Use Our Ductulator to Calculate Proper HVAC Ductwork Sizing
*Estimated in good faith, the duct size calculator is meant solely for generic, informative purposes. We do not guarantee the accuracy of this information. Please note that other external factors may affect or falsify the recommendations of this tool. For accurate values, consult a licensed heating and air conditioning professional or building engineer.
Our free HVAC duct calculator sizes ducts by CFM (cubic feet per minute) using either the friction rate or velocity method, for both metal and flex duct, in round or rectangular shapes. Correct duct sizing is as important as equipment sizing: undersized ducts choke airflow and strain the blower, while oversized ducts drop air velocity so rooms never get proper throw.
The standard residential friction rate is 0.05–0.10 inches of water column per 100 feet of duct. Flex duct has more internal resistance than smooth metal, so the same CFM typically needs a size larger in flex — the calculator handles that adjustment automatically.
| Duct Diameter | Approx. CFM |
|---|---|
| 6 in. | ~110 |
| 7 in. | ~160 |
| 8 in. | ~225 |
| 9 in. | ~300 |
| 10 in. | ~400 |
| 12 in. | ~680 |
| 14 in. | ~1,050 |
Values are approximate for smooth round metal duct; flex duct carries roughly one size less. Use the calculator above for your exact CFM, friction rate, and shape.
Cubic feet per minute — the volume of air a duct or fan moves. It is the fundamental unit of airflow in HVAC: every room’s heating and cooling delivery is ultimately a CFM number.
Start from the system total: about 400 CFM per ton of cooling. Then distribute by each room’s share of the total heating/cooling load — a room representing 10% of the load gets 10% of the CFM. A 3-ton system moves ~1,200 CFM total.
Roughly a 14-inch round metal duct at a 0.10 friction rate, or an equivalent rectangular trunk (about 20×8). At the more conservative 0.05 rate, step up a size. Enter your numbers above to see round and rectangular options side by side.
Metal delivers more air per diameter, lasts longer, and is easier to clean; flex is cheaper, faster to install, and fine for short branch runs when pulled tight. The practical rule: metal for trunks and long runs, flex for final connections — never compressed or kinked.
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