Duct Size Calculator
Size round and rectangular ducts from airflow. Enter CFM and a target velocity to get the round duct diameter, then the calculator converts it to a rectangular size for your available height using the ASHRAE equal-friction equivalent diameter — the correct conversion, not the equal-area shortcut that undersizes rectangular duct. It flags aspect ratios past 4:1 and prints a duct schedule line. Free, no login.
Airflow
Duct Size
Round: A = CFM ÷ velocity. Rectangular by ASHRAE equal-friction De = 1.30·(a·b)^0.625 ÷ (a+b)^0.25 (not equal area). Confirm vs Manual D.
Duct Sizing Schedule
Velocity-method duct sizing — prepared with the NORDIX Duct Size Calculator (nordixhq.com/tools/duct-size-calculator)
| Airflow (CFM) | Design velocity (FPM) | Round duct | Duct area (in²) | Rectangular (W × H) | Equiv. Ø (in) | Rect. velocity (FPM) |
|---|---|---|---|---|---|---|
| 400 | 900 | 9" Ø | 64 | 8.5" × 8" | 9 | 847.06 |
Basis: round duct from A = CFM ÷ velocity (continuity). Rectangular pairing by the ASHRAE equal-friction equivalent diameter De = 1.30·(a·b)0.625 ÷ (a+b)0.25 — not equal area. Velocity-method sizing is a planning aid; confirm against an ACCA Manual D (or equivalent friction-rate) design for the installed system.
How this calculator works
The round size is pure continuity — the cross-section that carries the flow at your chosen velocity. The rectangular pairing is where method matters:
A (ft²) = CFM ÷ velocity · d = √(4A ÷ π) · De = 1.30 × (a·b)^0.625 ÷ (a+b)^0.25
A rectangle with the same area as a round duct has more wall per unit of flow and higher friction, so equal-area conversion undersizes it. This calculator instead uses the ASHRAE equivalent-diameter relation (equal friction and flow) and solves for the width whose De matches the round duct at your chosen height. That equivalent rectangle is slightly larger in area than the round duct, so its actual velocity comes out a little lower than the velocity you entered — correct behavior, and the calculator shows it. Velocity-method sizing is a planning aid: an ACCA Manual D (or equivalent friction-rate) design governs the installed residential system.
Worked example
A supply trunk carrying 400 CFM at a 900 fpm target velocity, with 8 in. of joist depth available (the calculator’s defaults):
- Area: 400 ÷ 900 = 0.444 ft² = 64.0 in².
- Round diameter: √(4 × 64.0 ÷ π) = 9.0 in Ø.
- Equal-friction rectangle at 8 in. height: the width whose De matches 9.0 in. is 8.5 × 8 in (De of an 8.5 × 8 duct = 1.30 × (8.5 × 8)^0.625 ÷ (16.5)^0.25 ≈ 9.0 in).
- Actual rectangular velocity: 400 ÷ (8.5 × 8 ÷ 144) = 847 fpm — lower than 900 because the equal-friction rectangle carries slightly more area.
Enter the same inputs above (400 CFM, 900 fpm, 8 in. height) to reproduce it.
Reference: round duct capacity at 0.10 in. w.g. per 100 ft
The friction-rate view of the same physics — CFM each round galvanized duct carries at the classic 0.10 in./100 ft design rate, with the resulting velocity. Computed from the Darcy-Weisbach equation with the Colebrook friction factor at ASHRAE standard air (0.075 lb/ft³) and 0.0003 ft duct roughness — the same engine behind the duct sizing chart, rounded to the nearest 5 CFM.
| Round duct | CFM at 0.10"/100 ft | Velocity |
|---|---|---|
| 6" Ø | 110 CFM | 565 fpm |
| 8" Ø | 240 CFM | 685 fpm |
| 10" Ø | 435 CFM | 800 fpm |
| 12" Ø | 710 CFM | 900 fpm |
| 14" Ø | 1,070 CFM | 1,000 fpm |
| 16" Ø | 1,525 CFM | 1,090 fpm |
| 18" Ø | 2,080 CFM | 1,180 fpm |
| 20" Ø | 2,750 CFM | 1,260 fpm |
Rigid round metal duct only — flex duct moves far less air at the same friction rate, and altitude or temperature shifts air density.
Frequently asked questions
What velocity should I use to size a duct?
For residential supply trunks, 700–900 fpm is the usual planning range (the calculator defaults to 900); branch runouts are typically sized slower, around 600 fpm, and returns slower still to keep noise down. Commercial mains routinely run faster. Velocity is a noise-and-friction tradeoff: faster means smaller, cheaper duct but more static pressure and more sound. For residential design work, ACCA Manual D — sizing to a friction rate from the actual available static and effective length — is the authoritative method; velocity sizing is the quick first pass.
How do I convert a round duct to rectangular?
Not by matching area. A rectangle with the same cross-sectional area as a round duct has more wall per unit of flow and higher friction, so equal-area conversion undersizes the rectangular duct. This calculator uses the ASHRAE equivalent-diameter relation De = 1.30 × (a·b)^0.625 ÷ (a+b)^0.25 and solves for the width that gives your chosen height the same friction as the round size — which is why the rectangular duct it returns is a bit larger in area than the round one.
Why is the rectangular duct velocity lower than the velocity I entered?
Because the conversion is equal-friction, not equal-velocity. The equivalent rectangular duct has slightly more cross-sectional area than the round duct, so the same CFM moves through it a bit slower. That's correct behavior — the two ducts have matched friction loss per foot, which is what matters for the system — and the calculator shows the actual rectangular velocity so you can see the difference.
What is a good aspect ratio for rectangular duct?
Keep width-to-height at 4:1 or less where you can; the calculator flags anything beyond that. Flat, wide ducts have more perimeter per unit area, which means more sheet metal, more weight, and more friction for the same airflow — costs rise noticeably as the aspect ratio stretches. If you're being forced past 4:1 by a tight joist bay, consider two smaller ducts or a slightly deeper chase before accepting a very flat duct.
What is friction-rate duct sizing and how is it different from velocity sizing?
Friction-rate sizing — the ACCA Manual D approach — starts from the static pressure the blower actually has available, divides it by the total effective length of the longest run (fittings counted as equivalent length), and sizes every duct to that resulting friction rate, commonly landing around 0.08–0.1 in. w.c. per 100 ft in residential work. Velocity sizing, which this calculator currently uses, just picks a target speed. Velocity sizing is fine for quick takeoffs and sanity checks; friction-rate sizing is what verifies the duct system will actually deliver rated airflow on the installed blower.
Can I use these sizes for flex duct?
Treat them as rigid-metal sizes. Flex duct fully stretched already runs higher friction than galvanized, and even modest compression or sag multiplies the loss several times over — a compressed flex run at the same diameter can move far less air than the sheet-metal duct this calculator assumes. If flex is the material, pull it tight, support it properly, and size up (or check it against flex-specific friction data) rather than reading a metal-duct size straight across.
Method: round duct from A = CFM ÷ velocity (continuity); rectangular pairing by the ASHRAE equal-friction equivalent diameter De = 1.30 × (a·b)^0.625 ÷ (a+b)^0.25, solved for width at your height — not equal area. The capacity table is computed (Darcy-Weisbach, Colebrook friction factor, ASHRAE standard air, galvanized duct at 0.0003 ft roughness). Velocity targets are design conventions, not code; keep aspect ratios at 4:1 or less. Fittings, available static, and the equipment fan table set the real system — confirm against an ACCA Manual D or equivalent friction-rate design.
Pair with the duct sizing chart for the full CFM-by-friction-rate table, the rectangular to round conversion chart for every equal-friction equivalent, the duct friction loss chart for loss per 100 ft, or the CFM & Airflow Calculator to get the airflow target first.