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Rectangular Duct CFM Chart

How many CFM will a given rectangular duct carry? The chart gives the airflow capacity of 115 common duct sizes at the two friction rates used for residential and light-commercial design (0.08 and 0.10 in. wg per 100 ft), plus each size's equivalent round diameter and the velocity at 0.08. Sizes that link through are published, priced sizes you can order today.

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Duct size (W x D)	Equivalent round	CFM @ 0.08	CFM @ 0.10	FPM @ 0.08
4x4 in	4.4″	40	45	380
5x4 in	4.9″	55	65	400
6x4 in	5.3″	70	80	420
8x4 in	6.1″	100	115	450
10x4 in	6.7″	130	150	470
12x4 in	7.3″	165	185	490
14x4 in	7.8″	195	220	500
16x4 in	8.3″	225	255	510
18x4 in	8.7″	255	290	510
20x4 in	9.1″	290	325	520
22x4 in	9.5″	320	360	530
24x4 in	9.8″	355	395	530
26x4 in	10.1″	385	435	530
28x4 in	10.4″	415	470	540
30x4 in	10.7″	450	505	540
32x4 in	11.0″	480	540	540
36x4 in	11.5″	545	610	540
6x6 in	6.6″	120	135	490
8x6 in	7.6″	180	200	530
10x6 in	8.4″	235	265	560
12x6 in	9.1″	295	330	590
14x6 in	9.8″	355	395	610
16x6 in	10.4″	415	465	620
18x6 in	11.0″	475	535	630
20x6 in	11.5″	535	600	640
22x6 in	12.0″	595	670	650
24x6 in	12.4″	660	740	660
26x6 in	12.8″	720	810	660
28x6 in	13.2″	780	880	670
30x6 in	13.6″	845	950	680
32x6 in	14.0″	905	1,020	680
36x6 in	14.7″	1,030	1,160	690
8x8 in	8.7″	260	295	590
10x8 in	9.8″	350	395	630
12x8 in	10.7″	440	495	660
14x8 in	11.5″	535	600	690
16x8 in	12.2″	630	705	710
18x8 in	12.9″	725	815	720
20x8 in	13.5″	820	920	740
22x8 in	14.1″	915	1,030	750
24x8 in	14.6″	1,015	1,140	760
26x8 in	15.1″	1,115	1,250	770
28x8 in	15.6″	1,210	1,360	780
30x8 in	16.1″	1,310	1,475	790
32x8 in	16.5″	1,410	1,585	790
36x8 in	17.4″	1,605	1,805	800
10x10 in	10.9″	470	530	680
12x10 in	12.0″	600	675	720
14x10 in	12.9″	730	820	750
16x10 in	13.7″	860	970	780
18x10 in	14.5″	995	1,120	800
20x10 in	15.2″	1,135	1,275	820
22x10 in	15.9″	1,270	1,430	830
24x10 in	16.5″	1,410	1,585	850
26x10 in	17.1″	1,550	1,740	860
28x10 in	17.7″	1,690	1,900	870
30x10 in	18.3″	1,830	2,060	880
32x10 in	18.8″	1,970	2,215	890
36x10 in	19.8″	2,255	2,535	900
12x12 in	13.1″	765	860	760
14x12 in	14.2″	935	1,050	800
16x12 in	15.1″	1,110	1,245	830
18x12 in	16.0″	1,285	1,445	860
20x12 in	16.8″	1,465	1,650	880
22x12 in	17.6″	1,650	1,855	900
24x12 in	18.3″	1,835	2,060	920
26x12 in	19.0″	2,020	2,270	930
28x12 in	19.6″	2,205	2,480	950
30x12 in	20.2″	2,395	2,690	960
32x12 in	20.8″	2,585	2,905	970
36x12 in	21.9″	2,960	3,330	990
14x14 in	15.3″	1,145	1,290	840
16x14 in	16.4″	1,365	1,535	880
18x14 in	17.3″	1,590	1,790	910
20x14 in	18.2″	1,820	2,045	930
22x14 in	19.1″	2,050	2,305	960
24x14 in	19.9″	2,280	2,565	980
26x14 in	20.6″	2,520	2,830	1,000
28x14 in	21.3″	2,755	3,100	1,010
30x14 in	22.0″	2,995	3,370	1,030
32x14 in	22.7″	3,235	3,640	1,040
36x14 in	23.9″	3,720	4,180	1,060
16x16 in	17.5″	1,630	1,835	920
18x16 in	18.5″	1,905	2,140	950
20x16 in	19.5″	2,180	2,455	980
22x16 in	20.4″	2,465	2,770	1,010
24x16 in	21.3″	2,750	3,095	1,030
26x16 in	22.1″	3,040	3,420	1,050
28x16 in	22.9″	3,330	3,745	1,070
30x16 in	23.7″	3,625	4,075	1,090
32x16 in	24.4″	3,920	4,410	1,100
36x16 in	25.7″	4,520	5,080	1,130
18x18 in	19.7″	2,225	2,505	990
20x18 in	20.7″	2,560	2,875	1,020
22x18 in	21.7″	2,895	3,255	1,050
24x18 in	22.7″	3,235	3,640	1,080
26x18 in	23.5″	3,580	4,025	1,100
28x18 in	24.4″	3,930	4,420	1,120
30x18 in	25.2″	4,280	4,815	1,140
32x18 in	26.0″	4,635	5,215	1,160
36x18 in	27.4″	5,355	6,020	1,190
20x20 in	21.9″	2,940	3,310	1,060
22x20 in	22.9″	3,335	3,750	1,090
24x20 in	23.9″	3,735	4,200	1,120
26x20 in	24.9″	4,140	4,655	1,150
28x20 in	25.8″	4,545	5,115	1,170
30x20 in	26.6″	4,960	5,580	1,190
32x20 in	27.5″	5,375	6,045	1,210
36x20 in	29.0″	6,220	6,995	1,240
24x24 in	26.2″	4,765	5,355	1,190
26x24 in	27.3″	5,290	5,950	1,220
28x24 in	28.3″	5,825	6,555	1,250
30x24 in	29.3″	6,370	7,165	1,270
32x24 in	30.2″	6,920	7,780	1,300
36x24 in	32.0″	8,030	9,030	1,340

CFM rounded to the nearest 5, velocity to the nearest 10 FPM. Equal-friction method, galvanized sheet metal, straight run. A size works in either orientation (a 20x8 carries the same air as an 8x20). Fittings, flex and long runs reduce real-world capacity; confirm with your Manual D calculation.

Common questions

How many CFM does a 20x8 duct carry?

A 20x8 inch rectangular duct carries about 820 CFM at a 0.08 in. wg per 100 ft friction rate and about 920 CFM at 0.10, which is roughly 740 feet per minute at 0.08. Use the lower figure for return or quiet supply runs.

How many CFM does a 12x8 duct carry?

A 12x8 inch rectangular duct carries about 440 CFM at a 0.08 in. wg per 100 ft friction rate and about 495 CFM at 0.10, which is roughly 660 feet per minute at 0.08. Use the lower figure for return or quiet supply runs.

How many CFM does a 14x8 duct carry?

A 14x8 inch rectangular duct carries about 535 CFM at a 0.08 in. wg per 100 ft friction rate and about 600 CFM at 0.10, which is roughly 690 feet per minute at 0.08. Use the lower figure for return or quiet supply runs.

How many CFM does a 10x8 duct carry?

A 10x8 inch rectangular duct carries about 350 CFM at a 0.08 in. wg per 100 ft friction rate and about 395 CFM at 0.10, which is roughly 630 feet per minute at 0.08. Use the lower figure for return or quiet supply runs.

How many CFM does a 24x12 duct carry?

A 24x12 inch rectangular duct carries about 1,835 CFM at a 0.08 in. wg per 100 ft friction rate and about 2,060 CFM at 0.10, which is roughly 920 feet per minute at 0.08. Use the lower figure for return or quiet supply runs.

How is duct CFM capacity calculated?

Each size is converted to an equivalent round diameter, De = 1.30 (a x b)^0.625 / (a + b)^0.25, then the ASHRAE friction equation D = (0.109136 x CFM^1.9 / friction)^(1/5.02) is solved for CFM at the chosen friction rate. Always confirm against your Manual D load calculation.