Duct Sizing Calculator
Use this free online ductulator and CFM calculator to turn airflow and friction rate into round or rectangular duct sizes and check the resulting air velocity.
Duct Inputs
Allowed range: 0.05 to 0.50
What a Ductulator Does
A ductulator relates airflow, duct size, friction loss, and velocity. This online ductulator solves the standard galvanized-duct friction equation for diameter, then converts that diameter into practical round and rectangular options. It is useful for quick field checks and early layouts.
Choosing Friction Rate
Friction rate comes from available static pressure divided by total effective length. A starting value of 0.10 in. w.c. per 100 ft is common in preliminary residential work, but long runs, restrictive fittings, filters, and coils can require a different value.
Round vs. Rectangular
Round duct usually offers lower surface area, fewer leakage paths, and quieter airflow. Rectangular duct fits framing and tight mechanical spaces more easily, but aspect ratio and fitting geometry matter.
Common Round Duct Sizes at 0.10 Friction Rate
| Airflow | Calculated Diameter | Next Standard Size |
|---|---|---|
| 100 CFM | 5.8 in. | 6 in. |
| 200 CFM | 7.6 in. | 8 in. |
| 300 CFM | 8.8 in. | 9 in. |
| 400 CFM | 9.8 in. | 10 in. |
| 600 CFM | 11.5 in. | 12 in. |
| 800 CFM | 12.8 in. | 14 in. |
| 1,000 CFM | 13.9 in. | 14 in. |
| 1,200 CFM | 14.9 in. | 16 in. |
| 1,600 CFM | 16.6 in. | 18 in. |
| 2,000 CFM | 18.1 in. | 20 in. |
Use Manual D for the Full System
This duct sizing calculator is a sizing aid. A full ACCA Manual D duct design accounts for total effective length, fitting equivalent lengths, available static pressure, room airflow, balancing, registers, and equipment data.
Duct Sizing Questions
Keep approved duct work easy to price
Plenum Pricebook helps your team organize service and installation items after the design work is complete.
See Pricebook