Size supply registers and return and exhaust grilles from design airflow and face velocity.
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Application & Flow
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Sizing Results
Required Neck Area
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ft²
Required Area
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in²
Recommended Size
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inches (W×H)
Actual Neck Velocity
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FPM
Standard Sizes — CFM Capacity at Various Velocities
Size (W×H in)
Core Area (ft²)
CFM @ 250 FPM
CFM @ 400 FPM
CFM @ 500 FPM
CFM @ 600 FPM
CFM @ 750 FPM
Velocity at Your CFM
Grilles and registers are catalogued by nominal (core) size, and the velocity here is the core velocity, CFM ÷ (W × H). Free area is smaller — roughly 65–75% of the core for a bar grille, about 90% for an egg-crate — so the air leaves the openings faster than the core velocity. Ceiling diffusers are sized on neck velocity; check throw with the catalog Ak. Confirm noise (NC) and throw against the manufacturer’s performance table.
Inputs, results, and method — as an Excel workbook or plain CSV.
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This grille and register sizing calculator turns a design airflow and a target face velocity into a recommended grille size for supply ceiling diffusers, sidewall registers, return grilles, and exhaust or transfer grilles. Enter the CFM and pick the application; the tool shows a typical velocity range for that application, computes the required core (or neck) area, and picks the smallest standard size that covers it, with a table of the CFM each size carries at common velocities.
Face velocity is the lever that balances comfort, throw, and noise. Sizing too small drives velocity up and the grille becomes audible; sizing too large wastes wall or ceiling space and may short the throw. Working from a target velocity keeps every outlet inside a sensible acoustic band while still delivering the required air.
Formula & Method
Required core area
A (ft²) = Q (CFM) ÷ Vtarget (FPM)
In square inches
A (in²) = A (ft²) × 144
Size selection
smallest standard W×H (or square neck) with W·H ÷ 144 ≥ A
Actual velocity
Vactual = Q (CFM) ÷ (W·H ÷ 144)
The core relation is simply airflow divided by velocity, with the 144 factor converting square feet to square inches. Grilles and registers are catalogued and selected by nominal core size, so the velocity is the core velocity, CFM ÷ core area; the free area of a bar grille is only about 65–75% of the core (an egg-crate about 90%), so the jet leaving the openings is faster. Ceiling diffusers are sized the same way on their neck. When the airflow needs more than the largest listed size, the tool splits it across several grilles. The velocity ranges are rules of thumb for occupied spaces, not code values; confirm noise (NC) and throw against the manufacturer’s performance data, using the published Ak factor for ceiling diffusers.
Frequently Asked Questions
How is grille size calculated from CFM and face velocity?
The required core area equals the design airflow divided by the target velocity: area in square feet equals CFM divided by FPM, and times 144 gives square inches. Grilles and registers are catalogued by their nominal core size (width times height), so the tool picks the smallest standard size whose core area covers the requirement and reports the actual core velocity at that size. For a ceiling diffuser the same calculation uses the neck area and neck velocity.
What is free area and why isn’t it used for sizing?
Free area is the open part of the grille the air actually passes through after the bars, blades, and frame: roughly 65 to 75 percent of the core for a bar grille and about 90 percent for an egg-crate. Manufacturers’ selection tables and the usual velocity guidelines are based on the nominal core area, so the tool sizes on core area; the air leaves the free area faster than the core velocity. For ceiling diffusers, use the manufacturer’s Ak (effective area) factor to check throw.
What face velocity should I target?
As a rule of thumb, about 400 to 600 FPM neck velocity for ceiling diffusers, 500 to 750 FPM core velocity for sidewall supply registers, 400 to 500 FPM for return and exhaust grilles in occupied spaces, and 200 to 300 FPM for transfer and door grilles, which must stay quiet because they connect two rooms. Lower velocities are quieter; supply registers need enough velocity for throw. The tool flags a target outside the typical range for the selected application; the final check is the manufacturer’s NC rating at your airflow.
Why is a return grille usually larger than the supply diffuser for the same CFM?
Return and exhaust grilles are sized for lower velocities than supply outlets to keep them quiet, since they have no throw requirement to satisfy. A lower target velocity for the same airflow needs more core area, so the return grille comes out physically bigger than the supply diffuser handling the same CFM, and a transfer grille bigger still.
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Duct and pipe distribution sizing is planned for V2 — at launch, keep using this calculator for that step.
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.