The air
Duct shape
Work out the friction rateStatic pressure, duct length and bends
Enter the static pressure left after the filter, coil, registers and grilles. The friction rate then comes from static pressure × 100 ÷ total effective length.
More costs and pricingLabor buffer, sales tax, air balancing, overhead
See full breakdown
Duct size comes from airflow and speed: divide the airflow by the air velocity to get the area (Area = CFM ÷ FPM), then turn that area into a round diameter or a rectangle. Homes are usually designed at a friction rate of 0.05 to 0.10 in. w.g. per 100 ft, which comes from available static pressure × 100 ÷ total effective length. Plan on about 400 CFM per ton: a 2 ton system moves 800 CFM, which fits a 13 inch round metal trunk at 0.10. Flex duct has more friction than sheet metal, so it often needs one size up.
How to calculate duct size
A duct has to pass two checks: the air has to move slowly enough to stay quiet, and the run has to stay inside your friction budget. This duct size calculator runs both and keeps whichever one asks for the bigger duct.
| Size by velocity keeps the air quiet | Size by friction rate keeps static pressure in budget | |
|---|---|---|
| You enter | Airflow in CFM and the fastest velocity you will allow, in FPM | Airflow in CFM and a friction rate in in. w.g. per 100 ft |
| The math | Area (sq ft) = CFM ÷ FPM, × 144 for square inches | Friction loss per 100 ft = 0.109 × CFM1.9 ÷ diameter5.02, solved for the diameter |
| Round size | Diameter (in) = √(4 × area ÷ π), area in square inches | The diameter that lands exactly on your friction rate |
| Flex duct | Same area, since velocity does not depend on the duct wall | Counted at 1.5 times the friction of sheet metal, so the size comes out bigger |
| Rectangle or oval | Set one side and size the other until its equivalent round matches: 1.30 × (a × b)0.625 ÷ (a + b)0.25 for a rectangle | |
| Final size | Take the larger of the two diameters, then round up to the next whole inch | |
Worked example: a 2 ton system, sheet metal, 0.10 friction rate, 900 FPM limit
The friction method lands on the same 12.8 inches at 0.10, so the run goes in at 13 inches.
When to use this duct size calculator
Reach for this HVAC ductwork calculator whenever a run needs a size, a velocity check or a friction number and the paper ductulator is back in the truck.
- New installs: size every trunk and branch from the room CFM before any pipe gets cut. Getting it right on paper costs far less than tearing out a choked run after the drywall is up, and it keeps the blower from fighting the ducts for the life of the system.
- Renovations and additions: check whether the existing trunk can feed a new room or has to go up a size.
- Comfort and noise complaints: a room that never cools or a register that whistles often traces back to a run that is too small for its airflow. Switch to Airflow, enter the duct you found in the attic and see how much air it can really carry.
- Comparing system sizes: see how the trunk changes between a 2, 3 and 4 ton system.
- Energy efficient designs: an undersized run pushes static pressure up and makes the blower work harder, which costs efficiency and motor life. Check every run here before the layout is final.
- Round, rectangular or oval: turn a round size into a rectangle that fits a joist bay, or check what an old rectangular trunk equals in round.
- Flex or sheet metal: see how much bigger a flex run has to be for the same CFM.
- Pricing the duct job: HVAC contractors can switch to the Price the duct job tab and turn the run length, materials and crew hours into a quote with their own rate, markup and margin. The tab also shows what installed duct usually costs per linear foot, so a quote that sits far outside the normal range stands out before it goes to the customer.
- Checking a contractor's proposal: homeowners can confirm that the duct sizes on a bid make sense for the system going in.
Duct size chart: how much air each size carries
Typical capacities at a 0.10 friction rate and 600 to 900 FPM. Flex carries less than metal of the same diameter.
| Round size | Sheet metal | Flex duct |
|---|---|---|
| 4 in | 35 to 50 CFM | 25 to 40 CFM |
| 5 in | 60 to 80 CFM | 50 to 70 CFM |
| 6 in | 100 to 125 CFM | 75 to 110 CFM |
| 7 in | 150 to 175 CFM | 110 to 150 CFM |
| 8 in | 200 to 250 CFM | 150 to 200 CFM |
| 9 in | 270 to 325 CFM | 220 to 280 CFM |
| 10 in | 350 to 400 CFM | 300 to 360 CFM |
| 12 in | 525 to 700 CFM | 450 to 600 CFM |
| 14 in | 770 to 950 CFM | 650 to 850 CFM |
| 16 in | 1,100 to 1,300 CFM | 950 to 1,200 CFM |
| 18 in | 1,400 to 1,700 CFM | Not listed |
| 20 in | 1,750 to 2,000 CFM | Not listed |
| Rectangular duct | Round equivalent |
|---|---|
| 6 × 6 in | 6.6 in |
| 8 × 6 in | 7.6 in |
| 10 × 6 in | 8.4 in |
| 12 × 6 in | 9.1 in |
| 8 × 8 in | 8.6 in |
| 10 × 8 in | 9.8 in |
| 12 × 8 in | 10.7 in |
| 16 × 8 in | 12.2 in |
| 10 × 10 in | 10.9 in |
| 12 × 10 in | 12.0 in |
| 16 × 10 in | 13.7 in |
| 12 × 12 in | 13.1 in |
A rectangle loses the same pressure as its round equivalent at equal airflow, so read its capacity off the round column. Keep rectangles at 4 to 1 or squarer: a 24 × 4 duct has far more friction than a 12 × 8 of the same area.
How much airflow does each room need?
Work from the cooling load. One ton of cooling moves about 400 CFM, and each room gets its share of that total in proportion to its own load.
| System size | 1.5 ton | 2 ton | 2.5 ton | 3 ton | 4 ton | 5 ton |
|---|---|---|---|---|---|---|
| Airflow at 400 CFM per ton | 600 | 800 | 1,000 | 1,200 | 1,600 | 2,000 |
What friction rate and air velocity should you design to?
Residential systems usually sit between 0.05 and 0.10 in. w.g. per 100 ft, and 0.10 is the common default. A lower rate gives bigger, quieter ducts; a higher rate allows smaller ducts with more noise and more work for the blower.
| Application | Friction rate (in. w.g. per 100 ft) |
|---|---|
| Residential supply | 0.08 |
| Residential return | 0.06 |
| Commercial, low pressure | 0.08 to 0.10 |
| Commercial, medium pressure | 0.10 to 0.20 |
| Commercial, high pressure | 0.20 to 0.60 |
| Industrial | 0.30 to 1.00 |
How do elbows and fittings change the duct size?
Every elbow and takeoff adds resistance you can count as extra feet of straight duct. Total effective length is the straight run plus the equivalent length of every fitting on the longest run, and it sets the friction rate.
| Fitting | Equivalent length, sheet metal | Equivalent length, flex (× 1.5) |
|---|---|---|
| 45 degree elbow | 5 ft | 7.5 ft |
| 90 degree elbow | 10 ft | 15 ft |
| 180 degree return bend | 20 ft | 30 ft |
| Branch takeoff | 25 to 35 ft | 37.5 to 52.5 ft |
Counting only the straight pipe can understate the resistance of a run by 30% to 50%. Leave at least 2 ft of straight duct between a takeoff, fitting or register and the next bend.
What changes the duct size?
The same 800 CFM can need a 13 inch round metal run or a 14 inch flex run. These inputs move the answer.
| Factor | What changes the size | Typical effect |
|---|---|---|
| Airflow (CFM) | More air needs more area. | 400 CFM fits 10 in round; 1,200 CFM needs 16 in |
| Friction rate | A lower rate leaves less pressure for each foot of duct, so every run gets bigger. | A 10 in round carries about 419 CFM at 0.10 and 372 CFM at 0.08 |
| Velocity limit | Faster air allows a smaller duct but turns into noise at the registers. | Supply under 900 FPM, return under 700 |
| Duct material | Flex has a corrugated inside wall that adds friction. Sag, kinks and compression add more, so flex only performs when it is pulled tight. | 10% to 20% less CFM than metal of the same size; often one size up |
| Shape | Round has the lowest friction for its area. A rectangle fits framing cavities but loses more to friction. | A 12 × 8 rectangle equals a 10.7 in round |
| Fittings | Each elbow and takeoff counts as extra feet of duct. | A 90 degree elbow adds about 10 ft |
| Static pressure | The filter, coil, registers and grilles take their share of the blower's pressure first. The less that is left, the larger the ducts. | 0.29 left from a 0.5 blower in the example above |
| Long flex runs | Multiple bends or a long pull of flex adds friction quickly. | Go one size up past 25 ft or with several bends |
How do HVAC pros size ductwork?
ACCA Manual D is the standard for residential duct design. The field version comes down to six steps, and equal friction is the usual method for homes:
- Run the loadDo a Manual J load for the whole house and for each room.
- Find the tonnageDivide the whole house load by 12,000 BTU per ton. A 28,000 BTU load works out to 2.33, which rounds up to a 2.5 ton unit.
- Set system airflowTons × 400 CFM.
- Split it by roomRoom load ÷ house load × system CFM. A room with lots of windows or direct sun can need close to 2 CFM per square foot, so do not shortcut this one with floor area alone.
- Set the friction rateAvailable static pressure × 100 ÷ total effective length of the longest run.
- Size each runEnter the room CFM and friction rate, check velocity, and take the bigger size. Velocity reduction and static regain are the alternatives, mostly on larger commercial systems with long trunks.
The Size a duct tab handles steps 5 and 6.
How much does ductwork cost to install?
Installing ductwork averages about $9,293 for a 2,500 sq ft home with around 275 linear feet of duct, and most jobs land between $1,436 and $17,149. Material and access move the price more than anything else.
| Duct material | Replace existing, per linear ft | New, no ducts there now, per linear ft |
|---|---|---|
| Galvanized sheet metal | $21.38 to $62.36 | $19.51 to $58.95 |
| Galvanized spiral | $9.06 to $37.14 | $6.81 to $30.33 |
| Flex, uninsulated | $6.95 to $28.95 | $5.22 to $20.76 |
| Flex, insulated | $6.75 to $34.19 | $5.43 to $26.00 |
A 1,200 sq ft house runs about $689 to $8,232 and a 3,500 sq ft house $2,009 to $24,008. Permits usually add $50 to $400, and hauling away old duct $100 to $800.
What profit margin should an HVAC business aim for?
Set the margin in the Price the duct job tab against these. Gross margin covers the job itself, labor and materials only. Net margin is the slice that survives overhead, trucks, insurance and the owner's own salary.
| Margin | Industry average | Well run HVAC companies |
|---|---|---|
| Gross profit margin | 30% to 40% | 50% to 55%+ |
| Net profit margin | About 10% | 17% to 20%+ |
A net margin under 10% leaves a shop exposed to slow seasons, cash crunches and new competition.
HVAC owners: the first hot week decides who gets the duct work.
No cool calls pile up the moment the heat arrives, and a homeowner sweating through voicemail will book whoever picks up first. ServiceAgent answers every call day and night, books the visit into your calendar and follows up on open quotes, so the replacements and duct jobs land with your crew.
Book a free demoFrequently asked questions
Quick answers on duct sizes, airflow, friction rates and how this ductulator works.
Book a free demoWhat size duct do I need for a 12x12 room?
A 12 x 12 room is 144 sq ft, which works out to about 115 to 144 CFM at 1 CFM per 1 to 1.25 sq ft. At a 0.10 friction rate that is a 7 inch round sheet metal run. In flex, the 144 CFM end needs an 8 inch duct.
What size duct do I need for 5,000 CFM?
At a 0.10 friction rate, 5,000 CFM needs a round duct of about 25.6 inches, so 26 inches. If you also hold the air to 900 FPM, the size grows to 32 inches. Enter your own velocity limit above to see the size in between.
How do I calculate ductwork for an HVAC system?
Find the airflow first: tons of cooling × 400 CFM for the system, then room load ÷ house load × system CFM for each room. Divide each run's CFM by the velocity you will allow to get the area, convert that to a diameter, and check it against your friction rate. Use the bigger of the two sizes.
How many CFM per ton should I use?
About 400 CFM per ton of cooling is the usual baseline. Humid climates often drop to 350 CFM per ton to pull more moisture out of the air.
What friction rate should I use for duct sizing?
Residential systems usually sit between 0.05 and 0.10 in. w.g. per 100 ft, with about 0.08 for supply and 0.06 for return. For an exact number, take the available static pressure × 100 and divide by the total effective length of the longest run.
Does flex duct need to be bigger than sheet metal?
Usually, yes. Flex has a corrugated inside wall, so it is counted at about 1.5 times the friction of sheet metal and carries roughly 10% to 20% less air at the same diameter. Go one size up when a flex run has several bends or runs past 25 ft.
How accurate is this duct size calculator?
It uses the standard equal friction equation for round galvanized duct, the velocity method and the published equivalent diameter formulas for rectangular and oval duct. That is right for planning and checking residential and commercial runs. A final design should still follow ACCA Manual D for the actual house, equipment and fittings.
What is a ductulator?
A ductulator is the round slide rule HVAC techs use to match airflow, friction rate, velocity and duct size. This online ductulator does the same math and adds rectangular and oval sizes, flex duct and the friction rate from your static pressure.
Can I oversize ducts to cut noise?
Larger ducts slow the air down, which makes them quieter, but they cost more and an oversized run can drop velocity so far that air no longer reaches across the room. Size to your velocity limit instead of guessing big.
Does the duct calculator include installation costs?
The Size a duct tab only sizes the duct. The Price the duct job tab builds the cost: crew hours, rate, materials, markup and margin, compared with typical installed prices of $21.38 to $62.36 per linear foot for sheet metal and $6.75 to $34.19 for flex when replacing existing ducts.
Leads growth and product marketing for ServiceAgent at SaaS Labs, where he has worked in product marketing since 2020. Writes ServiceAgent's coverage of AI call answering, CRM and home services operations.
NATE certified Master HVAC Technician with 14 years in residential and light commercial HVAC. Former Senior Service Tech at Trane Austin, now reviews HVAC content for ServiceAgent.
Published by ServiceAgent, the AI front office for home service teams. Duct sizes and prices are estimates for planning. A final duct design should follow ACCA Manual D for the actual house, equipment and fittings, and every price should come from a written quote.