Enter the air your return has to carry, or the grille you have. You get the grille size by a maker's own core-area chart at the face velocity the design rules allow, or the air your grille carries, with a filter grille held to its filter's limit too.
Your return grille
A 20 × 30″ filter grille for 800 cfm800 cfm is 297 fpm through it, under ACCA's 300 · at Hart & Cooley's 450 fpm, a 20 × 20″
Quick answer: a return grille is sized by the air it carries: its core area × the speed through it. ACCA’s maximum is 500 fpm for a plain return grille and 300 fpm for a filter grille (Hart & Cooley’s own: 600 and 450). 800 cfm needs a 20 × 30″ filter grille at ACCA’s 300 fpm, or a 20 × 20″ at Hart & Cooley’s 450; a plain return grille, a 16 × 24″. A filter grille’s filter also caps it at 2 cfm a square inch.
How to use the return air grille size calculator
- Pick what you want: the size for an airflow, or what a grille you have carries.
- Pick the grille: a plain return grille, or a filter grille that holds a filter behind its face.
- Enter the airflow in cfm from your HVAC contractor’s design, or the grille’s size: the opening it fits, printed on it.
You get the grille size (and the sizes with the same core), the air speed through it, and the bar shows that speed against ACCA’s and Hart & Cooley’s limits.
The rule, and where it comes from
The chart. TRUaire’s performance data for its stamped ½″ louver return grilles and return filter grilles (models 122R, 170, 190 and 190RF) gives each size’s core area and the air it carries from 300 to 700 fpm. A 20 × 20 has a 1.81 sq ft core: 543 cfm at 300 fpm, 905 at 500. “Velocity measured 1″ from face. Filter grilles are tested without filters.”
The speed. Hart & Cooley’s engineering data, Sizing a Return: “Returns should be at 400-600 fpm maximum”, “Filter Returns should be at 450 fpm maximum”, and “ACCA recommends 300 max for filter grilles and 500 max for non-filter grilles”. “Low velocity reduces noise, especially on stamped face grilles.” The calculator sizes at ACCA’s and shows Hart & Cooley’s.
The filter. Hart & Cooley: “The rule of thumb is 2 cfm per square inch of filter size”. A 20 × 25 filter is 500 sq in: 1,000 cfm. On its filter grilles it adds: “Velocities higher than 500 FPM will decrease filter performance, increase flow resistance”.
A size not on the chart is read between the listed sizes by its nominal area, the interpolation Hart & Cooley prints for an unlisted size.
What a return grille carries, by size
| Grille | Core | Filter grille at ACCA’s 300 fpm | Its filter, 2 cfm a sq in | Plain grille at ACCA’s 500 fpm |
|---|---|---|---|---|
| 14 × 14″ | 0.9 sq ft | 270 cfm | 392 cfm | 450 cfm |
| 16 × 25″ | 1.81 sq ft | 543 cfm | 800 cfm | 905 cfm |
| 20 × 20″ | 1.81 sq ft | 543 cfm | 800 cfm | 905 cfm |
| 24 × 24″ | 2.61 sq ft | 783 cfm | 1,152 cfm | 1,305 cfm |
| 20 × 25″ | 2.34 sq ft | 702 cfm | 1,000 cfm | 1,170 cfm |
| 20 × 30″ | 2.69 sq ft | 807 cfm | 1,200 cfm | 1,345 cfm |
| 25 × 25″ | 2.93 sq ft | 879 cfm | 1,250 cfm | 1,465 cfm |
Core areas from TRUaire’s chart; the air is the core × the speed. Other makers’ cores differ a little: use their chart the same way.
Worked examples
A 20 × 30″ filter grille for 800 cfm: 297 fpm through its 2.69 sq ft core, under ACCA’s 300. At Hart & Cooley’s 450, a 20 × 20″.
A 16 × 24″ return grille for 800 cfm: 444 fpm, under ACCA’s 500 for a grille without a filter.
A 20 × 25″ filter grille carries 702 cfm: at ACCA’s 300 fpm; its 20 × 25 filter would take 1,000.
No single filter grille carries 2,000 cfm at ACCA’s 300 fpm: a 36 × 30″ does at Hart & Cooley’s 450, or use two returns.
Common mistakes
- Sizing by the face. A grille is named by the opening it fits; the air goes through its core, about two-thirds of that.
- Reusing a plain grille’s size for a filter grille. ACCA allows 300 fpm through a filter grille against 500 through a plain one: the filter grille is bigger for the same air.
- Forgetting the filter. A filter grille’s filter caps it at 2 cfm a square inch, whatever the grille could pass.
- One small return for a big system. Fast air through a stamped grille is noisy; Hart & Cooley: “Low velocity reduces noise, especially on stamped face grilles”.
More at Dreamy Home Style
The filter for a filter grille: the furnace filter size finder gives its actual size. The supply side: the floor register size finder. To plan a room around its vents, draw it to scale in the free 2D floor planner.
Return air grille questions
What size return air grille do I need?
The air it carries ÷ the speed allowed = the core area, then the grille with that core. ACCA’s maximum is 500 fpm through a plain return grille and 300 through a filter grille: 800 cfm takes a 16 × 24″ plain or a 20 × 30″ filter grille.
How do I use a return air grille size calculator?
Enter the airflow in cfm and pick a plain or filter grille; or enter your grille’s size to see the air it carries.
What are standard return air grille sizes?
From 6 × 4″ to 36 × 40″ in TRUaire’s chart: common ones are 14 × 14, 16 × 25, 20 × 20, 20 × 25, 20 × 30, 24 × 24 and 25 × 25, the filter sizes.
How many cfm can a 20 × 20 return grille handle?
543 cfm as a filter grille at ACCA’s 300 fpm (815 at Hart & Cooley’s 450); 905 cfm as a plain grille at ACCA’s 500. Its 20 × 20 filter caps it at 800.
What are return air filter grille sizes?
A filter grille takes a filter of its own size, 14 × 14″ to 25 × 25″ and up. At ACCA’s 300 fpm a 20 × 25″ filter grille carries 702 cfm; its filter would take 1,000.
Should a return grille be bigger than the supply registers?
Hart & Cooley: the air going back “must equal what is supplied”, so the return grilles together must carry all the supply air, at the lower speed a return allows.