Duct Calculator: Size Any Supply or Return Duct in Under a Minute
Get the right round or rectangular duct size from CFM, AC tonnage, furnace BTU, or room area. Built on the ASHRAE equal friction method with velocity checks for quiet operation.
๐จ Supply, return & exhaust
โก AC tonnage or furnace BTU
โฌค Round and rectangular
๐ Velocity-checked for silence
Step 1 of 7
in progress
What do you want to figure out?
Tap an option to jump straight to your answer.
๐จ How much air does this duct need to move?
Enter the design airflow in CFM (cubic feet per minute).
Bedroom branch: 80 to 150 CFM. Whole-house trunk: 600 to 2,000 CFM. Range hood: use the label CFM.
โก What kind of equipment?
Tap to continue.
โ How many tons is your system?
Tap to continue.
๐ฅ What is your furnace BTU input?
Use the input rating from the furnace label, not the output.
Typical residential: 40,000 to 120,000 BTU. Divide output BTU by AFUE (0.80 to 0.97) to get input.
๐ก What is your climate?
Climate affects CFM per ton. Tap to continue.
๐ Tell me about the room
Enter square footage and pick room type for the right CFM factor.
Length ร width. A 12ร12 bedroom is 144 sq ft.
What is this duct doing?
Supply and return ducts have different velocity limits for noise. Tap to continue.
โ What duct material?
Material changes friction resistance. Flex duct needs a bigger size than metal for the same airflow. Tap to continue.
๐ Design friction rate
Leave on Auto unless you have a Manual D calculation. Tap to continue.
Round or rectangular duct?
Round is most efficient. Rectangular fits tight spaces. Tap to continue.
๐ Describe your existing duct
Measure the inside dimensions. Insulation jacket does not count.
Tools I keep on the truck for duct jobs
An anemometer confirms the airflow you actually got. Good mastic keeps it. As an Amazon Associate this site earns from qualifying purchases.
Check Anemometer Prices on Amazon Insulated R8 Flex Duct Duct Mastic Sealant Duct Insulation Wrap๐ Reference & Guides
๐ Quick answers
โ How it works
๐ Velocity limits
๐ CFM chart
โก Size by tonnage
โ Flex vs metal
๐ Rectangular ducts
๐จ Range hood sizing
โ Common mistakes
โ FAQ
Duct Sizing Rules of Thumb (Quick Answers)
These are the numbers I give clients before pulling out the ductulator. The calculator above refines them for your exact situation.
- 3-ton AC (1,200 CFM) main trunk: 16-inch round metal, or 14ร10 rectangular.
- Standard bedroom (144 sq ft, 144 CFM): 7-inch round metal, 8-inch flex.
- Return duct for 3-ton system: 18-inch round or equivalent rectangular.
- Rule of thumb CFM: 400 CFM per ton for most of the US; 350 CFM in humid climates.
- Velocity limit for quiet operation: under 700 FPM in bedroom branches, under 900 FPM in trunk lines.
- Flex duct penalty: size up one standard size vs sheet metal for the same airflow.
Shop duct fittings and materials
Everything you need to run a new branch or extend a trunk. As an Amazon Associate this site earns from qualifying purchases.
Galvanized Round Duct Pipe Insulated Flex Duct Register Boots Takeoff Collarsโ How This Duct Calculator Works
This tool uses the equal friction method, the same approach behind the ASHRAE friction chart and the ductulator wheel every senior tech carries. You pick a design friction rate, and the calculator finds the smallest standard duct size that moves your CFM without exceeding that rate.
It then runs a second check on air velocity. A duct that passes the friction test can still whistle or roar if air moves too fast. The size you see is the smallest standard diameter that passes both tests simultaneously.
For rectangular ducts, dimensions are converted using the ASHRAE hydraulic equivalent diameter formula. A rectangular duct always needs more cross-sectional area than its round equivalent because corners create additional friction.
The tonnage mode converts equipment size to CFM first using the climate-adjusted CFM-per-ton value from ACCA Manual D, then runs the same friction and velocity calculation. The room mode uses the 1.0 CFM/sq ft design factor from ACCA Manual J with adjustments for room type.
๐ Air Velocity Limits That Keep Ducts Quiet
Velocity (FPM) is airflow divided by duct cross-sectional area. Exceed the limits below and you get hiss at registers, rumble in walls, and extra static that your blower has to fight.
| Duct application | Quiet target (FPM) | Maximum (FPM) |
|---|---|---|
| Supply trunk (residential) | 700 to 800 | 900 |
| Supply branch to bedroom | 500 to 600 | 700 |
| Supply branch to other rooms | 600 to 700 | 700 |
| Return trunk | 600 to 700 | 700 |
| Return branch | 500 to 600 | 600 |
| Exhaust (kitchen / bath) | 800 to 1,200 | 1,800 |
Bedrooms deserve the quiet end of every range. I have never had a callback because a duct was too quiet. I have had many because a duct was too loud.
๐ Round Duct CFM Capacity Chart
Values below are computed from the same ASHRAE friction formula the calculator uses. Metal column uses 0.08 in. wc/100 ft for trunks and 0.10 for branches. Flex column adds the 1.5 roughness correction factor.
| Diameter | Metal branch (0.10) | Flex branch (0.10) | Metal trunk (0.08) | Flex trunk (0.08) |
|---|---|---|---|---|
| 6 in | 109 CFM | 88 CFM | 97 CFM | 78 CFM |
| 7 in | 163 CFM | 132 CFM | 145 CFM | 117 CFM |
| 8 in | 232 CFM | 188 CFM | 207 CFM | 167 CFM |
| 9 in | 317 CFM | 256 CFM | 282 CFM | 228 CFM |
| 10 in | 419 CFM | 338 CFM | 372 CFM | 301 CFM |
| 12 in | 678 CFM | 548 CFM | 603 CFM | 487 CFM |
| 14 in | 1,019 CFM | 823 CFM | 906 CFM | 732 CFM |
| 16 in | 1,450 CFM | 1,172 CFM | 1,290 CFM | 1,042 CFM |
| 18 in | 1,978 CFM | 1,598 CFM | 1,760 CFM | 1,422 CFM |
| 20 in | 2,612 CFM | 2,110 CFM | 2,325 CFM | 1,878 CFM |
โก Duct Size by AC Tonnage (Trunk Sizing Chart)
These are the round metal trunk sizes that result from running the equal friction method at 0.08 in. wc/100 ft with a velocity check at 900 FPM. Use average climate (400 CFM/ton) unless you are in the humid Southeast or the dry Southwest.
| System size | Design CFM (avg) | Round trunk (metal) | Round trunk (flex) | Velocity @ metal size |
|---|---|---|---|---|
| 1.5 tons | 600 CFM | 12 in | 14 in | 764 FPM |
| 2 tons | 800 CFM | 14 in | 14 in | 789 FPM |
| 2.5 tons | 1,000 CFM | 14 in | 16 in | 986 FPM* |
| 3 tons | 1,200 CFM | 16 in | 18 in | 859 FPM |
| 3.5 tons | 1,400 CFM | 16 in | 18 in | 1,003 FPM* |
| 4 tons | 1,600 CFM | 18 in | 20 in | 907 FPM |
| 5 tons | 2,000 CFM | 20 in | 22 in | 916 FPM |
* Rows marked with an asterisk exceed the 900 FPM quiet target. Size up one diameter or use two parallel trunks for quiet operation.
Duct testing and sealing tools
Proper sealing after installation locks in your calculated airflow. As an Amazon Associate this site earns from qualifying purchases.
Shop Duct Mastic Sealant on Amazon Metal Foil Duct Tape (UL 181) Flex Duct Support Strapsโ Flex Duct vs Sheet Metal: When to Use Which
Flex duct is the right choice for the last 6 to 8 feet of a branch run because it absorbs vibration and makes register placement easy. It is the wrong choice for a long trunk or any run where you cannot keep it fully extended and supported every 4 feet.
A kinked or sagging flex duct loses dramatically more airflow than the friction formula predicts. A 90-degree kink in 8-inch flex duct can cut capacity by 50 percent or more. If you have to route a flex duct around an obstruction, use a sheet metal elbow at the turn and flex straight sections on either side.
๐ Rectangular Duct Sizing and the Equivalent Diameter Formula
The ASHRAE equivalent diameter formula converts a rectangular duct to its round equivalent for friction calculations: De = 1.30 ร (aรb)^0.625 / (a+b)^0.25, where a and b are the width and height in inches.
A key point: a 16ร8 rectangular duct has a cross-sectional area of 128 sq in, but its equivalent diameter is about 12.7 inches because the corners add friction. You always need more rectangular area than circular area to move the same CFM at the same friction rate. The aspect ratio (width-to-height) should stay below 4:1 for efficient duct design.
๐จ Range Hood and Exhaust Fan Duct Sizing
Kitchen range hoods need a straight, smooth, short duct run to perform well. The rule: use the duct size printed on the hood itself, never reduce it, and minimize elbows. Each 90-degree elbow adds an equivalent of 10 feet to the run length for a standard 6-inch duct.
For bath exhaust fans, the HVI standard (HVI 916) requires the duct to terminate outside, not into the attic. Minimum duct size is the fan's outlet diameter, typically 4 inches for fans under 80 CFM and 6 inches for fans above 100 CFM. IRC M1505 requires exterior termination with a back-draft damper.
Range hood and exhaust fan ducting
The right duct kit matters as much as the fan. As an Amazon Associate this site earns from qualifying purchases.
Range Hood Duct Kits Bath Fan Duct Kits Roof and Wall Vent Capsโ Common Duct Sizing Mistakes
- Matching the old duct size without question. The existing duct may have been wrong from the start or the system may have been upsized since installation. Always run the calculation.
- Using flex duct for long trunk runs. Flex duct loses significantly more airflow per foot than the label suggests when it is not perfectly straight. Use metal for any run over 8 feet.
- Ignoring velocity. Sizing only for friction rate can produce a duct that moves the CFM but sounds like an airport. Always check velocity against the table above.
- Reducing the range hood duct diameter. Using a 6-inch round duct kit on a 8-inch hood collar cuts airflow by roughly 40 percent. Use the size on the hood label.
- Not sealing joints. An unsealed duct system can lose 20 to 30 percent of conditioned air to the attic or crawl space. Use UL 181-rated mastic or metal foil tape, never standard cloth duct tape which fails at temperature.
- Using flex duct with sags and kinks. A properly sized flex duct that sags between supports loses so much airflow it effectively becomes undersized. Support every 4 feet and keep it fully extended.
- Forgetting the return side. Most residential systems are return-starved. The return duct must be as large as the supply trunk it serves, usually one size larger in practice.
โ Frequently Asked Questions
What size duct do I need for a 3-ton AC?
A 3-ton system moves 1,200 CFM at the standard 400 CFM-per-ton design value. For a main supply trunk in sheet metal at 0.08 in. wc/100 ft, that calls for 16-inch round duct, which moves air at 859 FPM - well within the 900 FPM quiet limit. In flex duct at the same friction rate, size up to 18 inches because flex has 50 percent more friction resistance than smooth metal. The calculator above handles this automatically and also offers rectangular equivalents if your space requires them.
What size flex duct do I need for a bedroom?
A standard 12x12 bedroom requires about 144 CFM (1.0 CFM per square foot). For an 8-inch flex duct at 0.10 in. wc/100 ft, the capacity is 188 CFM, which covers that bedroom with margin. For a longer run or a room with more heat gain (west-facing, lots of glass), size up to 10 inches. I rarely install less than 8-inch flex for any bedroom, even small ones, because a slightly oversized duct runs quieter and handles future load changes without modification.
What is the CFM per ton for HVAC?
The standard design value is 400 CFM per ton for most of the US. In humid climates like the Southeast and Gulf Coast, ACCA Manual D recommends 350 CFM per ton to allow the coil more contact time with the air, which improves dehumidification. In hot, dry climates like Arizona and Nevada, 450 CFM per ton maximizes sensible cooling capacity. The actual value depends on your system's coil, but 400 CFM is right for the vast majority of residential installations outside the humidity belt.
What is the friction rate for residential ductwork?
Residential ductwork is typically designed at 0.06 to 0.10 in. wc per 100 feet. The 0.08 value is the most common starting point and matches the default on most residential ductulator wheels. Return ducts, which are larger and slower, are often designed at 0.05 to 0.06. Short, direct branch runs can go up to 0.10. If you have done a full Manual D calculation and know your available static pressure and total effective length, use that number; otherwise, 0.08 for trunks and 0.10 for branches is a safe residential default.
How do I convert AC tonnage to CFM?
Multiply tons by the CFM-per-ton design value for your climate: 400 for most of the US, 350 for humid climates, 450 for hot dry climates. A 2.5-ton system in Atlanta (humid climate) would be designed for 2.5 ร 350 = 875 CFM. The same system in Phoenix (hot dry) would be 2.5 ร 450 = 1,125 CFM. For furnaces, the formula is: CFM = (BTU ร AFUE) / (1.08 ร temperature rise), where a typical 80,000 BTU 95% AFUE furnace with 55ยฐF rise produces about 1,295 CFM.
What is the maximum velocity for residential ductwork?
For quiet, residential-quality operation: supply trunk lines should stay below 900 FPM, supply branch runs to living areas below 700 FPM, and branches near bedrooms below 600 FPM. Return ducts should stay below 700 FPM for trunks and 600 FPM for branches. These limits come from ASHRAE Handbook of Fundamentals and ACCA Manual D. Exceeding them does not break the duct, but it creates audible airflow noise at registers and increases system static pressure, which reduces total airflow and efficiency.
Why do I need to size up when using flex duct?
Flex duct has a roughness factor of approximately 0.003 feet (versus 0.0001 for smooth sheet metal), which creates about 50 percent more friction resistance per foot than sheet metal at the same diameter. Practically, this means a 7-inch sheet metal branch that moves 145 CFM requires at least 8 inches in flex duct to move the same airflow at the same friction rate. The calculator applies a 1.5 friction correction factor to flex duct automatically. This is why you should never substitute flex duct of the same size for sheet metal in a design that was calculated for metal.
What is the ASHRAE equivalent diameter for rectangular ductwork?
The ASHRAE equivalent diameter formula is: De = 1.30 ร (aรb)^0.625 / (a+b)^0.25, where a and b are the width and height of the rectangular duct in inches. For a 16ร8 rectangular duct: De = 1.30 ร (128)^0.625 / (24)^0.25 = approximately 12.7 inches. This means a 16ร8 duct performs like a 12.7-inch round duct for friction purposes, not like a 16-inch round. The formula shows why aspect ratios above 4:1 become very inefficient - you end up with a lot of duct material that delivers relatively little airflow.
How do I size a return duct?
Use the same friction method as supply ducts but at a lower design friction rate - typically 0.05 to 0.06 in. wc/100 ft - and a stricter velocity limit of 700 FPM maximum. The total return CFM equals the total supply CFM for the zone. A common mistake is making the return duct the same size as the supply trunk: the lower friction rate and velocity limit mean the return almost always needs to be one standard size larger. For a 3-ton system at 1,200 CFM with a 0.06 friction rate, the return calls for 18 inches of round metal versus 16 inches for the supply trunk.
What duct size do I need for a 4-inch range hood?
If your range hood has a 4-inch outlet, you are limited to about 100 to 150 CFM of effective airflow regardless of what the fan motor can push, because a 4-inch round duct at 1,800 FPM maximum moves only 157 CFM. Most installation problems with residential range hoods come from reducing the duct at some point in the run. Use the outlet size printed on the hood as your minimum duct diameter for the entire run, install a smooth metal duct rather than flex, and terminate with a back-draft damper rated for cooking exhaust.
How do I add a duct run to an existing HVAC system?
First, confirm your system has available static pressure capacity for an additional run - an older system operating at or near its designed total effective length may not. Measure the existing trunk size at the proposed takeoff point, check the calculator's capacity mode to confirm the trunk can spare the CFM, then size the new branch for your room's load. Use a properly sized takeoff collar, run metal duct to within 6 feet of the register, then flex the final connection. Seal every joint with mastic and balance the new register with the damper in the boot.
What is Manual D duct design?
ACCA Manual D is the industry standard for residential duct system design, referenced in the International Residential Code (IRC) for HVAC installations. It defines how to calculate total effective length, available static pressure, and friction rate for each duct segment. The equal friction method in this calculator follows Manual D principles with standard residential defaults. For new construction, permitted work, or systems that repeatedly fail to meet comfort expectations, a full Manual D calculation from the equipment's actual static pressure ratings is the right approach rather than a rule-of-thumb calculator.
This calculator is for planning purposes. Duct sizing for new construction or permitted HVAC work should be based on a full ACCA Manual D calculation. All results use ASHRAE equal friction method with standard residential defaults. As an Amazon Associate this site earns from qualifying purchases.