Return Grille CFM Chart

At a 400 fpm face velocity, a 14x14 return grille handles about 353 CFM, a 20x20 about 722, a 25x20 about 902, and a 30x24 about 1300. The chart below gives three face velocities per size, because face velocity is the choice that decides whether the return is quiet or the one thing the homeowner complains about.

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Return grille CFM by size

Assumptions, so you can check this against a manufacturer sheet:

Free area 65 percent of the nominal opening, typical for a stamped residential return grille
Sizes are nominal, width x height, the way they are ordered
CFM is face velocity times free area, rounded down
No filter behind the grille. A loaded filter is a separate pressure drop and it is not in these numbers

Return grille CFM chart at three face velocities, 65 percent free area.
Size W x H, in.	Free area sq. in.	300 fpm CFM	400 fpm CFM	500 fpm CFM, loud
10 x 6	39	81	108	135
12 x 6	47	97	130	162
14 x 6	55	113	151	189
12 x 12	94	195	260	325
14 x 14	127	265	353	442
16 x 16	166	346	462	577
20 x 12	156	325	433	541
20 x 14	182	379	505	631
20 x 20	260	541	722	902
24 x 12	187	390	520	650
24 x 18	281	585	780	975
24 x 24	374	780	1040	1300
25 x 20	325	677	902	1128
30 x 20	390	812	1083	1354
30 x 24	468	975	1300	1625
36 x 24	562	1170	1560	1950

Pick the column, not just the row. 300 fpm for a return in or beside a bedroom, 400 fpm as the everyday default, 500 fpm only where nobody sits.

Supply register CFM by size

Supply registers run at a higher face velocity than returns. The air is throwing into the room through vanes rather than being pulled across an open grille, and the register is usually further from where anyone sits. Free area is lower, though, because the vanes take up more of the opening: 60 percent is typical for a double deflection residential register.

Supply register CFM chart at three face velocities, 60 percent free area.
Size W x H, in.	Free area sq. in.	400 fpm CFM	500 fpm CFM	600 fpm CFM, loud
4 x 10	24	66	83	100
4 x 12	29	80	99	119
4 x 14	34	93	116	140
6 x 10	36	100	125	150
6 x 12	43	120	150	180
6 x 14	50	140	175	210
8 x 14	67	186	233	280
10 x 6	36	100	125	150
10 x 8	48	133	166	200
12 x 6	43	120	150	180
12 x 8	58	160	199	239
14 x 6	50	140	175	210

These are the common residential floor, wall and ceiling sizes. Throw and spread depend on the vane pattern and are not in this table.

How to use this on a job

Start from the room's airflow, not from the size of the hole that is already there. Divide the CFM by your face velocity to get the free area you need in square feet, then find the first row in the table that meets it. Working the other way, from an existing grille, read across to see what it is actually capable of before you blame the equipment.

Returns are where this goes wrong most often, and it goes wrong quietly. An undersized return does not throw a code violation, it just makes the blower work against a restriction that never shows up on a walkthrough. What the homeowner reports is noise, or a system that cannot keep up on the hottest afternoon, and neither of those points at the grille.

Sizing the duct behind it is a different calculation with a different limit. Returns are sized on velocity rather than friction, and that is the return duct sizing chart.

Where these numbers come from

One equation: CFM equals free area times face velocity. Free area is the nominal opening times the free area ratio, which is the fraction of the grille that is actually a hole rather than a vane.

The free area ratio is the only soft number on this page, and it is the reason manufacturer sheets disagree with each other. Two grilles with the same nominal size can differ by fifteen percent depending on how the blades are pressed. The values here are typical, the free area column shows what they produce, and if you have the real figure for the model in your hand, scale the CFM by the ratio of the two.

These charts are free and always will be. If you also need to turn the job into a signable quote and an invoice, that is the paid tool this site pays for itself with. See what it costs.

Return grille and register questions

How many CFM does a 20x20 return grille handle?

About 722 CFM at a 400 fpm face velocity, assuming a 65 percent free area, which is typical for a stamped residential return grille. Push it to 500 fpm and it passes more air, but a return that close to a living space starts to be audible.

What size return grille do I need for 1200 CFM?

A 30 x 24 at 400 fpm, or two smaller grilles splitting the air between them. Two returns is usually the better answer in a house, because one grille big enough for a whole three ton system has to go somewhere and it is rarely somewhere anyone wants it.

What face velocity should I use for a return grille?

400 fpm is the everyday residential design value and it is the middle column here. 300 fpm is quiet and generous, for a return in a bedroom or a hallway right outside one. 500 fpm is the practical ceiling; past it you are trading noise for a smaller grille and the homeowner hears the trade.

Is a return grille really 2 CFM per square inch?

Close. That rule of thumb works out to 1.8 CFM per square inch of nominal grille at 400 fpm with a 65 percent free area, so calling it 2 is a fair field estimate that runs slightly generous. The chart above does the same arithmetic without the rounding.

Why does the manufacturer sheet give a different CFM than this chart?

Free area. Every number here assumes 65 percent free area for return grilles and 60 percent for supply registers, which are typical values, not your specific model. A dense bar grille or a filter frame can be well under that. The free area column is printed so you can substitute the real figure from the manufacturer sheet and scale the CFM directly.