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How Duct Sizing Works
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Correct duct sizing balances three factors: delivering enough airflow (CFM), keeping air velocity low enough to avoid noise, and maintaining acceptable static pressure loss (friction rate) so the blower can push air through the system.
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The Equal Friction Method
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The most common residential approach sizes every duct run to the same friction rate (pressure drop per 100 feet). Typical values are 0.06 to 0.10 inches of water gauge per 100 ft. This calculator uses a simplified form of the Darcy-Weisbach equation calibrated to standard galvanized steel duct.
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Round vs. Rectangular
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Round ducts are the most efficient shape -- they have the lowest surface area for a given cross-section, meaning less friction loss and material cost. Rectangular ducts fit into tight spaces but should be kept to a 4:1 aspect ratio or less (2:1 preferred) to maintain good airflow distribution.
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Equivalent Diameter
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A rectangular duct's equivalent round diameter is the round duct size that produces the same friction loss at the same airflow. The formula is:
Deq = 1.3 × (W × H)0.625 / (W + H)0.25
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Quick Reference
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| CFM | Round (in) | Typical Use |
|---|---|---|
| 100 | 6 | Single room branch |
| 200 | 7-8 | Large room branch |
| 400 | 10 | Small trunk |
| 800 | 12-14 | Main trunk |
| 1200 | 16 | Large main trunk |
| 2000+ | 18-20 | Commercial trunk |