How to Use This Ductulator
This ductulator calculates the correct duct diameter based on two inputs: the required airflow (CFM) and your target air velocity (FPM). The calculator outputs three results: the recommended round rigid duct diameter, the recommended flex duct diameter (sized larger to compensate for higher friction), and equivalent rectangular duct dimensions.
Step 1: Determine CFM
CFM (Cubic Feet per Minute) is the volume of air that needs to flow through the duct. For residential systems, each room has a CFM requirement based on its size, heat load, and the number of supply registers. The quick reference buttons above provide typical values. For precise CFM, use ACCA Manual J (heat load calculation) or consult your HVAC engineer.
Step 2: Choose Velocity
Air velocity determines how fast the air moves through the duct. Higher velocity allows smaller duct but increases noise and pressure drop. Standard ranges:
| Application | Main Duct (FPM) | Branch Duct (FPM) |
|---|---|---|
| Residential | 700-900 | 400-600 |
| Commercial (quiet) | 1,000-1,500 | 600-1,000 |
| Commercial (standard) | 1,500-2,000 | 1,000-1,500 |
| Industrial | 2,000-4,000 | 1,500-3,000 |
Step 3: Read Results
The calculator snaps to standard duct sizes (4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 24 inches). It also shows the exact calculated diameter so you can see how close the standard size is to the theoretical ideal. The flex duct recommendation is 15% larger than rigid to account for the corrugated inner surface that increases friction loss by 30-50%. See our flex duct sizes guide for detailed sizing tables by diameter.
Duct Sizing Principles
The fundamental equation behind every ductulator is: Area = CFM ÷ Velocity. For a round duct, Area = π × (D/2)², so D = √(4 × Area / π). This gives the minimum duct diameter to carry the required airflow at the target velocity.
In practice, you should always round up to the next standard size. An undersized duct increases velocity beyond the target, creating noise, excessive pressure drop, and reduced system performance. An oversized duct wastes material and space but does not hurt performance — when in doubt, size up.
Why Flex Duct Needs Larger Sizing
Flexible duct has a corrugated inner wall that creates significantly more friction than smooth rigid duct. At the same diameter and airflow, flex duct has 30-50% higher pressure drop. To deliver the same CFM with acceptable pressure drop, flex duct should be sized 1-2 inches larger than rigid duct.
This is why our calculator shows both rigid and flex recommendations. A system designed for 8-inch rigid duct should use 10-inch flex duct to maintain equivalent airflow performance. Additionally, flex duct must be pulled taut (fully extended) during installation — a compressed or sagging flex duct can lose 50% or more of its rated airflow capacity. See our installation guide for best practices.
Complete CFM Chart by Duct Size
For a comprehensive airflow capacity table covering every standard duct diameter from 4" to 24" at multiple velocities, see our dedicated flex duct CFM chart.