Air Duct Sizing Calculator
Enter airflow and target air velocity to estimate duct size for HVAC supply, return, or exhaust runs.
This is a planning tool. Final duct design should verify static pressure, friction rate, fitting losses, and local code requirements.
What this air duct sizing calculator does
This calculator gives you a fast first-pass duct size based on two values: airflow (CFM) and target velocity (FPM). It calculates the minimum duct cross-sectional area needed, then converts that area into round diameter and rectangular dimensions.
It is useful when you are sketching HVAC layouts, checking branch line sizing, or estimating if an existing duct run is undersized.
The core sizing formula
1) Required duct area
Area (ft²) = Airflow (CFM) / Velocity (FPM)
Once area is found in square feet, convert to square inches by multiplying by 144.
2) Convert area into duct size
- Round duct: Diameter is solved from area of a circle.
- Rectangular duct: Width and height are solved from area plus either aspect ratio or preferred width.
How to use the calculator correctly
- Enter the airflow for the run (from load calc, equipment schedule, or balancing plan).
- Choose a target velocity appropriate for noise and pressure drop goals.
- Select round or rectangular as your preferred shape.
- For rectangular ducts, either use an aspect ratio or set a preferred width.
- Click calculate and use the recommended nominal size as a starting point.
Typical velocity targets (rule-of-thumb)
- Main supply trunk: 700-1,200 FPM
- Supply branches near occupied spaces: 500-900 FPM
- Return ducts: 400-800 FPM
- Exhaust ducts: 800-1,500+ FPM (application-dependent)
Higher velocity can reduce duct size, but often increases fan energy, noise, and pressure drop. Lower velocity is generally quieter and more forgiving, but needs larger duct cross section.
Example
If you need 1,200 CFM at 900 FPM:
- Required area = 1,200 / 900 = 1.333 ft²
- Area in square inches = 1.333 × 144 = 192 in²
- Equivalent round diameter is about 15.6 inches, so a nominal 16-inch round duct is a practical pick.
For rectangular options, you could choose a size such as 16x12 in (192 in²) or a nearby standard size based on fitting availability and installed constraints.
Important design notes beyond sizing
Static pressure and friction rate matter
Two ducts with the same area can have different pressure losses depending on shape, roughness, length, and fittings. Always verify total external static pressure and fan capability.
Fittings can dominate pressure drop
Elbows, takeoffs, transitions, balancing dampers, and grilles can add significant resistance. Keep transitions smooth and avoid sharp turns where possible.
Aspect ratio impacts performance
Very “flat” rectangular ducts usually create more friction for the same area than round ducts. Moderate aspect ratios are typically preferred when space allows.
FAQ
Is this calculator good for residential and commercial work?
Yes, for preliminary sizing in both contexts. For final engineering and commissioning, include full pressure-drop calculations and manufacturer data.
Should I always choose the next larger size?
In many cases, yes—especially if noise is a concern. Oversizing slightly can lower velocity and reduce audible airflow noise.
Can I size the whole system with one velocity?
Usually no. Designers often use different velocity targets for trunks, branches, and return paths to balance noise, first cost, and fan power.