Check the math on the duct you have, then get the one measurement that settles the question.
13.5 inthe round duct an 8 × 20 in trunk behaves likeThat trunk has 160 square inches of opening. A 14 in round pipe has 153.9, 4% less, yet carries air better. De = 1.30(ab)^0.625 / (a+b)^0.25, Huebscher’s 1948 equal-friction relation.
The short answer
No chart can size a duct from airflow alone. Size also depends on the pressure left for the ducts and the run’s length, counting every elbow: that is ACCA Manual D.
What decides whether your ducts are big enough is one test: measured total external static pressure against your equipment’s rated maximum. On the defaults, an 8 × 20 in trunk behaves like a 13.5 in round duct.
Check your duct
Enter the size, the airflow and, if measured, the two static pressure figures. No duct size is issued. Nothing is emailed.
—What to do
—Velocity in this duct
—Free area
—Equal-friction equivalent
—Velocity pressure
—Static pressure headroom
What this assumed
—
No duct size is issued: that needs the friction rate and the run’s full length, which is ACCA Manual D. This is a screening tool, not a duct design. Duct, gas and electrical work is licensed work. HyreHVAC installs, services and sells nothing.
How do you read the duct calculator’s results?
The tool returns five descriptions and one verdict. Free area, velocity, velocity pressure and the circular equivalent are descriptions: simple math on the duct you typed in, which you can check with a calculator.
The headroom line is the only verdict. It appears only when you enter a measured static pressure and the equipment’s rated maximum. Leave those at zero and the tool says so instead of filling the gap with an average.
Ducts are big enough for one reason: the blower can push against them at the pressure it was rated for. That is a measurement, not something you can work out from dimensions.
EPA’s ENERGY STAR commissioning checklist sets out the test: two pressure-gauge (manometer) readings, return and supply, added together, then compared with the maximum in your equipment’s installation manual.
In DOE’s review of 44 field-study reports, low airflow showed up in more than 50% of home systems tested. Ask for those two numbers before you accept any claim that your ducts are too small, or big enough.
HyreHVAC analysis: read velocity as a trend and a sound, not a grade. Halve the area and velocity doubles, while velocity pressure roughly quadruples. That is what a squashed run does, and what you hear at a register. We publish no maximum velocity, because the tables behind those limits are proprietary.
Why can’t a CFM chart tell you the right duct size?
Because duct size depends on more than airflow. Search for a duct size and you get a table: 100 CFM needs 6 inches, 400 CFM needs 10, 1,200 CFM needs 16. The tables disagree with each other. That is the first clue.
Duct size depends on airflow and friction rate. The friction rate is the static pressure left for the ducts, after the equipment, coil and filter take their share, divided by the run’s total effective length.
Total effective length is the measured length plus an extra allowance for every elbow, branch, boot and transition. Two houses moving the same 1,200 CFM can need different ducts, because one run goes straight and the other turns four corners.
That method is Manual D: Residential Duct Systems, which sizes the duct system to deliver the airflow the equipment needs. It is proprietary, so we do not reproduce its charts. Chart tools quietly pick a friction rate, never say which, and apply it to your house.
HyreHVAC analysis: a duct size from CFM alone is not a shortcut version of Manual D. It is the most common error in home duct work, given a calculator.
Even federal design rules only name the method. UFC 3-410-01 says: “Use either the equal friction method or static regain method to design ducts.” There is no table, because there could not be one.
What do the duct terms mean?
Duct arguments go wrong when two people use one word for different things. These seven matter, in the order they come up.
Design HVAC fan airflow (CFM)
The airflow the blower should move. EPA’s HVAC Design Report records it at Item 5.2, and footnote 34 says it comes from the equipment maker’s performance data. Not from a rule of thumb or square footage.
Free area
The opening the air has. π d² ÷ 4 for a round duct, a × b for a rectangular one. The one number here nobody disputes.
Velocity (ft/min)
Airflow divided by free area. It is what the air is doing in the duct you have, and what you hear. Noise in a supply duct is usually a velocity problem first.
Velocity pressure (in. w.c.)
The pressure from the air’s motion. It rises with the square of velocity, so halving a duct’s area roughly quadruples it. We publish it because you can check the math.
Circular equivalent (De)
The round duct that behaves like your rectangular one at equal friction: De = 1.30 (ab)0.625 ÷ (a + b)0.25. Huebscher derived it in 1948. It lets you compare a basement trunk with a round branch fairly.
Total effective length
The straight length of a run plus an allowance for every fitting. No website can know it for your house. It is why a duct that looks “big enough” can still starve a room four elbows away.
Total external static pressure (in. w.c.)
Everything outside the equipment cabinet the blower pushes against: supply ducts, return ducts, coil, filter.
EPA’s footnote 36: “Design total external static pressure is the pressure corresponding to the Design HVAC fan airflow, inclusive of external components (e.g., evaporator coil, whole-house humidifier, or ≥ MERV 6 filter).”
It is measured, not calculated, and it decides whether your ducts are adequate.
Does duct shape matter as much as size?
Yes. A wide flat trunk looks roomy, but run the same area through the equal-friction equation and it is not. Here is one 160 square inch opening folded seven ways.
13 × 13 in — 1.0:114.2 in
10 × 16 in — 1.6:113.7 in
8 × 20 in — 2.5:113.5 in
6 × 26 in — 4.3:112.8 in
5 × 32 in — 6.4:112.6 in
4 × 40 in — 10.0:112.0 in
3.25 × 49 in — 15.1:111.5 in
Duct
Free area
Circular equivalent
Aspect ratio
Velocity at 1,200 CFM
13 × 13 in
169.0 sq in
14.2 in
1.0:1
1,022 ft/min
10 × 16 in
160.0 sq in
13.7 in
1.6:1
1,080 ft/min
8 × 20 in
160.0 sq in
13.5 in
2.5:1
1,080 ft/min
6 × 26 in
156.0 sq in
12.8 in
4.3:1
1,108 ft/min
5 × 32 in
160.0 sq in
12.6 in
6.4:1
1,080 ft/min
4 × 40 in
160.0 sq in
12.0 in
10.0:1
1,080 ft/min
3.25 × 49 in
159.3 sq in
11.5 in
15.1:1
1,085 ft/min
HyreHVAC calculation: circular equivalents from De = 1.30(ab)^0.625 / (a+b)^0.25 (Huebscher, 1948, as reproduced in the AIVC survey paper). Velocity is airflow ÷ free area. Areas differ slightly because real duct comes in whole inches.
The square duct and the flattest one are within nine square inches of each other, yet behave like round ducts 2.5 inches apart.
The survey paper concludes: “Pressure loss through a rectangular duct is significant higher than a volumetrically equal round one. The higher the aspect ratio, the higher-pressure loss in the rectangular system.”
Past about 4:1, the paper warns that the concept of “equivalent diameter” should not be applied either to low mean velocities of flow (non-turbulent case) or to the duct whose cross-section is far from circular, e.g. rectangular ducts with high aspect ratios. Read the last two rows as a direction, not a specification.
What does your static pressure headroom mean?
Enter a measured static pressure and the rated maximum, and the tool subtracts one from the other. That is the only verdict on this page.
Well below the rating
The blower is working inside its rating, so the ducts are doing their job, whatever they look like. You can stop here. A proposal to “upsize the ducts” is solving a problem you do not have.
Keep the reading. Have it retaken after any work in the air path: a new coil, a filter cabinet, a zoning damper, a new branch. Each one spends from the same budget.
Thin, or at the rating
With less than about a quarter inch of water column left, there is no room for anything new. A filter upgrade, humidifier or zone damper becomes a capacity decision.
The useful next step: ask which part is using the budget: return, supply, coil or filter. The ENERGY STAR checklist takes return and supply readings separately for this reason. A return reading twice the supply points to a return fix, which is usually cheaper.
Over the rating
The blower is pushing more than it was rated for. Airflow drops, and the usual problems follow: less cooling, a coil that can ice, a furnace that trips its high-limit switch, and a variable-speed blower burning power to keep up.
This is common. In DOE’s review of 44 field-study reports, low airflow showed up in more than 50% of home systems tested.
Field studies summarized by LBNL found about 0.5 in. of water for heating-only systems and 0.8 in. with a cooling coil. The DOE furnace test assumes 0.20 to 0.23 in.
The fix is a design job. This is where Manual D stops being abstract and becomes what you pay someone to do.
Where do you find each input?
1
Airflow through this duct (CFM)
If you have an HVAC design report, it is Item 5.2; for one branch, Item 5.5. Without one, use 400 CFM per ton: 1,200 for a 3-ton system. ENERGY STAR uses that number as a product boundary, not a design rule.
2
Round or rectangular
Round is flexible or spiral pipe. Rectangular is the folded sheet-metal trunk in a basement or a boxed-in soffit. Pick what you see. The tool converts either way.
3
Diameter, for a round duct
Read the inside diameter of the metal or the size printed on the flex jacket. Flexible duct is sold by inside diameter. You do not need to cut or open anything.
4
Depth, for a rectangular duct
The smaller side, usually set by the joist bay or soffit. It matters most, because it sets the aspect ratio. It is also the depth used when converting a round duct.
5
Width, for a rectangular duct
The larger side. With depth, it gives the free area, aspect ratio and circular equivalent.
6
Measured total external static pressure
Two manometer readings, return and supply, added together. A technician takes them at marked test holes with the blower at design speed (ENERGY STAR checklist Items 3.2 to 3.5). If nobody has, leave it at zero.
7
Rated maximum external static pressure
In your equipment’s installation manual, usually the highest pressure in the blower performance table. Search the model number plus “installation manual” on the maker’s site. No calculator can supply it.
What formulas does the duct calculator use?
Four lines of math, each checkable with a calculator. That is why they are the only things this page computes.
Geometry and velocity
Free area: round: A = π × d² ÷ 4, in square inches. Rectangular: A = a × b. Divide by 144 for square feet.
Velocity: V = CFM ÷ A (sq ft), in feet per minute. The definition of airflow, rearranged.
Circular equivalent: De = 1.30 (ab)0.625 ÷ (a + b)0.25, in inches. For a round duct and a set depth, the tool solves the same equation for the matching width.
Velocity pressure, worked out, not looked up
Pv = (V ÷ 4005)², in inches of water column, with V in feet per minute.
Standard air is 0.075 lb/ft³, or 0.002331 slug/ft³. Velocity pressure is ½ρV² with V in feet per second, so ½ × 0.002331 × (V/60)² = 3.238 × 10⁻⁷ V² pounds per square foot.
One inch of water column is 62.32 ÷ 12 = 5.193 pounds per square foot. Dividing gives Pv = 6.235 × 10⁻⁸ V², and the square root of its reciprocal is 4005.
HyreHVAC analysis: if a number cannot be traced to its assumptions, it does not belong in a tool you will quote to a contractor.
What are the most common duct sizing mistakes?
Sizing from a CFM chart and calling it a design
The chart assumed a friction rate it did not state and knows nothing about your run. It is right in houses that happen to match, wrong everywhere else, and you cannot tell which.
Judging a trunk by its area
A 4 × 40 in trunk has the same 160 square inches as a 10 × 16 in one, but behaves like a round duct nearly two inches smaller.
Fixing the supply side of a return problem
Small, leaky or too few returns cause many pressure problems, but complaints come from supply registers. Ask for return and supply readings separately, not just the total.
Adding equipment to a system with no pressure left
A higher-MERV filter, humidifier, UV cabinet or zoning damper each spends from the same budget. EPA’s footnote 36 counts the coil, humidifier and filter in it.
Replacing the equipment and keeping bad ducts
A new unit on ducts already over their rating buys a better efficiency rating and the same airflow problem: a smaller bill and the same cold back bedroom.
What should you ask your contractor about ducts?
None of this is homeowner work. It is a short list of numbers to ask whoever is quoting.
1
The measured total external static pressure, both halves
Return and supply readings at marked test holes, blower at design speed, added together (ENERGY STAR checklist Items 3.3 to 3.5, free to download). A contractor who measured it will have it in seconds.
2
The rated maximum from the installation manual
The reading means nothing alone. With the rating next to it, “your ducts are undersized” becomes a claim you can check.
3
Measured airflow against design airflow
Item 3.8 asks whether measured airflow is within ± 15% of design. Room by room, Item 4.2 asks for the greater of ± 20% or 25 CFM. A design airflow nobody checks is a wish.
4
For new or changed ductwork, the Manual D output
Item 5.1 of the design report reads: “Duct system designed for the equipment selected in Section 4, per ACCA Manual D.” If ducts are being added or replaced and no such document exists, ask where the sizes came from.
5
What the fix is, before you agree to it
A new trunk, a new return, a bigger filter cabinet and a blower speed change are four different jobs at four different prices. The reading should name the cause before the quote names the fix.
What changes the duct calculator’s answer most?
1. Whether anyone measured. With a measured pressure and a rated maximum, you get a verdict. Without them, a description. Nothing else comes close.
2. Free area, and so velocity. Going from a 14 in to a 10 in round duct at 1,200 CFM takes velocity from 1,123 to 2,200 ft/min, and velocity pressure from 0.079 to 0.302 in. w.c.: nearly four times higher.
3. Aspect ratio. Same area, flatter shape, smaller equivalent diameter. The seven-row table above shows the whole effect.
4. Airflow. It raises velocity in step and velocity pressure with its square. It is also the input most likely to be a guess.
Left out on purpose: your run’s total length and fittings, duct material, leaks, return layout, coil and filter pressure drop, and the blower’s real performance. Any of them would outweigh the inputs here.
HyreHVAC analysis: that is why the tool gives no duct size. It lacks the terms that would make one meaningful, and so does every chart that gives one anyway.
What doesn’t this duct tool know?
Your run. Not its length, fittings or effective length. Without those there is no friction rate, and without that, no duct size. Only Manual D designs a system.
Your leaks. Air that never reaches the room is invisible to geometry. DOE’s review found duct leakage in 90-100% of systems tested with ducts. Sealing recovered 33% of capacity in the studies it reviewed.
Your equipment. The blower curve, coil, filter and rated maximum pressure are all in your installation manual, not in a form.
Friction rates and velocity limits. Those tables belong to ACCA and ASHRAE and are proprietary. The velocity here is math on your own duct, to show the trend, not a pass mark.
It is not an instruction to do work. Building, cutting or sealing ductwork, drilling test holes, opening equipment, changing blower settings, and anything touching gas or line voltage is licensed work.
HyreHVAC does not install, service or sell HVAC equipment, and holds no data on quotes, invoices or finished jobs. Nothing here is a price we observed. That is also why these tools can tell you to keep the system you have.
What does a worked duct example look like?
Step
What happens
Result
The house
A three-bedroom ranch with a gas furnace, a cooling coil and a 3-ton outdoor unit. One rectangular basement trunk, 8 in deep by 20 in wide, with round branches. Nothing has been measured.
The defaults on this page
1. Airflow
No design report exists, so the common rule stands in: 3 tons × 400 CFM per ton. ENERGY STAR uses that number only in the definition of a “low-static blower coil system”, and repeated in the mid-static and mobile-home definitions: a boundary, not a design rule.
1,200 CFM, flagged as a stand-in
2. Free area
8 × 20 = 160 square inches, or 1.111 square feet. No assumptions.
160.0 sq in
3. Velocity
1,200 ÷ 1.111 = 1,080 feet per minute. Just the definition of airflow, rearranged.
1,080 ft/min
4. Velocity pressure
(1,080 ÷ 4005)² = 0.073 in. w.c. The 4005 is derived on this page from standard air, not looked up.
0.073 in. w.c.
5. Circular equivalent
De = 1.30 (ab)0.625 ÷ (a + b)0.25 with a = 8 and b = 20 gives 13.5 inches. The trunk has 160 square inches of opening but flows like a 13.5 in round pipe, which has 143.
13.5 in round, 2.5:1
6. The surprise
A 14 in round duct has 153.9 square inches, 4% less than the trunk, yet it is the bigger duct by equivalent diameter. Shape beats area.
14.0 in beats 13.5 in
7. The verdict field
Blank, because both pressure fields are zero. The tool says “Get static pressure measured before anything else” and does not grade the duct.
No verdict issued
8. The measurement arrives
A technician reads 0.8 in. w.c. total external static pressure at design fan speed. The installation manual rates the air handler at 0.50.
Measured 0.80, rated 0.50
9. The verdict
Now the subtraction works, and it is negative. The blower is pushing 0.30 in. w.c. more than it was rated for. Every figure above is now just context.
Over by 0.30 in. w.c.
10. What follows
A Manual D review of the system, starting with the return side, which nobody measures. Not a bigger outdoor unit, and not a filter upgrade.
A design question, not a form question
HyreHVAC calculation: each step is the engine running on the page defaults, then with one measurement added. The 0.8 in. w.c. in step 8 is the figure field studies found for systems with a cooling coil, as summarized in the LBNL report cited below.
Steps 1 to 7 are math and give no judgment. Step 8 is one visit with a pressure gauge, and it gives the whole answer. A page of geometry against one measurement: that is the point.
What changes when you change one input?
What changes from the defaults
Verdict
Velocity
Circular equivalent
Velocity pressure
Defaults: 1,200 CFM through an 8 × 20 in trunk, nothing measured
Get static pressure measured before anything else
1,080 ft/min
13.5 in round, at equal friction — 2.5:1 aspect ratio
0.073 in. w.c.
Shape: a 14 in round trunk instead, 4% less metal
Get static pressure measured before anything else
1,123 ft/min
8 × 21.8 in rectangular, at equal friction — 2.7:1 aspect ratio
0.079 in. w.c.
Depth: 6 × 26 in, the same trunk squashed for a joist bay
Get static pressure measured before anything else
1,108 ft/min
12.8 in round, at equal friction — 4.3:1 aspect ratio
0.076 in. w.c.
Depth: 10 × 16 in, the same area, less flattened
Get static pressure measured before anything else
1,080 ft/min
13.7 in round, at equal friction — 1.6:1 aspect ratio
0.073 in. w.c.
Depth: 4 × 40 in, the same 160 sq in, folded flat
Get static pressure measured before anything else
1,080 ft/min
12.0 in round, at equal friction — 10.0:1 aspect ratio
0.073 in. w.c.
Airflow: 800 CFM (a 2-ton system at 400 CFM/ton)
Get static pressure measured before anything else
720 ft/min
13.5 in round, at equal friction — 2.5:1 aspect ratio
0.032 in. w.c.
Airflow: 1,600 CFM (a 4-ton system at 400 CFM/ton)
Get static pressure measured before anything else
1,440 ft/min
13.5 in round, at equal friction — 2.5:1 aspect ratio
0.129 in. w.c.
A single 3.25 × 14 in wall stack carrying 100 CFM
Get static pressure measured before anything else
316 ft/min
6.9 in round, at equal friction — 4.3:1 aspect ratio
0.006 in. w.c.
Measured 0.35 in. w.c. against a rated 0.50
Thin headroom — worth a duct conversation
1,080 ft/min
13.5 in round, at equal friction — 2.5:1 aspect ratio
0.073 in. w.c.
Measured 0.45 in. w.c. against a rated 0.50
At the rating — no room for another restriction
1,080 ft/min
13.5 in round, at equal friction — 2.5:1 aspect ratio
0.073 in. w.c.
Measured 0.80 in. w.c. against a rated 0.50
Over the equipment rating — a duct problem, already
1,080 ft/min
13.5 in round, at equal friction — 2.5:1 aspect ratio
0.073 in. w.c.
Measured 0.62 in. w.c., rated maximum not looked up
Half the answer — find the rated maximum
1,080 ft/min
13.5 in round, at equal friction — 2.5:1 aspect ratio
0.073 in. w.c.
Airflow left blank
Enter the airflow this duct carries
—
13.5 in round, at equal friction — 2.5:1 aspect ratio
—
HyreHVAC calculation: every figure is the engine’s own output, run on September 5, 2026 with one input changed at a time.
Read the Verdict column down. It does not move for any change of duct size, shape or airflow. It moves only in the four rows where someone measured something. That is the finding.
Questions this calculator answers
What size duct do I need for 400 CFM?
No fixed size, despite what charts say. Duct size depends on airflow and friction rate: the pressure left for the ducts divided by the run’s total length, counting every fitting. The same 400 CFM branch can need different sizes in two houses. Check the duct you have here, then ask for a static pressure reading.
How many CFM per ton should my system move?
About 400 CFM per ton is the common rule. The only federal document we found that fixes it uses it inside ENERGY STAR product definitions, not as a design rule. Your real design airflow is Item 5.2 of an ENERGY STAR HVAC Design Report, taken from the maker’s performance data.
What is total external static pressure?
The resistance outside the equipment cabinet that the blower pushes against: supply ducts, return ducts, coil and filter. A technician measures it with a manometer at two test holes, return and supply, and adds the readings. It takes minutes. Drilling into the equipment’s air box is not a homeowner job.
Where do I find my equipment’s maximum static pressure?
In its installation manual, free from the maker by model number. It is usually the highest pressure column in the blower performance table. It belongs to your equipment, so no calculator can supply it. A contractor who says your ducts are too small should be able to name it.
Is round duct better than rectangular?
Yes, for pressure at equal area. The AIVC survey paper finds rectangular duct loses more pressure than an equal round one, and more as it gets flatter. On this page, 160 square inches as 13 × 13 behaves like a 14.2 in round; as 4 × 40, like 12.0 in.
How do I convert a rectangular duct to round?
Use the equal-friction equivalent: De = 1.30(ab)^0.625 / (a+b)^0.25, with a and b the two sides in inches. Huebscher derived it in 1948 and it is the form most used in the US. Above about a 4:1 ratio it becomes less reliable, so treat very flat ducts as a rough guide.
What air velocity is too high in a duct?
We do not publish a limit. The common limits come from ACCA Manual D and the ASHRAE Handbook, which are proprietary, and a made-up limit is worse than none. Velocity is airflow divided by area. Noise at a supply register is usually the first sign it is high.
Are my return ducts too small?
Often, yes. Returns cause many static pressure problems, but nobody checks them because complaints come from supply registers. The ENERGY STAR checklist takes return and supply as separate readings for this reason. A return reading much higher than the supply points to a return fix, which is usually cheaper.
Do I need new ductwork with a new air conditioner?
Only a pressure reading can say. If measured static pressure sits well below the new unit’s rated maximum, the ducts are fine. If it sits above, the new unit inherits the problem. Get the reading before choosing equipment. See the HVAC replacement cost calculator for the duct line.
Can I use this tool to design my ductwork?
No. It gives no duct size from airflow, publishes no friction rate or velocity limit, and knows nothing about your runs. It helps you read a proposal. Designing or changing ductwork, drilling test holes, opening equipment, or touching gas or line-voltage wiring is licensed work.
My contractor says my ducts are undersized. Is that true?
It may be. Low airflow showed up in more than 50% of systems in DOE’s review of 44 field studies. But you cannot check it without two numbers: measured total external static pressure and your equipment’s rated maximum. Ask for both, with return and supply shown separately.
Sources and methodology
Figures dated September 5, 2026. Last reviewed .
ENERGY STAR Single-Family New Homes National HVAC Design Report (Rev. 14) (US Environmental Protection Agency, ENERGY STAR, retrieved 2026-09-05. Version 3.1 / 3.2 / 3.3, Rev. 14, January 2025. Section 5 (Items 5.1 to 5.5) and footnotes 34 and 36. It records airflow and pressure, never a duct size.)
Manual J: Residential Load Calculation (Air Conditioning Contractors of America, retrieved 2026-09-05. Manual D is proprietary. This page names it as the method and reproduces none of its charts or tables.)
UFC 3-410-01, Heating, Ventilating, and Air Conditioning Systems (July 28, 2025) (US Department of Defense, Unified Facilities Criteria, via the Whole Building Design Guide, retrieved 2026-09-05. Quoted at UFC 3-410-01, Heating, Ventilating, and Air Conditioning Systems, July 28, 2025, §2-14.4. A federal design document that names the method and publishes no CFM-to-size table.)
Energy Implications of In-Line Filtration in California Homes (CEC-500-2013-081) (Walker, Faulkner, Dickerhoff and Turner, Lawrence Berkeley National Laboratory, for the California Energy Commission, retrieved 2026-09-05. Field static pressure of about 0.5 in. of water (heating only) and 0.8 in. (with a cooling coil), against a test rating of 0.20 to 0.23 in.)