Diffuser & Grille Sizing Chart
How to size supply diffusers, registers, and return grilles by face velocity and airflow. The core relationship is CFM = free area × face velocity — pick the velocity range for the application, and the size follows. Supply outlets run faster for throw; returns run slower to stay quiet. Values are manufacturer-typical, not code.
Face velocity ranges
| Application | Face velocity | Note |
|---|---|---|
| Residential supply register | 300–500 fpm | 250–275 fpm for very quiet rooms (bedrooms, theaters) |
| Commercial supply diffuser | 400–800 fpm | Up to ~1000 fpm for high throw; higher = more noise |
| Return grille (open) | 300–500 fpm | Size as large as possible to lower static and noise |
| Return filter grille | ≤ 400 fpm | Lower to protect the filter and keep noise down |
Typical CFM by round size
| Round size | Metal (CFM) | Flex (CFM) |
|---|---|---|
| 5" | 50 | 50 |
| 6" | 85 | 75 |
| 7" | 125 | 110 |
| 8" | 180 | 160 |
| 10" | 325 | 300 |
| 12" | 525 | 480 |
| 14" | 750 | 700 |
Velocity is the design lever
A diffuser has one job the sizing has to balance: deliver the air far enough to mix the room without being loud or drafty. That trade-off is face velocity. Push it up and you get more throw but more noise; drop it and the room goes quiet but the air may dump short. So sizing works from the velocity band for the application — CFM = free area (ft²) × face velocity (fpm) — rather than from a fixed CFM-per-size table, which is why the capacity numbers here are labeled velocity-dependent typicals. Manufacturer catalogs give the exact effective area (Ak) and throw for each model.
Returns follow the same physics in reverse and are the more common mistake. An undersized return starves the system and roars; the ~1 square foot per 350 CFM rule keeps the return velocity low enough to stay quiet. Get the airflow to size to from the CFM calculator.
Common questions
How do you size a supply diffuser?
Size it so its face velocity lands in the right range for the space: CFM = free area × face velocity. Residential supply registers run 300–500 fpm, commercial diffusers 400–800 fpm, and very quiet spaces (bedrooms, theaters) as low as 250–275. Divide the required CFM by the target velocity to get the free area, then pick a register whose effective (Ak) area matches.
How big should a return-air grille be?
A common rule of thumb is about 1 square foot of grille free area per 350 CFM — roughly 1 to 2 square feet of grille per ton. Returns are sized more generously than supplies (lower face velocity, 300–500 fpm) because an undersized return is a leading cause of noise and high static pressure. Filter grilles go lower still, around 300 fpm, to protect the filter.
Why is flex duct rated for less CFM than metal at the same size?
Flex duct has more internal friction than smooth metal, so at the same diameter it carries less air for the same pressure — typically you size flex one to two inches larger than metal for equal CFM. That is why the flex column runs below the metal column at every size.
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