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 (equal-friction method):
d = 0.6312 × (Q / √fr)^0.4- d = round duct inside diameter (inches)
- Q = airflow (CFM — cubic feet per minute)
- fr = friction rate (in.wg per 100 ft of duct)
- 0.6312 = empirical constant derived from the Darcy-Weisbach equation for air at standard conditions (0.075 lb/ft³) and galvanized steel roughness (ε ≈ 0.0003 ft), matching published ACCA Manual D ductulator tables
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 equal-friction formula (d = 0.6312 × (Q/√fr)^0.4) derived from the Darcy-Weisbach equation for standard air (0.075 lb/ft³) and galvanized steel duct. Results match published ACCA Manual D ductulator tables within ±0.1 inch across the 50–5000 CFM range. 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.