HVAC Duct Sizer Calculator
Matching the Pipe to the Flow
A duct that's too small for the air passing through it forces the blower to work harder, raises static pressure, and creates the whistling noise homeowners associate with a poorly designed system. The velocity method ties duct diameter directly to the airflow it needs to carry, at a target speed appropriate for residential ductwork. This calculator applies that method to recommend a standard round duct size.
The Formula
Diameter = √((4 × Area) / π) × 12
CFM is the required airflow in cubic feet per minute, and velocity is the target air speed in feet per minute — typically 600–900 ft/min for residential supply trunks and branches, with 700 ft/min as a common default. The calculated diameter is then rounded up to the nearest standard round duct size.
A Worked Example
| Step | Value |
|---|---|
| Cross-Sectional Area (400 ÷ 700) | 0.571 sq ft |
| Calculated Diameter | 10.24 in |
| Recommended Standard Size | 12" round duct |
The calculated 10.24" rounds up to the next standard size, since 10" duct isn't a common stocked increment above that point in most supply houses' round duct lines.
Standard Round Duct Sizes
| Available sizes | ||||||
|---|---|---|---|---|---|---|
| 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| 12 | 14 | 16 | 18 | 20 | 22 | 24 |
Where This Calculation Matters
- New duct runs — sizing supply and return branches to a specific room's calculated airflow requirement.
- Noise troubleshooting — undersized ducts push air velocity too high, and this calculation shows what size would bring velocity back into a normal range.
- Zoning projects — each new zone or branch needs its own duct sized to its share of total system airflow.
- Register and grille selection — airflow and duct size together inform the right register size to avoid excessive throw or noise.
How to Use This Calculator
- Enter the Required Airflow in CFM.
- Enter the Design Velocity in ft/min, or leave blank to use the default of 700 ft/min.
- Select Calculate to see the calculated diameter and the recommended standard round duct size.
Related Calculations
Find the CFM a space actually needs with the Airflow CFM Calculator, then check the resulting friction loss with the Duct Static Pressure Calculator.