Free · Airflow & ductwork
Duct Size Calculator
Enter CFM and friction rate to get the round duct size, the rectangular equivalent for the joist space you have, velocity against the limit for that duct's job, and a flex duct correction. Or flip it around and see what an existing duct can carry.
Design
Sum of the registers this run feeds. 400 CFM per ton is nominal.
Rectangular equivalent
Set to 0 to skip. Common: 8, 10, 12, 14 in.
Round duct
10"
Velocity
733 fpm
Limit ≈ 900 for a supply trunk
Friction at stock size
0.09
in. w.c. per 100 ft
Rectangular
12" × 8"
Exact width 10.1"
Rect velocity
600 fpm
At the stock width
Velocity is fine
Round duct capacity at 0.1 in. w.c./100 ft
| Diameter | CFM | Velocity (fpm) |
|---|---|---|
| 4" | 37 | 426 |
| 5" | 67 | 492 |
| 6" | 109 | 553 |
| 7" | 163 | 611 |
| 8" | 232 | 666 |
| 9" | 317 | 718 |
| 10" | 419 | 768 |
| 12" | 678 | 863 |
| 14" | 1,019 | 953 |
| 16" | 1,450 | 1,039 |
| 18" | 1,980 | 1,120 |
| 20" | 2,615 | 1,199 |
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How duct sizing works
The calculator uses the friction-loss equation for galvanized round duct that every ductulator is built on, then rounds up to a stock diameter and checks velocity against a limit for the duct's role.
Friction rate is the pressure the duct is allowed to lose per 100 feet of equivalent length. 0.10 is the ductulator default and fine for short, simple runs. Manual D usually lands residential systems at 0.06–0.08 once fittings are counted, so long runs with many elbows should use the lower rate and end up with bigger ducts.
Rectangular equivalents come from the ASHRAE equal-friction formula: De = 1.30 × (a·b)^0.625 ÷ (a + b)^0.25. Enter the height you have to fit in and the calculator solves the width, then rounds up to an even inch. A rectangular duct that matches a round one on friction is always larger in area, which is why its velocity comes out lower.
Velocity limits
| Duct | Residential max | Why |
|---|---|---|
| Supply trunk | 900 fpm | Noise at registers, static pressure |
| Supply branch | 700 fpm | Register noise |
| Return trunk | 700 fpm | Grille noise, filter loading |
| Return branch | 600 fpm | Grille noise |
Flex duct
Flex is sized on the same chart when it is pulled tight and supported every 4 feet, and almost no installed flex is. ASHRAE testing shows a few percent of compression can double the pressure drop. The flex toggle sizes at roughly half the friction rate, which in practice moves the answer one stock size up, and the capacity table re-derates to match. Keep runs short, pull it tight and strap it straight.
Need the CFM first? The CFM calculator gets airflow from tonnage, BTUs or air changes.
Frequently asked questions
What size duct do I need for 400 CFM?
A 10-inch round duct at 0.10 in. w.c. friction rate, running about 730 fpm. At 0.08 it is still 10 inches; at 0.06 it becomes 12 inches. In rectangular duct with 8-inch height, 12 × 8 is the equivalent.
How many CFM can a 6-inch duct carry?
About 105–110 CFM at 0.10 in. w.c. per 100 ft, 95 CFM at 0.08, and 80 CFM at 0.06. As flex duct, plan on 70–80 CFM. That is why a 6-inch branch serves one average register, not two.
What friction rate should I use?
0.10 in. w.c. per 100 ft for short, simple runs and quick checks. For a full Manual D design use the rate that comes out of the available static pressure divided by the total equivalent length, which usually lands between 0.06 and 0.08 in residential work.
How do I convert round duct to rectangular?
Use the equivalent diameter formula, not equal area. A 10-inch round duct is equivalent to 12 × 8, 10 × 10 or 16 × 6 rectangular on friction. Equal area would give you a smaller duct that loses more pressure because of the corners.
Is a bigger duct always safer?
Mostly, within reason. Oversized ducts cost more and can drop velocity so low that supply air does not throw across the room, but that takes a lot of oversizing. Undersized ducts raise static pressure, cut airflow and shorten blower life, and are the far more common problem.