Darcy-Weisbach with McGill AirFlow's friction factor and Table A.21 roughness; IRC M1601.1
Every duct chart and cardboard ductulator solves one equation, Darcy-Weisbach, and most hide one of its inputs: how rough the inside of the duct is. Fabric flex duct is listed at up to 40 times the roughness of galvanized spiral. This calculator sizes a round duct across the published roughness range of the material you are installing, checks itself against the design guide's worked examples, and prices the run.
Round duct size from airflow, friction rate and material
cfm
in. w.g. per 100 ft
ft
Round duct, standard air, straight runs. Fittings add equivalent length that you add to the run.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
Duct cross sections to scale
Ductwork prices
As published
Item
Low
High
Unit
Ductwork installed
$10
$40
per linear foot
Ductwork installed, retrofit with no existing ducts
The source's headline is $453 to $2,189, $1,252 on average, which is below the smallest row of its own table by house size ($1,400 to $4,500 for 100 to 250 ft), so the headline is left out and the per foot rate is used. Its page title reads 2025 on a page dated June 2026.
The source says aerosol sealing is for small to medium gaps; disconnected ducts or gaps larger than 5/8 in are sealed by hand.
Worked example
400 cfm through fabric flex duct at a friction rate of 0.10 in. w.g. per 100 ft, a 25 ft run.
Roughness. McGill Table A.21 lists fabric and wire flex duct at 0.0035 to 0.015 ft.
Diameter. Solving Darcy-Weisbach at 0.10 per 100 ft: 10.3 to 11.0 in, so a 11 in duct.
In that duct. 606 fpm, 0.073 to 0.100 in. w.g. per 100 ft.
Galvanized spiral would carry the same air at 9.6 to 9.7 in.
Price. 25 ft at $10 to $40 installed: $250 to $1,000.
The rough end of the flex range needs a larger duct than the smooth end. Which end an installed run sits at is not something the table says, and compression makes it worse.
diameter in inches, velocity in fpm, length in feet; ΔP in inches of water
The calculator solves for the diameter that gives your friction rate, at both ends of the material's roughness range.
Why this does not claim to be Manual D
The residential code, IRC M1601.1, sizes ducts by ACCA Manual D or other approved methods. Manual D is not free to read.
The friction physics under it is published, and McGill AirFlow's design guide prints both the equations and worked answers. This page reproduces those answers before it sizes anything, and asks you for the friction rate a Manual D design starts from.
Duct sizing chart: cfm by round duct size
How much air each round duct size carries at two friction rates, worked out with the same equation and roughness figures the calculator uses (McGill AirFlow, Equation A.13 and Table A.21). Each cell is a range because the roughness is a range: the low figure is the rough end of the material's listing, the high figure the smooth end.
Airflow in cfm, straight duct, standard air. Galvanized steel spiral seam, roughness 0.00018 to 0.00038 ft; flexible duct, all types of fabric and wire, 0.0035 to 0.015 ft, pulled straight
Round duct
At 0.08 in. w.g. per 100 ft
At 0.10 in. w.g. per 100 ft
Galvanized spiral, cfm
Flex duct, cfm
Galvanized spiral, cfm
Flex duct, cfm
4 in
32 to 33
25 to 29
37
28 to 32
5 in
59 to 60
45 to 52
67 to 68
50 to 58
6 in
96 to 98
72 to 84
109 to 111
81 to 95
7 in
146 to 149
109 to 127
165 to 168
122 to 142
8 in
209 to 213
155 to 180
236 to 241
173 to 203
9 in
287 to 293
211 to 246
324 to 332
236 to 276
10 in
380 to 389
278 to 326
430 to 441
311 to 365
12 in
621 to 636
449 to 527
701 to 720
503 to 591
14 in
939 to 964
674 to 792
1,060 to 1,088
754 to 888
16 in
1,342 to 1,374
957 to 1,127
1,515 to 1,551
1,071 to 1,264
18 in
1,833 to 1,879
1,305 to 1,539
2,065 to 2,121
1,460 to 1,724
20 in
2,423 to 2,486
1,722 to 2,032
2,729 to 2,806
1,926 to 2,277
Read it the way a Manual D design does. The friction rate comes first, from the blower's available static pressure, and the chart then gives the size. 0.08 and 0.10 are examples, not recommendations: at 0.10 a 6 in duct carries 109 to 111 cfm in galvanized spiral and 81 to 95 cfm in flex duct, which is why the same room can need a 6 in metal run or a 7 in flex run. Fittings add equivalent length, so a run with bends carries less than the straight figure at the same pressure.
Rectangular duct sizes and their round equivalents
A rectangular duct is sized through the round duct that has the same friction loss at the same airflow. McGill AirFlow's design guide gives the conversion as its Equation 1.10:
Deq = 1.30 × (a × b)0.625 ÷ (a + b)0.25, with a and b the two sides in inches.
The guide's own Sample Problem 1-11 converts a 12 x 45 in duct to 24 in; the equation gives 24.1.
Equivalent round diameter from McGill Equation 1.10; airflow at 0.10 in. w.g. per 100 ft in galvanized steel with longitudinal seams, 4 ft joints (roughness 0.00016 to 0.00032 ft, Table A.21)
Rectangular duct
Equivalent round
Airflow at 0.10, cfm
8 x 4 in
6.1 in
114 to 116
10 x 4 in
6.7 in
150 to 152
12 x 4 in
7.3 in
186 to 189
8 x 6 in
7.6 in
204 to 207
10 x 6 in
8.4 in
271 to 276
12 x 6 in
9.1 in
340 to 347
14 x 6 in
9.8 in
410 to 419
10 x 8 in
9.8 in
406 to 414
12 x 8 in
10.7 in
513 to 525
14 x 8 in
11.5 in
624 to 638
16 x 8 in
12.2 in
736 to 753
20 x 8 in
13.5 in
965 to 989
12 x 10 in
12.0 in
700 to 717
14 x 10 in
12.9 in
856 to 876
20 x 10 in
15.2 in
1,340 to 1,366
24 x 10 in
16.5 in
1,664 to 1,701
14 x 12 in
14.2 in
1,101 to 1,124
20 x 12 in
16.8 in
1,733 to 1,771
24 x 12 in
18.3 in
2,167 to 2,217
The equivalent is not the same area. A 12 x 6 in duct has 72 sq in of opening and an equivalent round of about 9.1 in, which has 66. The guide says to work out air velocity from the actual rectangular area, not from the equivalent round. Duct board is rougher than sheet metal (0.003 ft in Table A.21) and carries less at the same size.
The equation every duct chart is drawn from, checked against the book
A duct friction chart, a cardboard ductulator and every duct size calculator solve the same equation: Darcy-Weisbach, with a friction factor that depends on the duct's diameter, the air speed and how rough the inside surface is. McGill AirFlow's duct design guide prints it with a simplified friction factor (its Equation A.13) and works examples. This page runs the same arithmetic:
McGill AirFlow Duct System Design, 2003, printed answers against this page's arithmetic
Sample problem
McGill prints
This page
A-1: 18 in duct, 1,400 fpm, 150 ft, roughness 0.0003 ft
f = 0.0169, 0.20 in. w.g.
f = 0.0169, 0.20 in. w.g.
A-2: the same duct, roughness 0.01 ft
f = 0.0318, 0.38 in. w.g.
f = 0.0318, 0.38 in. w.g.
1-8: 500 cfm at 0.20 in. w.g. per 100 ft
9 or 9.5 in, from the chart
9.2 in at roughness 0.0003 to 0.0005 ft
1-8: 1,500 cfm at 0.20 in. w.g. per 100 ft
14 in, from the chart
13.8 to 14.0 in at roughness 0.0003 to 0.0005 ft
1-8: 5,000 cfm at 0.20 in. w.g. per 100 ft
22 in, from the chart
21.8 to 22.0 in at roughness 0.0003 to 0.0005 ft
1-8: 15,000 cfm at 0.20 in. w.g. per 100 ft
33 in, from the chart
33.1 to 33.5 in at roughness 0.0003 to 0.0005 ft
The chart answers are read by eye and the book says so, which is why 9 or 9.5 inches is a fair reading of 9.2. The equation is what a calculator should use, and the roughness is the input most of them hide.
The same 400 cfm in nine kinds of duct
400 cfm through a 10 inch round duct, roughness from McGill Table A.21, friction in inches of water
Duct material
Roughness, ft
Friction per 100 ft
Round size for 0.10 per 100 ft
Galvanized steel, spiral seam, 12 ft joints
0.00018 to 0.00038
0.084 to 0.088
9.6 to 9.7 in
Galvanized steel, longitudinal seams, 4 ft joints
0.00016 to 0.00032
0.084 to 0.087
9.6 to 9.7 in
Galvanized steel, longitudinal seams, 2.5 ft joints
0.0005
0.090
9.8 in
Aluminum
0.00015 to 0.0002
0.083 to 0.084
9.6 in
PVC plastic pipe
0.00015 to 0.0003
0.083 to 0.086
9.6 to 9.7 in
Fibrous glass duct, rigid
0.003
0.116
10.3 in
Fibrous glass duct liner, air side with facing
0.005
0.128
10.5 in
Flexible duct, metallic
0.004 to 0.007
0.123 to 0.138
10.4 to 10.6 in
Flexible duct, all types of fabric and wire
0.0035 to 0.015
0.119 to 0.165
10.3 to 11.0 in
Fabric flex duct has the widest range in the table, 0.0035 to 0.015 ft, so the same airflow and friction rate need anything from 10.3 to 11.0 in of duct. McGill does not say where in that range a given product or installation sits.
And that is flex duct pulled straight. The Building America Solution Center, run by PNNL for the Department of Energy, on installing it:
Compressing the duct or using excessive length will noticeably increase friction losses.
It gives the length rule as no more than 4 percent longer than the span, and cites Manual D for fittings:
For example, a 90-degree bend in a flex duct adds an additional 15 feet of equivalent length to the duct system at a velocity of 700 feet per minute.
The calculator sizes straight duct. Add the equivalent length of every bend and fitting to the run before you use its total.
The price guide's ductwork figures do not agree with each other
HomeAdvisor's ductwork guide, updated June 20, 2026, makes three statements that cannot all be true of the same job:
As published by HomeAdvisor, updated June 20, 2026
Statement
What it gives
Against
Headline cost
$453 to $2,189, $1,252 average
its own table starts at $1,400 for the smallest house
Duct needed
one linear foot per 10 to 15 sq ft
67 to 200 ft for 1,000 to 2,000 sq ft; the table says 100 to 250
Cost by house size
$1,400 to $4,500 for 100 to 250 ft
$14 to $18 a foot, inside its $10 to $40 installed rate
The per foot rate is the figure the calculator uses, applied to the length you enter. The headline range looks like a smaller job than a house's ductwork, and the page does not use it.
Where the friction rate and the airflow come from
Airflow for a room is its share of the house's load: a room with a tenth of the Manual J load gets a tenth of the blower's airflow. The heat load calculator gives a whole-house heating load.
The friction rate is what is left of the blower's static pressure after the coil, filter, registers and grilles, spread over the longest run with its fittings. The equipment's fan table gives the static pressure; the default here is only an example.
Velocity is shown so you can check it against the equipment or register maker's limits for noise.
The sources, and what Manual D adds
Read 2026-09-13.
The residential code, in Virginia's adoption of the 2021 IRC:
Duct systems serving heating, cooling and ventilation equipment shall be installed in accordance with the provisions of this section and ACCA Manual D, the appliance manufacturer's installation instructions or other approved methods.
Duct System Design Guide, First Edition, McGill AirFlow Corporation, 2003. Equations 1.5, A.10 to A.14, Sample Problems 1-5 to 1-8, A-1 and A-2, and Table A.21 duct surface roughness. Equation 1.10 (equivalent round diameter of rectangular duct) and Sample Problem 1-11, pages 1.13 and 1.14, read 2026-10-03.
Virginia Residential Code 2021, Chapter 16 Duct Systems, International Residential Code 2021 as adopted with amendments in Virginia, read through UpCodes with the jurisdiction and edition stated on the page. Section M1601.1, duct design to ACCA Manual D or other approved methods.
No Excessive Coiled or Looped Flex Ducts, Building America Solution Center, Pacific Northwest National Laboratory for the US Department of Energy. Pull flex duct taut; length no more than 4% over the span; compression raises friction.
No Kinks or Sharp Bends in Flex Duct Installation, Building America Solution Center, Pacific Northwest National Laboratory for the US Department of Energy. Equivalent length of a 90 degree flex bend, citing Manual D Appendix 3.
How Much Does Ductwork Installation Cost?, HomeAdvisor staff writer, reviewed by HomeAdvisor editor Ryan Noonan, updated June 20, 2026. Installed cost per linear foot, material cost per linear foot and the table by house size.
Manual D starts from the blower: the static pressure the air handler can deliver at its design airflow, less the coil, filter, registers and grilles, spread over the longest run including its fittings. That gives the friction rate this calculator asks you for. Each room's airflow comes from its share of the Manual J load.
Frequently asked questions
How do I calculate duct size?
Choose the airflow and a friction rate, then solve Darcy-Weisbach for the diameter, using the roughness of the duct material. At 400 cfm and 0.10 in. w.g. per 100 ft, fabric flex duct needs about 10.3 to 11.0 inches and galvanized spiral about 9.6 to 9.7.
What size duct for 400 cfm?
It depends on the friction rate and the material. At 0.10 in. w.g. per 100 ft, this page gives 9.6 to 9.7 inches for galvanized spiral duct and 10.3 to 11.0 for fabric flex duct, so an 11 inch flex or a 10 inch metal duct.
What size duct for 100 cfm?
At 0.10 in. w.g. per 100 ft, a 6 in galvanized spiral duct carries 109 to 111 cfm and a 6 in flex duct 81 to 95 cfm, from the duct sizing chart on this page. At 0.08 the 6 in carries 96 to 98 cfm in metal and 72 to 84 cfm in flex, so a flex run can need 7 in.
How many cfm does an 8 inch duct carry?
At 0.10 in. w.g. per 100 ft, 236 to 241 cfm in galvanized spiral and 173 to 203 cfm in fabric flex duct pulled straight. At 0.08, 209 to 213 and 155 to 180. Both come from Darcy-Weisbach with McGill AirFlow's roughness figures.
What round duct equals a rectangular duct?
McGill AirFlow's Equation 1.10: 1.30 x (a x b)^0.625 / (a + b)^0.25. A 12 x 8 in duct comes out at 10.7 in and a 14 x 6 in at 9.8 in. The guide's Sample Problem 1-11 turns 12 x 45 in into 24 in.
Is flex duct rougher than metal duct?
Yes. McGill AirFlow's Table A.21 lists fabric and wire flex duct at 0.0035 to 0.015 ft of roughness against 0.00018 to 0.00038 for galvanized spiral duct. The Building America Solution Center adds that compressing flex duct or leaving extra length noticeably increases friction.
What friction rate should I use?
The one a Manual D design gives: the blower's available static pressure after the coil, filter and grilles, over the longest run including fittings. It is specific to the equipment and the layout, so this page asks for it rather than assuming one.
How much does a 90 degree bend add?
The Building America Solution Center, citing Manual D Appendix 3, gives 15 feet of equivalent length for a 90 degree bend in flex duct at 700 feet per minute. Add fittings to the run length before reading the total friction.
How much does ductwork cost?
HomeAdvisor's June 2026 guide gives $10 to $40 per linear foot installed, $30 to $50 in a retrofit with no existing ducts. Its headline $453 to $2,189 is below the smallest row of its own table by house size, so this page uses the per foot rate.
How do I size a return air duct?
The same way as a supply duct. Take the airflow the return carries and the friction rate available to it, then solve for the diameter using the roughness of the material. Count the return grille and filter against the static pressure, and add the return's fittings to its run length before reading the friction.
How much does air conditioner duct cleaning cost?
HomeAdvisor's June 2026 guide gives $271 to $509 for a typical air duct cleaning job, $25 to $50 per vent, or $90 to $125 per hour for a cleaning crew. At the per vent rate a house with 15 vents costs $375 to $750, above the service's typical range. Pest, mold or duct repair add-ons run $200 to $3,400. A furnace and an air conditioner that share a blower share the same ducts.
How much does HVAC duct cleaning cost?
HomeAdvisor's June 2026 guide gives $271 to $509 for a typical job and $25 to $50 per vent.
How much does it cost to install an HVAC system with ductwork?
HomeAdvisor's June 2026 guide gives $5,000 to $12,500 for a new system and $1,000 to $2,700 for ductwork. HomeAdvisor's June 2026 guide gives $10 to $40 per linear foot for ductwork installed.
How much does whole house duct cleaning usually cost?
HomeAdvisor's June 2026 guide gives $271 to $509 for a whole-house duct cleaning and $25 to $50 per vent.
Check these numbers yourself
Duct System Design Guide, First EditionMcGill AirFlow Corporation, 2003. Equations 1.5, 1.10 and A.10 to A.14, Sample Problems 1-5 to 1-8, 1-11, A-1 and A-2, and Table A.21 duct surface roughness.
Virginia Residential Code 2021, Chapter 16 Duct SystemsInternational Residential Code 2021 as adopted with amendments in Virginia, read through UpCodes with the jurisdiction and edition stated on the page. Section M1601.1, duct design to ACCA Manual D or other approved methods.
No Excessive Coiled or Looped Flex DuctsBuilding America Solution Center, Pacific Northwest National Laboratory for the US Department of Energy. Pull flex duct taut; length no more than 4% over the span; compression raises friction.
No Kinks or Sharp Bends in Flex Duct InstallationBuilding America Solution Center, Pacific Northwest National Laboratory for the US Department of Energy. Equivalent length of a 90 degree flex bend, citing Manual D Appendix 3.
How Much Does Ductwork Installation Cost?HomeAdvisor staff writer, reviewed by HomeAdvisor editor Ryan Noonan, updated June 20, 2026. Installed cost per linear foot, material cost per linear foot and the table by house size.
How every figure here is verified: Sources & Method. Who builds this: Lucas Pradella, and the method is on About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-13. Codes are amended locally; confirm against the edition your jurisdiction enforces.
This is not a Manual D duct design. It sizes one straight round duct from an airflow and friction rate you supply. The code section is quoted from Virginia's 2021 adoption, read on 13 September 2026; your jurisdiction may differ. Prices are HomeAdvisor's, not this site's.