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.

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.