Standard Round Duct Sizes

Metric spiral duct — EN 1506

EN 1506 standard metric round duct diameters with area and airflow range
Diameter	Area	Perimeter	Flow at 3 m/s	Flow at 8 m/s
63 mm	0.0031 m²	198 mm	9 L/s	25 L/s
80 mm	0.0050 m²	251 mm	15 L/s	40 L/s
100 mm	0.0079 m²	314 mm	24 L/s	63 L/s
125 mm	0.0123 m²	393 mm	37 L/s	98 L/s
160 mm	0.0201 m²	503 mm	60 L/s	161 L/s
200 mm	0.0314 m²	628 mm	94 L/s	251 L/s
250 mm	0.0491 m²	785 mm	147 L/s	393 L/s
315 mm	0.0779 m²	990 mm	234 L/s	623 L/s
355 mm	0.0990 m²	1,115 mm	297 L/s	792 L/s
400 mm	0.1257 m²	1,257 mm	377 L/s	1,005 L/s
450 mm	0.1590 m²	1,414 mm	477 L/s	1,272 L/s
500 mm	0.1963 m²	1,571 mm	589 L/s	1,571 L/s
560 mm	0.2463 m²	1,759 mm	739 L/s	1,970 L/s
630 mm	0.3117 m²	1,979 mm	935 L/s	2,494 L/s
710 mm	0.3959 m²	2,231 mm	1,188 L/s	3,167 L/s
800 mm	0.5027 m²	2,513 mm	1,508 L/s	4,021 L/s
900 mm	0.6362 m²	2,827 mm	1,909 L/s	5,089 L/s
1000 mm	0.7854 m²	3,142 mm	2,356 L/s	6,283 L/s
1120 mm	0.9852 m²	3,519 mm	2,956 L/s	7,882 L/s
1250 mm	1.2272 m²	3,927 mm	3,682 L/s	9,817 L/s

The flow columns are the band a size sensibly covers, not a limit. Three metres per second is quiet and generous; eight is the top of the general commercial band. Pick within the band using the velocity chart for the application.

Imperial spiral duct

Imperial standard round duct diameters with area and airflow range
Diameter	Area	Metric equivalent	Flow at 600 fpm	Flow at 1600 fpm
4 in	0.087 ft²	102 mm	52 CFM	140 CFM
5 in	0.136 ft²	127 mm	82 CFM	218 CFM
6 in	0.196 ft²	152 mm	118 CFM	314 CFM
7 in	0.267 ft²	178 mm	160 CFM	428 CFM
8 in	0.349 ft²	203 mm	209 CFM	559 CFM
9 in	0.442 ft²	229 mm	265 CFM	707 CFM
10 in	0.545 ft²	254 mm	327 CFM	873 CFM
11 in	0.660 ft²	279 mm	396 CFM	1,056 CFM
12 in	0.785 ft²	305 mm	471 CFM	1,257 CFM
14 in	1.069 ft²	356 mm	641 CFM	1,710 CFM
16 in	1.396 ft²	406 mm	838 CFM	2,234 CFM
18 in	1.767 ft²	457 mm	1,060 CFM	2,827 CFM
20 in	2.182 ft²	508 mm	1,309 CFM	3,491 CFM
22 in	2.640 ft²	559 mm	1,584 CFM	4,224 CFM
24 in	3.142 ft²	610 mm	1,885 CFM	5,027 CFM
26 in	3.687 ft²	660 mm	2,212 CFM	5,899 CFM
28 in	4.276 ft²	711 mm	2,566 CFM	6,842 CFM
30 in	4.909 ft²	762 mm	2,945 CFM	7,854 CFM
32 in	5.585 ft²	813 mm	3,351 CFM	8,936 CFM
34 in	6.305 ft²	864 mm	3,783 CFM	10,088 CFM
36 in	7.069 ft²	914 mm	4,241 CFM	11,310 CFM
38 in	7.876 ft²	965 mm	4,725 CFM	12,601 CFM
40 in	8.727 ft²	1,016 mm	5,236 CFM	13,963 CFM
42 in	9.621 ft²	1,067 mm	5,773 CFM	15,394 CFM
44 in	10.559 ft²	1,118 mm	6,336 CFM	16,895 CFM
46 in	11.541 ft²	1,168 mm	6,925 CFM	18,466 CFM
48 in	12.566 ft²	1,219 mm	7,540 CFM	20,106 CFM
52 in	14.748 ft²	1,321 mm	8,849 CFM	23,597 CFM
56 in	17.104 ft²	1,422 mm	10,263 CFM	27,367 CFM
60 in	19.635 ft²	1,524 mm	11,781 CFM	31,416 CFM
64 in	22.340 ft²	1,626 mm	13,404 CFM	35,744 CFM
72 in	28.274 ft²	1,829 mm	16,965 CFM	45,239 CFM
80 in	34.907 ft²	2,032 mm	20,944 CFM	55,851 CFM
88 in	42.237 ft²	2,235 mm	25,342 CFM	67,579 CFM
96 in	50.265 ft²	2,438 mm	30,159 CFM	80,425 CFM

The metric equivalent column is the exact conversion, not a standard size — 16 inches is 406 mm, which is not an EN 1506 diameter. That mismatch is the point of the next section.

Why the two lists do not meet

Metric spiral duct follows a preferred-number series: each size is roughly 1.12 to 1.25 times the one below it, so the steps are proportional rather than absolute. That is why the gaps grow — 25 mm between 100 and 125, but 100 mm between 900 and 1000. Imperial sizing grew from whole-inch shop practice instead, in one-inch steps to 12 inches and two-inch steps beyond, which produces a series that starts finer and coarsens differently.

The practical consequence appears on international projects. A design done in inches and issued to a metric fabricator produces schedules full of 356 and 406 mm duct, neither of which is stocked. The fix is not conversion but redesign: size the duct in the system it will be purchased in, using the standard list for that market, and let the two schedules differ. A 400 mm duct and a 16 inch duct are not the same duct, and pretending otherwise puts a 1.5 per cent area error into every branch of the system.

Notes for specification and coordination

Nominal is internal. Spiral sizes refer to the air path. Add the sheet thickness for the outside, then the insulation, then clearance for hangers before booking void space.
Fittings follow the same list. Bends, tees and reducers are made for standard diameters. A non-standard duct means non-standard fittings, which is where the cost actually lands.
Round beats rectangular where it fits. Least perimeter for the area means less metal, less insulation and less friction. Reach for rectangular when the void forces it, not by default.
Above about 1000 mm, check handling. Manufacture is rarely the constraint; getting the section through the building and hanging it usually is.

Frequently asked questions

What are the standard round duct sizes?

In metric markets, spiral duct follows EN 1506: 63, 80, 100, 125, 160, 200, 250, 315, 355, 400, 450, 500, 560, 630, 710, 800, 900, 1000, 1120 and 1250 mm. In imperial markets the range runs 4 to 12 inches in one-inch steps, then 14 to 48 in two-inch steps, then 52, 56, 60, 64, 72, 80, 88 and 96 inches. Sizes outside these lists exist but are made to order and priced accordingly.

Why do metric and imperial duct sizes not line up?

Because they were standardised separately, from different starting points. EN 1506 follows a preferred-number progression — each size roughly 1.12 to 1.25 times the one below — while the imperial list grew from whole-inch practice. The result is that 400 mm sits between 15 and 16 inches, and 16 inches is 406 mm. Converting a size from one system to the other almost never lands on a diameter you can buy, so design in the system the duct will be purchased in.

What airflow does each duct size carry?

It depends entirely on the velocity you design to, which is why this page shows a band rather than a single figure. The table below gives the flow at 3 m/s — quiet, generous, typical of a branch near occupied space — and at 8 m/s, the top of the general commercial band. The same 400 mm duct carries 380 L/s at the quiet end and 1005 L/s at the busy end, and both are legitimate answers to different questions.

Is duct diameter measured inside or outside?

Nominal spiral duct sizes refer to the internal diameter, which is what matters for the air. The outside diameter is slightly larger by the sheet thickness, and with insulation the overall dimension grows considerably — a 400 mm duct with 50 mm insulation needs 500 mm of clear space plus support. Coordinate the void on the outside dimension, size the air on the inside one.

Should I use round or rectangular duct?

Round wherever the void allows it. For the same air and the same pressure loss, round duct needs less metal, less insulation, fewer stiffeners and less labour than any rectangular equivalent, because it encloses the most area for the least perimeter. Rectangular exists because ceiling voids are shallow, not because it is better. Where the void is tight but the friction penalty of a flat rectangular section is unwelcome, flat oval sits between the two.

What is the largest round duct available?

Spiral machines commonly run to 1250 mm in metric and 96 inches in imperial, but the practical ceiling is usually transport and handling rather than manufacture. Above roughly 1000 mm, sections get awkward to move through a building and heavy to hang, and rectangular or field-assembled construction often becomes more economic despite the friction penalty.

Metric spiral duct — EN 1506

Imperial spiral duct

Why the two lists do not meet

Notes for specification and coordination

Frequently asked questions

Related Calculators