HVAC Duct Sizing Chart & Calculator
Enter your airflow (CFM) and friction rate to instantly find the correct round or rectangular duct size. The built-in chart shows the full sizing table at your chosen friction rate. All calculations use the ACCA Manual D equal-friction method — the industry standard for residential duct design.
Inputs
Default 0.08 = ACCA Manual D residential standard
Formula & Variables
Round duct diameter (ASHRAE equal-friction method):
d = (0.109136 × Q^1.9 / fr)^(1 / 5.02)- d = round duct inside diameter (inches)
- Q = airflow (CFM — cubic feet per minute)
- fr = friction rate (in.wg per 100 ft of duct)
- This is the ASHRAE Fundamentals round-duct friction-loss equation (Huebscher form) for galvanized steel and standard air (0.075 lb/ft³), solved for diameter. It reproduces published ACCA Manual D / ASHRAE friction-chart values — e.g. a 6" round duct carries ≈ 100 CFM at ~0.085 in.wg/100 ft.
Rectangular equivalent diameter (ASHRAE Fundamentals, equal-friction):
D_eq = 1.30 × (a × b)^0.625 / (a + b)^0.25- D_eq = equivalent round diameter (inches)
- a, b = rectangular duct width and height (inches)
Velocity: v (ft/min) = Q / (π × (d/24)²)
Duct Sizing Chart — CFM to Round Duct Diameter
At standard 0.08 in.wg/100ft friction rate, galvanized steel round duct. Highlighted row = closest match to your entered CFM.
| CFM | Calculated dia. (in) | Nearest standard (in) | Velocity (ft/min) | Velocity rating |
|---|
Standard sizes: 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 24 in. For whole-house design, consult a certified HVAC professional and a Manual D calculation.
How to Use This Duct Sizing Calculator
Enter the airflow in CFM for the duct section you are sizing. For branch ducts, this is the CFM delivered to a single room or zone. For trunk ducts, it is the total CFM downstream of that point in the system.
Leave the friction rate at 0.08 in.wg/100ft unless you have a specific reason to change it (see FAQ below). Select "Flexible duct" if you are sizing flex — the calculator automatically adds 2 inches to account for corrugation friction loss per ACCA Manual D guidance.
To get a rectangular duct size, enter a preferred duct width. The calculator returns the required height using the ASHRAE equivalent-diameter formula: Deq = 1.30 × (a×b)0.625 / (a+b)0.25.
Need to calculate CFM from room dimensions first? Use the CFM calculator. For flex duct specifics, see the flex duct sizing chart. For return air duct sizing, see the return air calculator.
Frequently Asked Questions
The ACCA Manual D standard for most residential systems is 0.08 in.wg per 100 ft of equivalent duct length. This assumes roughly 0.50 in.wg total external static pressure with typical losses for filters, coils, and grilles. If you have a high-efficiency filter or a long duct run, use 0.06. For shorter, simpler systems you can use 0.10.
The calculator uses the ASHRAE Fundamentals round-duct friction equation (Huebscher form), d = (0.109136 × Q^1.9 / fr)^(1/5.02), for standard air (0.075 lb/ft³) and galvanized steel duct. This is the same equal-friction relationship behind published ACCA Manual D / ASHRAE ductulator tables. Results are a sizing estimate — for whole-house design, a licensed HVAC contractor should run a full Manual D calculation.
Yes. Always round up (never down) when selecting a standard duct size. Undersized duct increases static pressure, reduces airflow, causes noise, and makes the blower work harder. The nearest standard sizes are 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, and 24 inches.
Branch ducts: 600–700 ft/min is comfortable and quiet. Trunk ducts can run 800–1000 ft/min. Above 1000 ft/min in branches or 1200 ft/min in trunks you may notice air noise. Very low velocity (below 400 ft/min) can cause stratification issues in cooling mode.
ACCA Manual D recommends sizing flexible duct 1–2 inches larger than equivalent rigid metal to compensate for the higher friction of the corrugated inner liner. This calculator adds 2 inches when "Flexible duct" is selected, which is the conservative (safe) approach. Always fully extend and support flex duct — compressed or sagging flex dramatically increases friction loss.
Guides & Resources
Everything you need to understand duct sizing from first principles — friction rates, Manual D, flex duct derating, and more.