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.
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.
Hello there, these are standard values HVAC technicians use. We will try to give some context as recommended, thank you.
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 .
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.
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.
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.