Understanding your result
The headline is the round duct to use, from the sizes sold (4 to 24 inches), or the rectangular duct if you asked for one, with its velocity and friction in the sub-line. The table shows the two sizes the calculator weighed against each other, the exact diameter for your friction rate and the diameter at which the air would reach the velocity limit for the duct type, and says which governed. Most branch ducts are governed by friction; large trunks and returns are often governed by velocity, and the note explains what that means for noise and pressure.
The velocity and friction in the chosen stock size are the figures to carry forward: the stock size is bigger than the exact one, so the actual friction is lower than the design rate and the actual velocity lower than the limit. If the friction in the stock size falls well below 0.05 in wc per 100 ft, the note suggests checking the size down.
For a rectangular duct the result includes the equivalent round diameter of the duct you will build and its aspect ratio; above 4:1 the duct wastes metal and the note says so. The airflow for a branch comes from the CFM per room calculator; the friction rate for the house comes from the equivalent length calculator; and the duct velocity calculator checks an existing duct the other way round.
How we calculate this
The friction formula is the standard fit to the ASHRAE friction chart for clean galvanised steel duct at standard air, the chart every duct calculator wheel reproduces; its results match the wheel to within a few percent across residential sizes. The velocity limits are the quiet-operation conventions of residential design (ACCA Manual D and common practice). The equivalent diameter formula is ASHRAE’s; it makes a rectangular duct equal to a round one in both airflow and friction, which is why the equivalent has more area than the round duct it replaces.
The duct size chart tabulates CFM against round and rectangular sizes at 0.08 and 0.10 in wc per 100 ft, and the unit conversions page covers CFM to m³/h, inches to millimetres, in wc to pascals and FPM to m/s, which the metric mode uses.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Friction formula | ΔP = 0.109136 × Q^1.9 ÷ D^5.02 | Fit to the ASHRAE friction chart for galvanised duct, standard air |
| Velocity limits | 900 FPM trunk, 700 branch, 600 return | Residential quiet-design conventions (ACCA Manual D, industry practice) |
| Stock round sizes | 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24 in | Residential round duct and flex duct sizes sold in the US |
| Rectangular rounding | Even inches | Sheet-metal shop practice |
| Equivalent diameter | ASHRAE 1.30 (ab)^0.625 ÷ (a+b)^0.25 | ASHRAE Handbook, Fundamentals, duct design |
| Default friction rate | 0.10 in wc per 100 ft | Common residential design rate; derive yours from the equivalent length calculator |
Assumptions last reviewed October 8, 2026.
The calculator sizes a single duct for a single airflow. It does not design a trunk-and-branch system, where the trunk is sized for the sum of the branches downstream of each takeoff, nor does it account for flex duct’s extra friction (use the equivalent length calculator’s flex option, or go up a size), for altitude, or for dirty or lined duct. The guide to duct sizing basics walks through a whole system, and the guide to duct leakage explains why a well-sized leaky duct still fails.
Two worked examples
A branch carrying 400 CFM
400 CFM at 0.10 in wc per 100 ft, treated as a supply trunk (900 FPM limit), round.
- Friction size: (0.109136 × 400^1.9 ÷ 0.10)^(1/5.02) = 9.83 in
- Velocity size at 900 FPM: √(400 × 144 × 4 ÷ (π × 900)) = 9.03 in
- Governed by friction → stock size 10 in round
- In the 10 in duct: 733 FPM, 0.092 in wc per 100 ft
- Rectangular equivalents: 6 × 16, 8 × 12, 10 × 10, 12 × 8 in
The stock 10-inch duct runs a little under the design rate, which is the normal outcome of rounding up. As a branch to a single room 400 CFM is a lot of air; the CFM per room calculator would normally split a living room this size across four registers on 6- or 7-inch branches.
A trunk in a 10 inch joist space
1,200 CFM (a 3-ton system) at 0.08 in wc per 100 ft, supply trunk, rectangular with one side fixed at 10 in.
- Friction size: 15.57 in; velocity size at 900 FPM: 15.64 in
- Governed by velocity → stock round 16 in
- In 16 in round: 859 FPM, 0.070 in wc per 100 ft
- Rectangular: second side 22.3 in → 10 × 24 in (equivalent to 16.5 in round), 720 FPM, aspect ratio 2.4:1
At 1,200 CFM the friction and velocity sizes nearly coincide, and the 10 × 24 rectangular trunk that fits the joist space moves the air at a comfortable 720 FPM. The trunk will be reduced after each takeoff as the airflow falls; each reduced section is sized the same way for the CFM that remains.
Where to find your inputs
Airflow. For a branch, the room’s CFM from the CFM per room calculator; for a trunk, the sum of everything downstream, or 400 CFM per ton for the main trunk.
Friction rate. 0.08 to 0.10 in wc per 100 ft if you have nothing better; the equivalent length calculator derives the right rate from your blower and your longest run.
Duct type. Trunk, branch or return, which sets the velocity limit. Returns run slowest because they are the loudest path to the living space.
Fixed side. The joist depth (usually 7.25 or 9.25 in for 2 × 8 and 2 × 10 joists) or the height of the soffit the duct will run in.
Common mistakes
- Sizing rectangular ducts by equal area. A rectangle needs more area than the round duct it replaces; use the equivalent diameter.
- Using flex sizes from a sheet-metal chart. Flex has about 50% more friction when tight and far more when sagging; go up a size or count it as 1.5 ft per foot.
- Ignoring velocity. A duct that passes the friction check can still whistle; trunks and returns are often velocity-limited.
- Keeping the trunk one size to the end. Reduce it after each takeoff, or the far registers get little air and the near ones too much.
- Guessing the friction rate. It comes from the blower’s static pressure and the longest run; a long flex system may only afford 0.05.
- Perfect sizing, leaky joints. Seal every joint with mastic; a 20% leak wastes more than any sizing error.
Questions people ask
- What size duct do I need for 400 CFM?
- At the common design friction rate of 0.10 inches of water per 100 ft, a 10-inch round duct, with air moving at about 730 feet per minute; at 0.08 the exact size is 10.3 in and most designers would still use 10 in and accept a slightly higher rate. Rectangular equivalents are about 8 × 12 or 6 × 16 inches. A 9-inch duct would carry 400 CFM at 900 FPM and 0.16 in wc per 100 ft, which is too fast and too lossy for a branch.
- What friction rate should I use for residential ducts?
- 0.08 to 0.10 inches of water column per 100 feet of equivalent length is the usual design range for houses with a standard 0.5 in wc blower, and it is what most duct calculators assume. The correct rate for a specific house is the blower's available static pressure divided by the total equivalent length of the longest run, which the equivalent length calculator works out; houses with long flex runs and many fittings may need 0.06, and short sheet-metal systems can use 0.12.
- How many CFM can a 6 inch duct carry?
- About 100 to 120 CFM at a friction rate of 0.08 to 0.10 in wc per 100 ft, with air at 500 to 600 FPM, which is why 6-inch is the standard branch to a single register. Pushed to 160 CFM a 6-inch duct runs at 815 FPM and 0.22 in wc per 100 ft, noisy and starved. The duct size chart lists the comfortable capacity of each size.
- How do I convert a round duct to rectangular?
- By equivalent diameter, not equal area. A rectangular duct has more surface for the same area and loses more pressure, so the equivalent of a 10-inch round duct is not 78 sq in of rectangle but about 8 × 12 (96 sq in). The ASHRAE formula is De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. The calculator solves it for the second side when you fix the first, and rounds to the even inches sheet-metal shops work in.
- What is the maximum velocity for a residential duct?
- Common limits are 900 feet per minute in supply trunks, 700 in branches and 600 in returns, kept low so the ducts and registers stay quiet; registers and grilles use lower face velocities still. Commercial systems run 1,200 to 2,000 FPM because noise is tolerated and ducts are stiffer. When the friction size would exceed the limit, the calculator sizes for velocity instead and says so.
- Does flex duct need a bigger size than sheet metal?
- Yes. Flex duct's ribbed interior has higher friction than smooth metal, about 50% more when it is pulled tight and far more when it sags or is compressed. Designers either go up one size on flex branches or treat each foot of flex as 1.5 ft of equivalent length in the friction-rate calculation, which is what the equivalent length calculator does. Kinked or sagging flex can halve the airflow of a run.