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-Inch Rectangular Ducts Size Chart (60-775 CFM)
|6″ Duct||6″ CFM|
8-Inch Rectangular Ducts Size Chart (90-1,500 CFM)
|6″ Duct||6″ CFM|
10-Inch Rectangular Ducts Size Chart (120-2,350 CFM)
|10″ Duct||10″ CFM|
12-Inch Rectangular Ducts Size Chart (150-3,050 CFM)
|12″ Duct||12″ 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.