Preliminary numbers for estimating and for checking a drawing. The grille data are one maker's stamped face return grille. The equipment manufacturer, the grille maker's catalog and the local authority having jurisdiction come first.

Maintained by BuildSolver · Updated October 20, 2026

Calculator · HVAC ducts

Return air grille size calculator

A return grille is sized on its net area, not its nominal size. 1,000 CFM at 400 FPM needs 2.5 sq ft, which reads like a 20 x 18. It is not enough. In Hart & Cooley's table a 20 x 20 carries 720 CFM at 400 FPM and a 30 x 18 968, so neither passes. A 24 x 24 carries 1,032 CFM and a 30 x 20 1,074 CFM. Enter the CFM and this page does the division, reads the table and checks the face velocity against the Hart & Cooley and ACCA limits. For the return duct behind the grille, use the duct sizing calculator.

Choose to size a return grille for an airflow, to check a listed Hart & Cooley size, or to check a grille of another make from its net area.
For sizing, enter the total return CFM and the number of grilles, and leave the face velocity blank for the default. For a check, enter the airflow through the one grille and its size.
The result shows the net area needed, the smallest listed sizes that carry the airflow with their pressure loss, and the face velocity against the H&C and ACCA limits.

Hart & Cooley 672, 674 and 673 return grilles, standard sizes only. Other makes go through the check from net area.

What to work out

Size finds the grille for a CFM. The two checks take a grille you have and give its face velocity.

Grille type

A filter grille holds a filter behind the face, so its limits are lower. A plain return grille is the usual choice on a wall, a ceiling or a floor.

Return airflow, CFM

To size: the total return airflow, which H&C says has to equal or exceed the supply. To check: the airflow through this one grille.

Number of return grilles

The airflow is shared equally between them. With several returns the real shares depend on the duct and the balancing.

Face velocity to size to, FPM (optional)

Leave blank for 400 FPM on a non-filter grille or 300 FPM on a filter grille. ACCA's maximums, as H&C quotes them, are 500 and 300.

Example result for the default inputs

1,000 CFM over 1 non-filter grille at 400 FPM (this page's default for a non-filter grille).

Airflow per grille

1,000 CFM

Net area needed, Ak

2.500 sq ft

Smallest listed size that passes

24 x 24

Calculation details
Airflow per grille	All 1,000 CFM goes through the one grille.
Net area needed, Ak	1,000 CFM / 400 FPM = 2.500 sq ft, or at least 360 sq in. of net area per grille. H&C's equation is CFM = Ak x face velocity.
Why nominal size is not enough	Dividing as above and reading 360 sq in. as the grille size comes up short. H&C's Ak is 64 to 68 percent of a grille's nominal area, so look for a nominal size of about 526 to 563 sq in.
Smallest size that passes	24 x 24, Ak 2.58 sq ft: 1,032 CFM at 400 FPM, Ps 0.032 in. w.c.
Next size up	30 x 20, Ak 2.69 sq ft: 1,074 CFM at 400 FPM, Ps 0.031 in. w.c.
Next size up	30 x 24, Ak 3.21 sq ft: 1,286 CFM at 400 FPM, Ps 0.031 in. w.c.
Largest size that falls short	30 x 18, Ak 2.42 sq ft: 968 CFM at 400 FPM, 32 CFM under the 1,000 CFM needed.
Face velocity against the limits	400 FPM is within the 500 FPM ACCA maximum for a non-filter grille. H&C's maximum is 600 FPM.

For estimation purposes only. Consult a licensed engineer for final designs.

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How the grille size is worked

A return grille is sized on its net area, the free area the air actually uses, and on the speed the air has at its face. Hart & Cooley publish the net area, Ak, of each size of their return grilles with the CFM and pressure loss measured at four face velocities. The simple division of CFM by velocity gives the net area you need. The table tells you which size has it.

Airflow per grille: CFM per grille = total return CFM / number of grillesHart & Cooley says the total CFM capacity of the return grilles must equal or exceed the total supply. 1,200 CFM over two grilles is 600 CFM each.
Face velocity: V = CFM / Ak (FPM, Ak in sq ft)H&C's Equation of Airflow, CFM = Ak x face velocity. 1,000 CFM through a 30 x 20 (Ak 2.69 sq ft) is 372 FPM, and through a 20 x 20 (Ak 1.80) 555 FPM, read against the table's CFM columns. The plain division gives 556 for the 20 x 20; the table's rounding accounts for the difference.
Net area needed: Ak = CFM per grille / face velocity (sq ft, times 144 for sq in.)1,000 CFM at 400 FPM needs 2.5 sq ft, which is 360 sq in. of net area.
Size from the table: the smallest Ak whose table CFM at that velocity is at least the CFM per grilleAt 400 FPM a 20 x 20 carries 720 CFM and a 30 x 18 968. A 24 x 24 carries 1,032 CFM and a 30 x 20 1,074 CFM. Between the table's columns the CFM is read in a straight line.
Pressure loss: Ps from the table; between columns Ps = Ps1 x (V / V1)^n, outside 300 to 600 FPM Ps = Ps0 x (V / V0)^2A 20 x 20 reads 0.033 in. w.c. at 400 FPM. Between adjacent columns Ps rises as velocity to a power from 1.46 (the 30 x 30, 500 to 600 FPM) to 2.12 (the 6 x 6, 300 to 400 FPM). Outside the tested range the square is used, which overstates the loss of the larger sizes above 600 FPM, so it is an estimate kept in reserve.
Filter size: a filter grille's filter is its nominal size; nominal sq in. >= CFM per grille / 2H&C's rule of thumb is 2 CFM per sq in. of filter. 1,000 CFM needs 500 sq in., and the 30 x 30 grille picked for 300 FPM holds a filter of 900 sq in., so the face velocity decides. H&C's filter size table gives a 20 x 25 for 1,000 CFM, which is about 421 to 450 FPM across the net area and softer than the 300 FPM ACCA maximum. This page takes the stricter one.

1,000 CFM through one return grille at 400 FPM

The airflow each size carries at 400 FPM, from the Hart & Cooley table, against the 1,000 CFM needed. The net area needed is 2.5 sq ft.

Return grille sizes against 1,000 CFM at 400 FPM
Size	Nominal, sq in.	Ak, sq ft	CFM at 400 FPM	Against 1,000 CFM
20 x 20 in.	400	1.80	720	280 short
20 x 25 in.	500	2.24	898	102 short
30 x 18 in.	540	2.42	968	32 short
24 x 24 in.	576	2.58	1,032	passes, 32 spare
30 x 20 in.	600	2.69	1,074	passes, 74 spare
30 x 24 in.	720	3.21	1,286	passes, 286 spare

Green Building Advisor works this example with the Manual T division, which gives a 20 x 18. That size is not in the table, and the 20 x 20 that is does not make the grade. He goes up the table: the 30 x 18 does not quite get there, so he takes the next size, the 30 x 20. The 24 x 24 further down the same table also passes, at 1,032 CFM, with 2.58 sq ft of net area against 2.69. Both work. Which one fits is a question of the wall or ceiling opening. Pressure loss at 400 FPM is 0.032 in. w.c. for the 24 x 24 and 0.031 for the 30 x 20.

The airflow of common sizes at the four velocities Hart & Cooley tested, and the pressure loss at 400 FPM.

Return grille airflow in CFM at 300, 400, 500 and 600 FPM for common sizes
Size	Ak, sq ft	300 FPM	400 FPM	500 FPM	600 FPM	Ps at 400, in. w.c.
12 x 12 in.	0.66	197	263	329	395	0.034
14 x 14 in.	0.89	267	357	446	535	0.034
16 x 16 in.	1.16	348	464	580	696	0.033
20 x 20 in.	1.80	540	720	900	1,081	0.033
24 x 24 in.	2.58	774	1,032	1,290	1,548	0.032
30 x 20 in.	2.69	806	1,074	1,343	1,611	0.031
30 x 30 in.	4.00	1,201	1,602	2,002	2,403	0.030

Hart & Cooley puts returns at 400 to 600 FPM at most and filter returns at 450 FPM at most, and quotes ACCA at 500 FPM for a non-filter grille and 300 FPM for a filter grille.

Standards and data sources

Hart & Cooley Engineering Data, 672/674 Return Air Grille and 673 Return Air Filter Grille - the 56 sizes from 6 x 4 to 36 x 8 with Ak, and CFM and pressure loss Ps at face velocities of 300, 400, 500 and 600 FPM. The filter grille footnote: tested without filters, 2 CFM per sq in. of nominal filter area, recommended face velocity 300 to 450 FPM.
Hart & Cooley, Using the Engineering Data - the definition of Ak, the equation CFM = Ak x face velocity, the return limits (400 to 600 FPM, filter returns 450 FPM), the ACCA figures it quotes (500 FPM non-filter, 300 FPM filter), the rule of 2 CFM per sq in. and the filter size table.
Green Building Advisor, Designing Duct System Vents for Good Airflow (Allison Bailes, 2017) - the Manual T way of sizing a non-filter return, CFM divided by a face velocity of 400 FPM, and the worked 1,000 CFM example checked against the H&C table. The same article gives the net free area of metal (70 to 90 percent) and wooden (50 percent or less) grilles.
ACCA Manual T, Air Distribution Basics - the standard for choosing and placing supply outlets and return inlets. The standard itself is sold by ACCA. This page uses the Manual T figures only as H&C and Green Building Advisor report them.

Limitations of this calculator

One family of grilles. The table is Hart & Cooley's stamped face 672/674 return grille and 673 filter grille. Another make, a bar grille, a louvered or a wooden grille has its own Ak. Use the check from net area for those, with the figure from that maker's catalog.
Ak is not the free area and not the nominal size. In the table Ak is 64 to 68 percent of the nominal size. The Ak of one catalog edition can differ from another, so use the edition of the grille you are buying.
The table's CFM is rounded. It differs from Ak x velocity by up to 3 CFM, and this page judges a size on the table's own CFM.
H&C tested the grilles at 300 to 600 FPM, with the velocity measured 1 in. from the face. Between those columns the CFM is read in a straight line and the Ps from a power fitted to the neighbours. Outside them the CFM follows Ak x velocity and the Ps is scaled by the square of the velocity, an estimate that errs on the high side above 600 FPM, and the result says so.
Ps is for a grille without a filter. A filter grille was tested without filters, and a filter in it adds its own resistance, which climbs as it loads.
Noise (NC) is not worked. H&C notes that low velocity reduces noise, most of all on stamped face grilles, but it gives no NC figure for these return grilles.
The airflow is shared equally between the grilles you enter. The real share of each return depends on the duct behind it and on balancing.
A transfer grille, a jumper duct and a door undercut are not covered. They move air between rooms and are sized on a pressure difference.
The return duct behind the grille is a separate job. The duct sizing calculator sizes the duct and the duct velocity and friction calculator checks one round run.
Airflow from 25 to 10,000 CFM, up to 12 grilles, and a velocity to size to from 100 to 1,000 FPM. A check refuses only a face velocity above 1,000 FPM; a large grille on a small airflow is a valid, low-velocity result. That is this page's working range, not a standard.

Common mistakes when sizing a return grille

Using the nominal size as the open area. 1,000 CFM at 400 FPM is 2.5 sq ft, or 360 sq in., which reads like a 20 x 18. In the H&C table a 20 x 20 has 1.80 sq ft of net area and carries 720 CFM at 400 FPM, and a 30 x 18 carries 968. The size that carries 1,000 CFM is larger than the sum suggests.
Stopping at the first size that passes. A 30 x 20 carries 1,074 CFM at 400 FPM, and it is the size the Green Building Advisor example picks once the 30 x 18 falls short. The 24 x 24 further down the same table carries 1,032 CFM with less net area, 2.58 sq ft against 2.69. Run the whole table, and then pick for the wall or ceiling opening you have.
Putting the whole system on one small grille. 1,000 CFM through a 20 x 20 is 555 FPM, above the 500 FPM ACCA maximum for a non-filter grille. Through a 16 x 16 it is 862 FPM, above H&C's 600 FPM maximum. A return that is too small is noisy and holds back the airflow.
Taking the filter grille from the filter size table. H&C's filter size table gives a 20 x 25 for 1,000 CFM and a 20 x 20 for 800 CFM. That is 2 CFM per sq in. of filter, about 421 to 450 FPM across the net area of the grille, over the 300 FPM ACCA maximum. At 300 FPM the grilles are a 30 x 30 and a 30 x 20, and each holds a filter of its own size.
Swapping a metal grille for a wooden one. Green Building Advisor puts the net free area of a metal grille at 70 to 90 percent and a wooden grille at 50 percent or less. A decorative grille of the same nominal size has less open area, so the face velocity through its open area climbs. Take its Ak from the maker.
Leaving the filter out of the pressure loss. The Ps in the H&C table is for the grille tested without a filter. A filter grille with a filter in it, and a loaded filter more so, has a higher loss, and the blower sees it as return static pressure.

Common questions about return grille size

How do you size a return air grille?

Divide the airflow by the face velocity you want. That gives the net area. 1,000 CFM at 400 FPM is 2.5 sq ft. Then find a size whose net area, Ak, in the maker's table carries the airflow at that velocity. In the Hart & Cooley table a 24 x 24 carries 1,032 CFM at 400 FPM and a 30 x 20 carries 1,074 CFM.

What face velocity should a return grille be sized to?

Hart & Cooley says returns should be at 400 to 600 FPM at most and filter returns at 450 FPM at most. It quotes ACCA at 500 FPM at most for a non-filter grille and 300 FPM at most for a filter grille. Green Building Advisor sizes a non-filter grille at a maximum of 400 FPM. This page sizes to 400 FPM for a non-filter grille and 300 FPM for a filter grille unless you type another figure.

How many CFM can a 20x20 return grille handle?

In the Hart & Cooley 672 table a 20 x 20 return grille has 1.80 sq ft of net area. It carries 540 CFM at 300 FPM, 720 at 400, 900 at 500 and 1,081 at 600. That is only 720 CFM at 400 FPM, less than the 1,000 CFM that the nominal 400 sq in. might suggest. The frame and fins take the rest.

What size return do I need for a 3 ton system?

H&C's filter size table works at 400 CFM per ton, so 3 tons is 1,200 CFM. Sized for 400 FPM, that needs 3.0 sq ft of net area, and the smallest size in the H&C table that passes is a 30 x 24 at 1,286 CFM. Two grilles of 600 CFM each can be a 24 x 14. The airflow your system really moves is on the equipment data, and it can be more or less than 400 CFM per ton.

Why is a return grille bigger than its airflow suggests?

Air passes only through the open part. The size on the grille is the nominal size, and the air only uses the net area. In the H&C table Ak is 64 to 68 percent of the nominal area. Green Building Advisor adds that metal grilles have 70 to 90 percent net free area and wooden ones 50 percent or less.

How big should a filter return grille be?

A filter grille holds a filter of its own size, so one size has to pass both ways. By H&C's rule of thumb the filter passes 2 CFM per sq in., so 1,000 CFM needs 500 sq in. of filter. The face velocity should be 300 FPM at most by ACCA, which for 1,000 CFM takes a 30 x 30 grille with a 30 x 30 filter, or two grilles of 500 CFM each, a 16 x 24. H&C's filter size table is softer: it gives a 20 x 25 for 1,000 CFM and a 20 x 20 for 800 CFM, about 421 to 450 FPM across the net area. This page takes the stricter 300 FPM, so for 800 CFM it shows a 30 x 20.

Can I use two smaller return grilles instead of one large one?

Yes. The total capacity of the return grilles has to equal or exceed the supply, and this page shares the airflow equally between them. H&C says you cannot really oversize a single point return, and that with several return locations balancing matters more, so each one pulls its own share of air from the rooms it serves.

What does Ak mean in a grille table?

Ak is the net area in square feet. Hart & Cooley measure it in the lab as the area across the face when air is forced through the opening. The airflow is Ak times the face velocity, so a larger Ak carries more air at the same velocity. It is not the nominal size. Nor is it the open slot area.

How is this different from the duct sizing calculator?

This page sizes the grille at the end of the return, from the CFM and the face velocity. The duct sizing calculator sizes the ducts that feed it, and the duct velocity and friction calculator works out the friction and velocity of one round duct.