Duct Sizing Chart: CFM Tables for Flex and Round Duct
The reference tables contractors and serious DIYers actually search for, plus the friction rate that quietly makes every chart different.
A 10-inch round sheet metal duct carries roughly 360 CFM. A 10-inch flex duct in the same run carries closer to 305 CFM. A 12-inch round handles about 520 CFM; a 12-inch flex, about 440. Every figure on this page assumes a friction rate of 0.1 inches of water column per 100 feet of equivalent length, which is the standard residential design point per ACCA methodology, and the reason two duct sizing charts on the internet often disagree.
- Round metal duct meaningfully outperforms flex of the same nominal diameter, because the corrugated inner liner of flex generates more friction than smooth sheet metal.
- Residential systems are typically designed at 0.1 IWC per 100 feet, inside an acceptable range of 0.06 to 0.18 IWC per 100 ft per ACCA methodology.
- Total system airflow follows the 400 CFM per ton rule, so a 3-ton system needs about 1,200 CFM of supply and the same 1,200 CFM back through the returns.
- Return ducts are undersized more often than supplies. They should be sized for lower velocity, which means larger sheet metal, not the same size as the supply.
- Flex only hits its chart CFM when it is pulled taut. Sag, kinks, and hard bends can effectively double resistance.
What follows is the reference itself. The tables come first because that is what you came here for. The mechanics behind them come after, along with the parts that charts leave out.
Duct sizing chart: CFM by duct size
The table below lists working CFM for round sheet metal duct and flexible duct at a 0.1 IWC per 100 ft friction rate, which is the residential design assumption most charts you will find online are built on. Flex figures assume the duct is pulled tight, not sagged between supports.
| Duct Diameter | Round Metal CFM | Flex Duct CFM |
|---|---|---|
| 4" | 50 | 40 |
| 5" | 80 | 70 |
| 6" | 120 | 100 |
| 7" | 170 | 145 |
| 8" | 230 | 195 |
| 9" | 290 | 245 |
| 10" | 360 | 305 |
| 12" | 520 | 440 |
| 14" | 720 | 610 |
| 16" | 970 | 825 |
| 18" | 1,270 | 1,080 |
| 20" | 1,620 | 1,375 |
| 22" | 2,020 | 1,715 |
| 24" | 2,480 | 2,105 |
Here is the part most charts leave out. The values above are ceilings for a straight run at a specified friction rate. Every elbow, boot, takeoff, and transition in the actual run adds equivalent length, and equivalent length shrinks the CFM that duct can carry in the real world. A 10-inch flex line with two hard elbows and a boot is not delivering 305 CFM. It is delivering whatever is left after the fittings eat their share.
How many CFM does your system need?
Residential cooling systems are designed around 400 CFM per ton of nominal capacity. A 3-ton unit needs about 1,200 CFM of total airflow through the supply trunk, and the return path has to move the same volume back to the air handler. The acceptable range runs from 350 to 450 CFM per ton depending on climate and how much dehumidification the system needs to do, per standard residential HVAC design practice.
| System Size | Total CFM | Round Metal Trunk | Typical Rectangular Trunk |
|---|---|---|---|
| 1.5 ton | 600 | 12" | 14 x 8 |
| 2 ton | 800 | 14" | 16 x 8 |
| 2.5 ton | 1,000 | 14" | 18 x 8 |
| 3 ton | 1,200 | 16" | 20 x 8 |
| 3.5 ton | 1,400 | 16" | 22 x 8 |
| 4 ton | 1,600 | 18" | 24 x 8 |
| 5 ton | 2,000 | 20" | 26 x 8 |
The 400 CFM per ton figure is a starting point, not gospel. In a humid climate like Houston or Charleston, a designer may deliberately push the design closer to 350 CFM per ton so the coil runs colder, dwells longer, and pulls more moisture out of the air. In a dry climate like Phoenix, 450 CFM per ton makes sense because latent load is minimal and sensible cooling is everything.
The rule breaks down for variable-speed equipment, which modulates airflow across the entire operating range, and for zoned systems where different zones call for different flows. If you are working with an inverter compressor and a variable-speed blower, size the ductwork to handle the manufacturer's maximum rated airflow, not the nominal tonnage.
Why do flex duct and round duct carry different CFM?
Flexible duct has a corrugated inner liner. Sheet metal is smooth. Air moving through the corrugations hits thousands of small ridges that generate turbulence and drag, which is why flex duct carries meaningfully less air than rigid duct of the same nominal diameter. To hit the same CFM, flex generally needs to be one nominal size larger than the round metal it is replacing.
That is the number you get on paper. Here is what happens in an attic.
Flex duct only performs to its rated CFM when it is stretched taut, supported at short intervals, and routed with gentle sweeping turns. A sagged run of 10-inch flex behaves like a series of small accordions, each one throwing another chunk of static pressure at the blower. Compressed at a support strap, kinked around a truss, or wadded up behind a boot, its effective area shrinks and the corrugations bite deeper into the airflow. Field measurements routinely show that a poorly installed flex run can double the pressure loss the chart predicts.
Rigid round metal or rectangular duct with sealed seams is dramatically more forgiving. It moves the CFM the chart says it will move, in a straight line or around a fitting, whether the installer had a good day or not. On long trunk runs and any main supply line, metal is the better material every time. Flex is best reserved for the last few feet into a boot, kept short and pulled tight.
How do you size a return air duct?
Return air duct capacity has to equal supply capacity. A system moving 1,200 CFM of supply must have 1,200 CFM of return to give the blower somewhere to pull air from. Returns are sized larger than the equivalent supply because they are designed for lower velocity, typically 400 to 800 feet per minute versus 800 to 1,200 FPM on the supply side, per standard ASHRAE ventilation design practice.
The short version: an undersized return is the most common airflow problem in American homes, and it makes an otherwise-good system feel weak.
| System CFM | Round Metal Return | Rectangular Return | Filter Grille Face |
|---|---|---|---|
| 600 | 14" | 16 x 8 | 20 x 20 |
| 800 | 16" | 20 x 8 | 20 x 25 |
| 1,000 | 18" | 22 x 8 | 25 x 25 |
| 1,200 | 18" | 24 x 8 | 25 x 30 |
| 1,400 | 20" | 26 x 8 | 25 x 30 |
| 1,600 | 20" | 30 x 8 | 30 x 30 |
| 2,000 | 22" | 32 x 10 | 30 x 36 |
The symptoms of an undersized return are familiar to any tech who has spent a summer chasing comfort complaints: whistling at the filter grille, a filter that gets sucked into a bowl shape, doors that slam themselves shut when the blower kicks on, and a house that feels stuffy no matter how cold the supply air is. All of those are the blower fighting a return path that cannot feed it enough air.
One more thing on filter grilles. A useful rule of thumb: size the face for no more than 2 CFM per square inch of net free area, and less than that if the filter is a deep-pleat MERV 13 or higher per EPA filter guidance. A 20-by-25 return grille is 500 square inches gross, and a good chunk of that is louver and frame. Do the math on the actual open area, not the outside dimensions.
What is a ductulator and do you still need one?
A ductulator is a circular slide rule that solves the same duct sizing equation the tables above use. Line up CFM against friction rate on the outer wheels, and the tool reveals duct diameter, rectangular equivalent, and air velocity through a small window. Digital duct calculators, whether on a phone or in design software, do exactly the same math with fewer moving parts.
Do you still need the physical wheel? Honestly, no. But most experienced techs keep one in the truck anyway, because it does not need a battery, it does not need signal, and it lets you sanity-check a chart in about four seconds. That is worth something on the roof of a strip mall in July.
The more important point: a ductulator or calculator only sizes one section at a time. Neither one designs the system. Both assume you have already worked out how much CFM each branch needs and what friction rate the whole system was designed around. Feed it garbage inputs and it will produce a very precise, very wrong duct size.
How do you calculate duct size yourself?
Sizing a duct system correctly follows ACCA Manual D methodology and requires four inputs: the CFM each branch needs to deliver, the available static pressure from the blower, the total effective length of the longest run, and the friction rate calculated from those two values. Miss any one of them and the whole exercise is guesswork.
Here is the sequence, honestly.
A proper Manual J residential load calculation gives you the heating and cooling load per room in BTUs, which converts directly to CFM per room. Without Manual J, distributing total system CFM by room square footage is an approximation, not a design.
Read the blower performance chart in the air handler manual. Total external static is typically 0.5 IWC for a residential unit. Subtract pressure drops for the coil, filter, humidifier, and any accessories. What is left is available static pressure for the ductwork.
Walk the longest supply run and the longest return run. Add the physical length in feet. Then add equivalent lengths for every elbow, tee, boot, and transition using Manual D fitting tables. The equivalent length of fittings is where most DIY calculations quietly go wrong.
Friction rate equals available static pressure times 100, divided by total effective length. The result should fall between 0.06 and 0.18 IWC per 100 ft. If it lands outside that range, either the ducts are trying to do too much or the blower is oversized for the layout.
Using the calculated friction rate, size each trunk section and branch to carry its required CFM. Keep supply main velocities in the 800 to 1,200 FPM range and return main velocities lower, ideally under 800 FPM. This is where the chart at the top of this page earns its keep.
Meet Jim, who tried this on his own last year. Jim added a bonus room over his garage, ran 25 feet of 6-inch flex to a new supply boot, and could not figure out why the room stayed 8 degrees warmer than the rest of the house. Jim did the CFM math correctly. What Jim did not do was add equivalent length for the tee at the trunk, two elbows getting around a truss, the boot at the end, and the fact that the flex was not pulled tight. The 6-inch line was carrying about half the air the chart said it would. This is why professionals price the design work separately from the sheet metal.
When should you have ductwork professionally designed?
Any time the ductwork is new, being significantly modified, or attached to new equipment, professional design pays for itself. A poorly designed duct system can strip a large share of the rated efficiency off a top-tier air handler, which shows up on the utility bill every month for the life of the system. Manual J sizes the equipment. Manual D sizes the ducts. Skipping either one is how a brand-new 4-ton unit ends up cooling the guest room to 65 and the master to 78.
Retrofits are the case where design matters most, because the existing duct layout was rarely built for the equipment now attached to it. Older systems ran at higher static pressure with smaller returns and undersized branches, and modern high-efficiency blowers do not tolerate that abuse well. If you are replacing a system and reusing the ducts, someone needs to verify the ducts can actually deliver the airflow the new blower expects.
For homeowners planning a full duct redesign or a new-construction system, AC Direct offers a ductwork design service that produces a Manual D layout you or your installer can build to. If you also need the equipment and the sheet metal, the full ac ductwork catalog includes trunks, boots, boards, and takeoffs sized to the drawings. For context on how these pieces fit into the rest of the system, our overview of the parts of hvac system is a useful map.
A duct sizing chart cannot design a duct system. It can tell you what a single section of duct will carry at a specified friction rate, and that is enormously useful. It cannot tell you what friction rate to design around, what happens at every fitting, or whether the flex up in the attic is pulled tight. Those decisions are the design, and the design is what makes the equipment work.
Frequently Asked Questions
What size duct do I need for 1,200 CFM?
At the standard residential friction rate of 0.1 IWC per 100 ft, 1,200 CFM needs roughly a 16-inch round metal trunk, or an 18-inch flex duct if flex is being used for the main run. That figure assumes a straight section without fittings, so the actual sizing for a real layout will typically bump up one size to account for elbows and takeoffs.
How much CFM can a 6-inch flex duct handle?
A 6-inch flex duct carries approximately 100 CFM at a 0.1 IWC per 100 ft friction rate, pulled taut, in a short straight run. Add typical fittings and any sag, and the real-world delivery drops fast. This is why 6-inch flex is best used for bedrooms with modest loads and kept under 20 feet of total run length.
Is flex duct or rigid duct better?
Rigid metal duct moves more air per inch of diameter, is unaffected by installation quality, and is dramatically more durable. Flex duct is cheaper, faster to install, and useful for the last few feet into a boot. For main trunks and long branch runs, rigid wins. For flexible connections in tight spaces, flex is fine when installed correctly and pulled tight.
Why are my return ducts smaller than my supply ducts?
They should not be. Return ducts should be larger than supplies for the same CFM because they are designed for lower air velocity to prevent noise and reduce static pressure. If your returns are smaller than your supplies, or roughly equal in size, the return path is almost certainly restrictive and the blower is working harder than it should to move design airflow.
What is a good friction rate for residential duct design?
Residential duct systems are typically designed at 0.08 to 0.12 IWC per 100 ft, with 0.1 being the standard reference point used by most published charts. The acceptable range spans 0.06 to 0.18 IWC per 100 ft. Values above that suggest the ducts are undersized for the blower; values below suggest oversized ducts or a blower that cannot generate enough pressure.
Can I use a duct calculator app instead of hiring a designer?
A duct calculator sizes one section at a time given inputs you provide. It does not calculate room-by-room loads, measure equivalent lengths of fittings, or balance an entire branched system. For a single addition or a straightforward branch swap, an app is fine. For a whole-house design or a system that replaces existing equipment, a Manual D layout by a qualified designer is the professional standard.
