Duct size chart — CFM to duct size

A printable duct size chart running from 100 to 10,000 CFM at the friction rates most low-velocity systems are designed to. Each row gives the required round diameter, the nearest standard size, two rectangular equivalents and the velocity that results — so a size can be picked straight off the page.

Basis of these tables. Standard air at sea level and 20 °C, density 1.2043 kg/m³, viscosity 18.134×10⁻⁶ Pa·s, galvanised spiral steel at ε = 0.09 mm. Friction is solved with Darcy-Weisbach and the Altshul-Tsal friction factor per ASHRAE Fundamentals Chapter 21 — these are computed values, not a scan of a printed chart. Every figure is generated by the same code that drives the ductulator, so the chart and the calculator cannot disagree.

Imperial — CFM to round duct size

Round duct size and velocity by airflow in CFM at three friction rates
Airflow	0.06 in.wg/100 ft		0.08 in.wg/100 ft		0.10 in.wg/100 ft
Airflow	Size	fpm	Size	fpm	Size	fpm
100 CFM	7 in	374	7 in	374	6 in	509
150 CFM	8 in	430	8 in	430	7 in	561
200 CFM	9 in	453	8 in	573	8 in	573
250 CFM	10 in	458	9 in	566	9 in	566
300 CFM	10 in	550	10 in	550	9 in	679
400 CFM	11 in	606	11 in	606	10 in	733
500 CFM	12 in	637	12 in	637	11 in	758
600 CFM	14 in	561	12 in	764	12 in	764
800 CFM	14 in	748	14 in	748	14 in	748
1,000 CFM	16 in	716	16 in	716	14 in	935
1,250 CFM	18 in	707	16 in	895	16 in	895
1,500 CFM	18 in	849	18 in	849	16 in	1,074
2,000 CFM	20 in	917	20 in	917	18 in	1,132
2,500 CFM	22 in	947	22 in	947	20 in	1,146
3,000 CFM	24 in	955	22 in	1,136	22 in	1,136
4,000 CFM	26 in	1,085	26 in	1,085	24 in	1,273
5,000 CFM	28 in	1,169	28 in	1,169	26 in	1,356
6,000 CFM	30 in	1,222	30 in	1,222	28 in	1,403
8,000 CFM	34 in	1,269	32 in	1,432	30 in	1,630
10,000 CFM	38 in	1,270	36 in	1,415	34 in	1,586

Size is the next standard spiral diameter at or above the calculated requirement, so the velocity shown is always a little below the target — that is rounding up, and it is the safe direction to round.

Metric — L/s to round duct size

Round duct size and velocity by airflow in litres per second at three friction rates
Airflow	0.6 Pa/m		0.8 Pa/m		1.0 Pa/m
Airflow	Size	m/s	Size	m/s	Size	m/s
50 L/s	200 mm	1.59	160 mm	2.49	160 mm	2.49
75 L/s	200 mm	2.39	200 mm	2.39	200 mm	2.39
100 L/s	250 mm	2.04	200 mm	3.18	200 mm	3.18
150 L/s	250 mm	3.06	250 mm	3.06	250 mm	3.06
200 L/s	315 mm	2.57	315 mm	2.57	250 mm	4.07
250 L/s	315 mm	3.21	315 mm	3.21	315 mm	3.21
300 L/s	315 mm	3.85	315 mm	3.85	315 mm	3.85
400 L/s	355 mm	4.04	355 mm	4.04	315 mm	5.13
500 L/s	400 mm	3.98	400 mm	3.98	355 mm	5.05
650 L/s	450 mm	4.09	400 mm	5.17	400 mm	5.17
800 L/s	500 mm	4.07	450 mm	5.03	450 mm	5.03
1,000 L/s	500 mm	5.09	500 mm	5.09	450 mm	6.29
1,250 L/s	560 mm	5.08	560 mm	5.08	500 mm	6.37
1,600 L/s	630 mm	5.13	560 mm	6.50	560 mm	6.50
2,000 L/s	710 mm	5.05	630 mm	6.42	630 mm	6.42
2,500 L/s	710 mm	6.31	710 mm	6.31	630 mm	8.02
3,200 L/s	800 mm	6.37	800 mm	6.37	710 mm	8.08
4,000 L/s	900 mm	6.29	800 mm	7.96	800 mm	7.96
5,000 L/s	900 mm	7.86	900 mm	7.86	900 mm	7.86

Metric sizes follow EN 1506. Note that they do not interleave with the imperial list — 400 mm sits between 15 and 16 inches, so a duct converted from one system to the other will usually not land on a size that is actually made.

Rectangular equivalents at common void depths

Where the ceiling void will not take a round duct, these are the rectangular sizes that carry the same air at the same friction rate. Widths are on the standard 50 mm construction increment, and the aspect ratio is shown because a size that matches on friction but sits at 6:1 is a size that costs money.

Rectangular duct equivalents at 200, 300 and 400 mm depth
Airflow	Round	200 mm deep	300 mm deep	400 mm deep
100 L/s	200 mm ø	200 × 200 (1.0:1)	—	—
200 L/s	315 mm ø	450 × 200 (2.3:1)	300 × 300 (1.0:1)	—
300 L/s	315 mm ø	450 × 200 (2.3:1)	300 × 300 (1.0:1)	—
500 L/s	400 mm ø	750 × 200 (3.8:1)	500 × 300 (1.7:1)	—
800 L/s	450 mm ø	1,000 × 200 (5.0:1)	600 × 300 (2.0:1)	450 × 400 (1.1:1)
1,200 L/s	500 mm ø	1,300 × 200 (6.5:1)	750 × 300 (2.5:1)	550 × 400 (1.4:1)
1,800 L/s	630 mm ø	2,250 × 200 (11.3:1)	1,250 × 300 (4.2:1)	900 × 400 (2.3:1)
2,500 L/s	710 mm ø	3,050 × 200 (15.3:1)	1,700 × 300 (5.7:1)	1,150 × 400 (2.9:1)
3,500 L/s	800 mm ø	4,150 × 200 (20.8:1)	2,250 × 300 (7.5:1)	1,500 × 400 (3.8:1)
5,000 L/s	900 mm ø	5,650 × 200 (28.3:1)	3,000 × 300 (10.0:1)	2,000 × 400 (5.0:1)

Sized at 0.8 Pa/m. Any ratio above 4:1 should be treated as a prompt to find more void depth. Use the shape converter for a size not listed here.

What a chart cannot tell you

A size chart answers one question — what diameter delivers this flow at this friction rate — and it answers it well. Three things it cannot know, and all three have ended up on drawings because somebody trusted the chart alone.

Where the duct runs. The same 400 mm duct at 6 m/s is unremarkable in a plant room and unacceptable above a consulting room. The velocity column is the one to check against the application, not the size column.
What the air is. These tables are standard air. At altitude, in a hot extract, or on a cold outdoor intake, the density differs and so does the pressure the fan sees.
What the duct is made of. Steel only. A flexible connection sized from this chart will be badly undersized, because flexible duct is more than thirty times rougher than spiral steel.

Frequently asked questions

How do I read a duct size chart?

Find the row for your airflow, then read across to the friction rate your system is designed to. The diameter shown is the next standard round size at or above the calculated requirement, and the velocity beside it is what that standard size actually produces. Check that velocity against the band for your application before accepting the size — a chart cannot know whether the duct runs past a bedroom or a plant room.

What friction rate should I use with this chart?

For commercial low-velocity supply, 0.08 to 0.10 in.wg per 100 ft (0.8 to 1.0 Pa per metre) is the usual band, and the middle column is the common default. Use the lower column on long runs where fan energy dominates over the life of the system, or where a low sound level is specified. The higher column saves sheet metal at the cost of noise and fan power.

Why does the chart jump from 14 inches straight to 16?

Because 15 inch spiral duct is not a standard size. The imperial list runs in one-inch steps to 12 inches and then in two-inch steps, so a calculated requirement of 14.3 inches rounds up to 16. That jump is why the calculated requirement is shown alongside the standard size — knowing you needed 14.3 and are getting 16 tells you how much margin you have, which a chart showing only the standard size hides.

Do the metric and imperial charts give the same answers?

They give the same physics but different sizes, because the two markets manufacture different diameters. EN 1506 metric spiral runs 315, 355, 400, 450 mm; imperial spiral runs 12, 14, 16, 18 inches, which is 305, 356, 406, 457 mm. The lists interleave rather than align, so converting a metric answer to inches will rarely land on a size you can actually buy. Work in the system the duct will be bought in.

Does this chart apply at altitude or for hot air?

Not directly. It is generated for standard air at sea level and 20 °C. Pressure loss scales with density, so at 2000 metres the real loss is roughly 22 per cent below what these tables show, and in a hot extract it is lower still. The duct size barely moves, but the fan pressure does. For a job away from these conditions, use the ductulator with your actual temperature and elevation.

Can I use this chart for flexible duct?

No. These tables assume galvanised spiral steel at 0.09 mm absolute roughness. Fully extended flexible duct is around 3.0 mm — thirty-three times rougher — and compressed flexible duct is worse again. A flexible run sized from a steel chart will be substantially undersized. Size flexible duct with the calculator, selecting the flexible material.

Last updated: 26 July 2026