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This duct size calculator finds the right duct size for an airflow. Enter the CFM (or the system size in tons) and the design friction rate, and it calculates the exact round duct diameter, the next standard size, the air velocity and an equivalent rectangular duct for a given height.
How to Use the Duct Size Calculator
- Enter the airflow in CFM, or the system size in tons (400 CFM per ton is the usual rule).
- Enter the design friction rate. Homes are commonly designed at 0.08–0.10 in. w.c. per 100 ft.
- Optionally enter a duct height to get an equivalent rectangular size.
Duct Sizing Formula
Friction (in. w.c./100 ft) = 0.109136 × CFM^1.9 ÷ Diameter^5.02
This ASHRAE-based approximation for round galvanized steel duct is solved for the diameter. The rectangular equivalent uses the Huebscher equation: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25.
Example
400 CFM at 0.08 in. w.c. per 100 ft needs a 10.3 in round duct, so use a 12 in duct (velocity about 510 ft/min) or an 8 × 16 in rectangular duct.
Round Duct Capacity Chart
Approximate airflow at 0.08 in. w.c. per 100 ft:
| Round duct | Approx. CFM |
|---|---|
| 6 in | 97 |
| 7 in | 145 |
| 8 in | 207 |
| 10 in | 372 |
| 12 in | 603 |
| 14 in | 906 |
| 16 in | 1,290 |
| 18 in | 1,760 |
| 20 in | 2,325 |
Recommended Velocities
| Duct | Typical residential velocity |
|---|---|
| Supply trunk | 700–900 ft/min |
| Supply branch | 500–700 ft/min |
| Return | 500–700 ft/min |
Higher velocities cause noise; lower velocities need larger ducts. Flexible duct has more friction than metal, so size it larger or keep runs short and straight.
Sources
Frequently Asked Questions
What size duct for 400 CFM?
At 0.08 in. w.c. per 100 ft, about a 10–12 in round duct or an 8 × 16 in rectangular duct.
How many CFM per ton?
Central air systems typically move about 400 CFM per ton of cooling (350–450 CFM depending on climate).
What friction rate should I use for duct design?
0.08–0.10 in. w.c. per 100 ft is common for homes; Manual D sets it from the blower’s available static pressure.
How many CFM does a 6 inch duct carry?
Roughly 100 CFM at 0.08 in. w.c. per 100 ft in smooth metal duct, and less in flex duct.
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