Ductwork Sizing Calculator
Size HVAC ducts for a US home. Enter the airflow (CFM) and design targets to get the required round duct diameter, the actual air velocity (FPM) and friction rate, plus rectangular and flex-duct equivalents — with size tables and a downloadable PDF report.
| Round | Velocity | Friction/100ft | Status |
|---|---|---|---|
| Enter airflow to see sizes | |||
| Height | Width | Velocity | Equiv round |
|---|---|---|---|
| Enter airflow to see rectangular sizes | |||
How to use the ductwork sizing calculator
Four quick steps turn an airflow in CFM into the round duct size you need, with its actual air velocity and friction rate, plus rectangular and flex-duct equivalents. You also get size tables and a downloadable PDF report.
Enter the airflow
Type the CFM this duct carries (about 400 CFM per ton for a trunk), and pick trunk, branch or return.
Choose the duct type
Round metal, rectangular metal or insulated flex. The headline size switches to match your choice.
Set design targets
Pick a friction rate (0.10 is standard) and a noise cap on velocity, from 600 FPM (very quiet) to 1,200 FPM.
Read results & save a PDF
See the size, velocity, friction, rectangular and flex options, size tables and a chart. Then download the report.
How duct size is calculated
The recommended size is the smallest standard round duct that stays at or below both your friction rate and your velocity cap. Friction uses the ASHRAE galvanized-duct equation. Here is the exact math.
Worked example: 3-ton supply trunk, 1,200 CFM, 0.10 in/100 ft, 900 FPM cap
14″ round: velocity 183.35 × 1,200 ÷ 14² ≈ 1,123 FPM · friction ≈ 0.14 (too fast, too much friction)
16″ round: velocity 183.35 × 1,200 ÷ 16² ≈ 859 FPM · friction 0.109 × 1,200^1.9 ÷ 16^5.02 ≈ 0.07 (OK)
Cross-section: π ÷ 4 × (16 ÷ 12)² ≈ 1.40 ft²
Rectangle: 10 × 23 (De ≈ 16.2″) · flex: 16 + 1 = 17"
≈ a 16″ round trunk (859 FPM, 0.07 in/100 ft), or a 10×23″ rectangle, or 17″ flex. At a 700 FPM cap it steps up to 18″.
Duct sizing charts
Lookups for round-duct capacity, the recommended size by CFM, rectangular equivalents and system airflow. All values use the same equations and standard sizes as the calculator, for smooth galvanized metal duct.
| Round size | Trunk (0.10, 900 FPM) | Branch / return (0.10, 700 FPM) | Quiet (0.06, 600 FPM) |
|---|---|---|---|
| 4″ | 37 | 37 | 28 |
| 5″ | 67 | 67 | 51 |
| 6″ | 109 | 109 | 83 |
| 7″ | 163 | 163 | 125 |
| 8″ | 232 | 232 | 178 |
| 9″ | 317 | 309 | 242 |
| 10″ | 419 | 382 | 320 |
| 12″ | 678 | 550 | 471 |
| 14″ | 962 | 748 | 641 |
| 16″ | 1,257 | 977 | 838 |
| 18″ | 1,590 | 1,237 | 1,060 |
| 20″ | 1,964 | 1,527 | 1,309 |
| 22″ | 2,376 | 1,848 | 1,584 |
| 24″ | 2,827 | 2,199 | 1,885 |
The lower of the friction limit and the velocity limit. Small ducts are limited by friction, larger ones by velocity. For flex, use the next size up.
| Airflow | 0.10 · 900 FPM | 0.10 · 700 FPM | 0.08 · 700 FPM | 0.06 · 600 FPM |
|---|---|---|---|---|
| 50 CFM | 5″ | 5″ | 5″ | 5″ |
| 75 CFM | 6″ | 6″ | 6″ | 6″ |
| 100 CFM | 6″ | 6″ | 7″ | 7″ |
| 150 CFM | 7″ | 7″ | 8″ | 8″ |
| 200 CFM | 8″ | 8″ | 8″ | 9″ |
| 300 CFM | 9″ | 9″ | 10″ | 10″ |
| 400 CFM | 10″ | 12″ | 12″ | 12″ |
| 600 CFM | 12″ | 14″ | 14″ | 14″ |
| 800 CFM | 14″ | 16″ | 16″ | 16″ |
| 1,000 CFM | 16″ | 18″ | 18″ | 18″ |
| 1,200 CFM | 16″ | 18″ | 18″ | 20″ |
| 1,600 CFM | 20″ | 22″ | 22″ | 24″ |
| 2,000 CFM | 22″ | 24″ | 24″ | 26″ |
Smallest standard round metal size meeting both targets. Typical room branches are 50–200 CFM; trunks are the system total.
| Round | 6″ high | 8″ high | 10″ high | 12″ high | 14″ high | Best fit |
|---|---|---|---|---|---|---|
| 6″ | 6×6 | — | — | — | — | 6×6 |
| 8″ | 6×10 | — | — | — | — | 6×10 |
| 9″ | 6×12 | 8×9 | — | — | — | 6×12 |
| 10″ | 6×15 | 8×11 | — | — | — | 6×15 |
| 12″ | 6×23 | 8×16 | 10×13 | — | — | 8×16 |
| 14″ | — | 8×22 | 10×17 | 12×14 | — | 8×22 |
| 16″ | — | 8×30 | 10×23 | 12×19 | 14×16 | 10×23 |
| 18″ | — | — | 10×30 | 12×24 | 14×20 | 10×30 |
| 20″ | — | — | 10×37 | 12×30 | 14×25 | 12×30 |
Same friction as the round size, rounded up to whole inches. “—” means the shape would be taller than wide or flatter than 4:1. Best fit is closest to 2.5:1.
| System size | Airflow (~400 CFM/ton) | Round trunk | Flex trunk | Rect. trunk |
|---|---|---|---|---|
| 1.5 ton | 600 CFM | 12″ | 13″ | 8×16 |
| 2 ton | 800 CFM | 14″ | 15″ | 8×22 |
| 2.5 ton | 1,000 CFM | 16″ | 17″ | 10×23 |
| 3 ton | 1,200 CFM | 16″ | 17″ | 10×23 |
| 3.5 ton | 1,400 CFM | 18″ | 19″ | 10×30 |
| 4 ton | 1,600 CFM | 20″ | 21″ | 12×30 |
| 5 ton | 2,000 CFM | 22″ | 23″ | 14×30 |
At 0.10 in/100 ft and a 900 FPM cap. High-efficiency or variable-speed systems may use 350–450 CFM per ton. Check the equipment’s spec sheet.
| Setting | Typical use |
|---|---|
| 0.10 in/100 ft | classic rule-of-thumb friction rate |
| 0.08 in/100 ft | common result of a Manual D static-pressure budget |
| 0.06 in/100 ft | larger, quieter, lower-energy ducts |
| 900 FPM | supply trunks |
| 700 FPM | branches and returns |
| 600 FPM | very quiet rooms and bedrooms |
The duct-role chip shows a typical target velocity (800 FPM for trunks, 600 for branches and returns) for reference. Sizing uses the friction rate and velocity cap you set.
Everything the calculator works out
From one airflow and a couple of design targets, the calculator sizes the duct in all three common forms and shows how nearby sizes compare.
Key figures behind the math
Built for planning HVAC ductwork
From a new room addition to a quick check on a contractor’s plan, the same calculator turns airflow into a duct size.
Dealing with a room that won’t cool, noisy registers or high static pressure, and wants to know whether the ducts are undersized.
- Enter the room’s branch CFM
- Compare with the existing size
- Bring the PDF to your HVAC pro
Adding a room or finishing a basement and needs the right branch size, and a rectangle that fits in a joist bay.
- Use the rectangular equivalents
- Upsize 1″ if using flex
- Keep the velocity cap at 700
Wants a fast size check on site, or a simple way to show a customer why the trunk needs to be bigger.
- Set the Manual D friction rate
- Compare round vs rectangle
- Hand the PDF to the client
7 tips for sizing ductwork
Ducts that are too small are the most common cause of noisy, uneven and inefficient HVAC. These points keep your sizing realistic. It’s still a per-duct estimate, not a system design.
Ductwork sizing calculator FAQ
The airflow, friction and duct-size questions people ask most about residential ductwork.
Pick the smallest standard round size that stays within both your friction rate and your velocity (noise) limit.
At the default 0.10 in/100 ft and 900 FPM, a 1,200 CFM supply trunk needs a 16″ round duct. It runs at about 859 FPM and 0.07 in/100 ft. A 14″ duct would run 1,123 FPM and 0.14 in/100 ft, which is too fast and too much friction. A 100 CFM branch at a 700 FPM cap takes a 6″ duct (about 509 FPM).
The calculator also gives the rectangular and flex-duct equivalents for the same airflow.
At a 0.10 in/100 ft friction rate and a 700 FPM branch limit, round metal duct carries roughly 6″ ≈ 109 CFM, 8″ ≈ 232, 10″ ≈ 382, 12″ ≈ 550 and 14″ ≈ 748 CFM.
Small ducts are limited by friction; larger ducts are limited by velocity. Raise the velocity cap to 900 FPM and a 14″ trunk handles about 962 CFM.
Flex duct carries less for the same nominal size, which is why it’s usually upsized by an inch.
0.10 in. w.c. per 100 ft is the classic rule of thumb. 0.08 is common when a Manual D static-pressure budget comes out tighter, and 0.06 gives larger, quieter ducts.
In a proper ACCA Manual D design, the friction rate comes from the blower’s available static pressure divided by the total effective length of the longest run, including fittings.
A lower friction rate means bigger ducts and less fan energy. A higher one means smaller ducts but more static pressure and noise.
Common residential limits are about 900 FPM for supply trunks, 700 FPM for branches and returns and 600 FPM for very quiet designs. 1,200 FPM is a practical maximum.
Faster air is louder at registers and grilles and raises static pressure. Returns are usually kept slower because the grille sits in the living space.
The calculator upsizes the duct whenever it would exceed your chosen cap.
Use the equal-friction equivalent diameter: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. A rectangle with the same De has the same friction at the same airflow.
A 16″ round matches about 10×23, 12×19, 14×16 or 8×30. The calculator picks the option closest to a 2.5:1 aspect ratio and keeps it at 4:1 or less.
Flatter ducts use more metal and have more friction for the same area. Only go wide and shallow when the space requires it.
Flex duct’s corrugated inner liner creates much more friction than smooth metal, and more still if it sags, kinks or isn’t fully stretched.
The calculator adds 1 inch to the round metal size for flex. A 16″ metal trunk becomes a 17″ flex, and a 6″ branch becomes 7″.
Keep runs short and straight, pull the liner tight, support it every few feet, and avoid tight bends.
A common rule of thumb is about 400 CFM per ton of cooling. A 2-ton system moves about 800 CFM, 3 tons about 1,200 CFM and 4 tons about 1,600 CFM.
Each room’s branch gets a share of that total, based on its heating and cooling load from an ACCA Manual J calculation.
Enter the trunk’s total CFM for the main, and each room’s CFM for its branch.
No. It sizes one duct at a time from an airflow and design targets, using the ASHRAE galvanized-duct friction equation.
A complete ACCA Manual D design starts from a Manual J load, then budgets the blower’s available static pressure across filters, coils, registers and fittings, and uses the total effective length of each run.
Use this tool to estimate and check sizes, then have a licensed HVAC contractor confirm the design.
Estimate Only, Not a Duct Design: Round sizes use the ASHRAE galvanized-duct friction equation (friction = 0.109136 × CFM^1.9 ÷ D^5.02), and rectangular equivalents use the equal-friction equivalent-diameter formula. Flex sizes are a simple 1-inch upsize. The calculator sizes one duct at a time from the airflow and targets you enter. It is not an ACCA Manual J load calculation or a Manual D system design, and it does not account for total effective length, fittings, filters, coils, registers, available static pressure or duct leakage. Ductwork must meet your local mechanical and energy codes. Confirm sizes with a Manual D design and a licensed HVAC contractor before buying or installing. For planning purposes only.