HVAC Duct Sizing Chart
Reference chart showing the required round duct inside diameter for common airflow rates. Select a friction rate below — the chart updates instantly. Standard round sizes: 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 24 in.
Chart Settings
Duct Sizing Chart at 0.08 in.wg/100ft
Formula: d = 0.6312 × (Q / √fr)0.4 — equal-friction method, galvanized steel round duct, standard air (0.075 lb/ft³). As tabulated in ACCA Manual D / ASHRAE Fundamentals (as of June 2026).
| CFM | Calculated dia. (in) | Use this size (in) | Velocity (ft/min) | Velocity note |
|---|
Always round up to the next standard size. For flex duct, add 2 inches to the "Use this size" column.
How to Read This Duct Sizing Chart
The Calculated dia. column shows the exact mathematical result of the equal-friction formula for that CFM at the chosen friction rate. The Use this size column shows the nearest standard round duct size at or above the calculated value — this is what you actually order and install.
The Velocity column shows air speed in feet per minute when the standard-size duct is used. Residential guidelines (ACCA Manual D / ASHRAE): branch ducts 600–700 ft/min, trunk ducts 800–1000 ft/min. Above 1200 ft/min you may hear air noise at registers.
For the full interactive calculator (including rectangular duct sizing), see the duct sizing calculator. To calculate required CFM from room dimensions, use the CFM calculator.
Frequently Asked Questions
At the standard 0.08 in.wg/100ft friction rate, 200 CFM requires a calculated diameter of about 8.3 inches — so you would use a standard 9-inch round duct. At 0.10 friction rate it drops to approximately 7.7 inches, so an 8-inch duct is sufficient.
At 0.08 in.wg/100ft, 100 CFM calculates to about 6.6 inches — use a 7-inch duct. At a tighter 0.10 friction rate, a 6-inch duct can handle 100 CFM.
A 6-inch round duct at the 0.08 friction rate handles approximately 75–85 CFM before velocity becomes excessive. Some sources show up to 100 CFM at 0.10 friction rate, but velocity will be above 900 ft/min which can cause noise in residential applications.
Find your required CFM in the left column, then read across to the diameter column at your chosen friction rate. Always select the nearest standard size that is equal to or larger than the calculated diameter. Never round down — an undersized duct increases static pressure and reduces airflow.
The standard duct sizing chart applies to rigid galvanized steel round duct. For flexible duct, add 1–2 inches to the metal diameter — the corrugated inner liner creates significantly more friction than smooth metal.