SettingsSettings for this calculationUS
The airflow the grille needs to pass.
This is the design airflow for the return (or supply) opening being sized.
The desired air velocity across the grille's gross face, chosen for acceptable noise and comfort.
Lower target velocities reduce noise and drop but require a larger grille; higher velocities allow a smaller grille at the cost of more noise and pressure drop.
The percentage of the grille's gross face that is actually open to airflow.
A grille's blades or louvers block part of its face, so its effective open area is less than its overall (gross) size — this percentage is published by the manufacturer for each model.
Required gross grille area
1.14 sq ft
Free area percentage varies by grille model, blade spacing, and finish — use the specific grille manufacturer's published free-area rating for an accurate result, not an assumed typical value.
They open the calculator with your figures already in it
Return Air Grille Free Area Sizing Calculator: 1.14 sq ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Gross grille area = Airflow ÷ Target Face Velocity ÷ Free Area Fraction, since a grille's core (blades/louvers) blocks a portion of its face — standard grilles are commonly 60-75% free area, with the exact figure published by the grille manufacturer for each model.
Inputs used
- Airflow
- 400 CFM
- Target face velocity
- 500 ft/min
- Grille Free Area (%, Manufacturer-Published, Typically 60-75%)
- 70
Confidence note: Free area percentage varies by grille model, blade spacing, and finish — use the specific grille manufacturer's published free-area rating for an accurate result, not an assumed typical value.
What this calculation does not cover
- Returns an area, not a grille. You buy a catalogue face size in fixed increments, and free area does not hold constant as those dimensions change — a long narrow grille loses proportionally more of its face to frame and end margins than a square one of the same area, so the free-area percentage has to be re-read off the model actually selected once a size is picked.
- Face velocity is a stand-in for noise, not a measurement of it. What a grille sounds like at a given airflow comes from the manufacturer's tested NC data for that model, and a balancing damper mounted behind the core is a separate and usually louder source — a grille that meets the velocity target with a damper half closed can be the loudest thing in the room.
- Leaves out the pressure drop the return path actually sees. The grille's own loss is small; a filter behind it is not, and a filter-back return sized only for face velocity hands the blower a restriction that grows as the filter loads. Return paths are where systems quietly lose their airflow, and the governing number is the static pressure drop across the whole opening, filter included.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Gross grille area = Airflow ÷ Target Face Velocity ÷ Free Area Fraction, since a grille's core (blades/louvers) blocks a portion of its face — standard grilles are commonly 60-75% free area, with the exact figure published by the grille manufacturer for each model.
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