷ face velocity ÷ number of grilles. Nominal sizes are compared by their nominal area, for example 20 × 25 = 500 sq in, or 3.47 ft². A common airflow rule of thumb is roughly 350–400 CFM per ton of cooling.
Worked example
A 3-ton system at 1,200 CFM and 300 fpm needs 4 ft² (576 sq in). One grille would need a 24x24x1 or larger; two grilles each need 2 ft² (288 sq in), met by 12x24x1 or 14x24x1.
Illustrative note (2026-10-29): This example was worked on 2026-10-29 for illustration.
Limits
Grille free area, duct size, noise and the blower's static-pressure budget all matter. Adding or resizing returns is design work for a qualified HVAC technician.