HVAC Duct Sizing Chart (50-3,050 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.

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.

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