Flex And Round Duct Sizing Charts CFM (4,6,8,10,12-Inch CFM Duct Charts)

Nobody can properly size the ductworks without a duct sizing chart. HVAC duct sizing chart answers one simple question:

“What size should my ducts be?”

The dimensions of ducts (be it flex, metal, round, or rectangular) determines the airflow (measured in CFM).

Example: How many CFM does a 6-inch duct have? A rectangular 6×6-inch duct has an airflow of 110 CFM. Bigger 6×12-inch ducts can handle 270 CFM airflow.

The bigger the crosssection of ducts, the bigger CFM; that’s pretty obvious.

The smallest 6×4 ducts can handle 60 CFM airflow. The biggest 42×12 ducts with 504 sq inch crosssection can handle 3000+ CFM airflow.

cfm duct sizing for round and rectangular ducts
Example of big metal round and rectangular ducts (3,000+ CFM airflow).

To figure out exactly how big ducts you need for central air conditioning systems, we have prepared complete CFM duct charts for all different kinds of ducts:

  • Duct sizing chart for flex ducts (round ducts with 5-20 inch diameter).
  • Duct sizing chart for metal ducts (round ducts with 5-20 inch diameter).
  • 4-inch rectangular CFM duct chart (from 6x4 to 24x4 duct sizes).
  • 6-inch rectangular CFM duct chart (from 4x6 to 30x6 duct sizes).
  • 8-inch rectangular CFM duct chart (from 4x8 to 36x8 duct sizes).
  • 10-inch rectangular CFM duct chart (from 4x10 to 40x10 duct sizes).
  • 12-inch rectangular CFM duct chart (from 4x12 to 42x12 duct sizes).

You can use these ASHRAE duct sizing charts if you’re an HVAC technician or a DIY enthusiast:

CFM Sizing Chart For Flex Round Ducts (50-1,700 CFM)

Duct Size (Inches) Flex Duct Airflow (CFM)
5 inches 50 CFM
6 inches 75 CFM
7 inches 110 CFM
8 inches 160 CFM
9 inches 225 CFM
10 inches 300 CFM
12 inches 480 CFM
14 inches 700 CFM
16 inches 1,000 CFM
18 inches 1,300 CFM
20 inches 1,700 CFM

A common question here is “What size duct is needed for 1000 CFM?”. If you’re using flex round ducts, you need ducts with a 16-inch diameter.

Note: For everybody using metal duct calculators, you should input Flex duct = .05″ to get an accurate calculation.

If you are replacing rectangular ducts with round ducts, you will have to determine the equivalent diameter of the round ducts. You can find the details about rectangular to round duct transformation here.

CFM Sizing Chart For Metal Round Ducts (50-2,000 CFM)

Duct Size (Inches) Metal Duct Airflow (CFM)
5 inches 50 CFM
6 inches 85 CFM
7 inches 125 CFM
8 inches 180 CFM
9 inches 240 CFM
10 inches 325 CFM
12 inches 525 CFM
14 inches 750 CFM
16 inches 1,200 CFM
18 inches 1,500 CFM
20 inches 2,000 CFM

You can see that, compared to flex ducts, metal ducts can handle more airflow. For example, 20-inch flex ductwork can handle 1,700 CFM airflow while 20-inch metal ductwork can handle 2,000 CFM airflow.

Note: For everybody using metal duct calculators, you should input Round metal pipe = .06″ to get an accurate calculation.

Let’s look at how much airflow rectangular-shapred ducts can handle:

4-Inch Rectangular Ducts Size Chart (60-330 CFM)

4″ Duct 4″ CFM
6×4 60 CFM
8×4 90 CFM
10×4 120 CFM
12×4 150 CFM
14×4 180 CFM
16×4 210 CFM
18×4 240 CFM
20×4 270 CFM
22×4 300 CFM
24×4 330 CFM

6-Inch Rectangular Ducts Size Chart (60-775 CFM)

6″ Duct 6″ CFM
4×6 60 CFM
6×6 110 CFM
8×6 160 CFM
10×6 215 CFM
12×6 270 CFM
14×6 320 CFM
16×6 375 CFM
18×6 430 CFM
20×6 490 CFM
22×6 540 CFM
24×6 600 CFM
26×6 650 CFM
28×6 710 CFM
30×6 775 CFM

8-Inch Rectangular Ducts Size Chart (90-1,500 CFM)

6″ Duct 6″ CFM
4×8 90 CFM
6×8 160 CFM
8×8 230 CFM
10×8 310 CFM
12×8 400 CFM
14×8 490 CFM
16×8 580 CFM
18×8 670 CFM
20×8 750 CFM
22×8 840 CFM
24×8 930 CFM
26×8 1,020 CFM
28×8 1,100 CFM
30×8 1,200 CFM
32×8 1,300 CFM
34×8 1,400 CFM
36×8 1,500 CFM

10-Inch Rectangular Ducts Size Chart (120-2,350 CFM)

10″ Duct 10″ CFM
4×10 120 CFM
6×10 215 CFM
8×10 310 CFM
10×10 430 CFM
12×10 550 CFM
14×10 670 CFM
16×10 800 CFM
18×10 930 CFM
20×10 1,060 CFM
22×10 1,200 CFM
24×10 1,320 CFM
26×10 1,430 CFM
28×10 1,550 CFM
30×10 1,670 CFM
32×10 1,800 CFM
34×10 1,930 CFM
36×10 2,060 CFM
38×10 2,200 CFM
40×10 2,350 CFM

12-Inch Rectangular Ducts Size Chart (150-3,050 CFM)

12″ Duct 12″ CFM
4×12 150 CFM
6×12 270 CFM
8×12 400 CFM
10×12 550 CFM
12×12 680 CFM
14×12 800 CFM
16×12 950 CFM
18×12 1,100 CFM
20×12 1,250 CFM
22×12 1,400 CFM
24×12 1,600 CFM
26×12 1,750 CFM
28×12 1,950 CFM
30×12 2,150 CFM
32×12 2,300 CFM
34×12 2,450 CFM
36×12 2,600 CFM
38×12 2,750 CFM
40×12 2,900 CFM
42×12 3,050 CFM

All in all, it’s important to understand what size ducts you need to properly plan the ductwork. You can also check more information about CFM and duct sizing in the ASHRAE duct fitting database.

If you’re interested in how quickly the air is moving in your ducts, you can use these two duct velocity calculators. On top of that, if you have the airflow numbers expressed in SCFM (Standard Cubic Feet Per Minute), you can check this SCFM to CFM conversion.

9 thoughts on “Flex And Round Duct Sizing Charts CFM (4,6,8,10,12-Inch CFM Duct Charts)”

  1. Doesn’t this only work for a given static pressure? If my air handler has a 500 CFM blower and I use a 10″ round pipe, it doesn’t mean my system is a 325 CFM system. It will be putting out more than 325. Will be some loss due to the added restriction in size but air will also move faster than if it was an 12″ pipe. I may want the air to move faster like if going through a ventilated attic or slower to reduce noise. Charts are useless without context. Not even getting into equivalent length and fittings, etc. Please add some context for the assumptions of these values.

    • Stymie,

      Thank you for that observation my initial question was at what velocity are these ratings calculated at? As a designer, I need to know velocities, I tend to have higher velocities in areas where it does not impact the bldg. owner

    • This is true available static pressure of air mover IWC, fittings loss,total run supply and return = tel total equivalent length .

  2. How would do the math when cutting insulation at work to take and insulate the spiral round and rectangular duct so my insulation will be nice and snug to my ductwork ?

    • Hello Bryan, that math is often quite confusing. You usually follow the principle that the cross-section of ducts should be the same all along the ducts. That’s just an estimate that doesn’t include accounting for Reynolds number, airflow velocity, and so on but it’s rather simple to use without losing accuracy too much. Hope this helps.

  3. I had two rooms and I made them one and now the room is 210 square feet it has 28 in vents in it but it seems to be warm in that room should I increase the vent size

    • Hello Mark, when you are combining two rooms into one, you usually can keep the same duct sizes if both rooms had ducts. However, if one of these rooms was without ducts, the ducts in the second room might not be able to provide sufficient CFMs for the new combined room. If you see that the room is warm, it is not necessarily caused by ducts being too small. It might be that you have a little too small AC unit or that these extremely high temperatures are just too much for your current AC system. Hope this helps a bit.

  4. I Have Been In Ac Business For 38 Years I Have Seen It Change So Many Times One Time A Mfg Says Do This The Next Time Do That. We Heard One Time To Engineer You Duct Runs On A .10 Static Then That Is Only Good For Metal, Then They Came Back And Said No It Is Good On Flex Also Because The Spiral In Flex Acts Like A Riffled Barrel Now They Are Saying Flex Needs To Be Engineered On A .04 Static. If You Have Customers That Cry About Noise Do The .04 Static But Some Customers Complain They Can’t Feel The Air. Then Engineer On A .08 Static. Truth Is It Is Impossible To Satisfy Everyone.


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