For 100 CFM, a typical bedroom branch, a 6 in duct runs at 0.085 in. w.g. per 100 ft, just over 0.08, so the calculator suggests 7 in.
How the duct size calculator works
DuctSizeCalc uses the equal-friction method. For round galvanized duct carrying standard air, friction loss per 100 ft = 0.109136 × CFM1.9 ÷ D5.02 (D in inches), a fit to the ASHRAE friction chart that ductulators are based on. Solving for D gives the diameter at your friction rate. A second diameter keeps the velocity at or below your limit: D = 12 × √(4 × CFM ÷ (π × fpm)). The larger of the two is rounded up to a standard round size. Rectangular sizes use the Huebscher equivalent diameter, De = 1.30 × (a·b)0.625 ÷ (a + b)0.25, at heights of 6, 8, 10 and 12 inches.
Round duct size by airflow (0.08 in. w.g. per 100 ft, 900 fpm max)
| Airflow | Round size | Velocity | Friction /100 ft |
|---|---|---|---|
| 50 CFM | 5 in | 367 fpm | 0.057 in. w.g. |
| 75 CFM | 6 in | 382 fpm | 0.049 in. w.g. |
| 100 CFM | 7 in | 374 fpm | 0.039 in. w.g. |
| 150 CFM | 8 in | 430 fpm | 0.044 in. w.g. |
| 200 CFM | 8 in | 573 fpm | 0.075 in. w.g. |
| 300 CFM | 10 in | 550 fpm | 0.053 in. w.g. |
| 400 CFM | 12 in | 509 fpm | 0.037 in. w.g. |
| 600 CFM | 12 in | 764 fpm | 0.079 in. w.g. |
| 800 CFM | 14 in | 748 fpm | 0.063 in. w.g. |
| 1000 CFM | 16 in | 716 fpm | 0.049 in. w.g. |
| 1200 CFM | 16 in | 859 fpm | 0.07 in. w.g. |
Flexible duct has more friction than metal duct, especially when it is not pulled tight, and every elbow, boot and register adds pressure drop, so the friction rate you design to should come from the blower's available static pressure divided by the total effective length (ACCA Manual D). Results are estimates for planning, not a substitute for a proper duct design.
Frequently asked questions
What size duct for 100 CFM?
A 6 in round duct carries 100 CFM at about 509 fpm and 0.085 in. w.g. per 100 ft; a 7 in duct brings it under 0.04.
What size duct for 1,200 CFM?
About 15.6 in at 0.08 in. w.g. and 900 fpm, so a 16 in round trunk (859 fpm), or 18 × 12 in rectangular.
How many CFM per ton?
About 400 CFM per ton of cooling is the common design figure; humid climates often use 350.
Is DuctSizeCalc free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.