Example data table
| Scenario | Airflow | Target velocity | Estimated round diameter | Sample rectangle | Notes |
|---|---|---|---|---|---|
| Main supply | 1200 CFM | 800 fpm | ~14.7 in | 16 × 10 in | Balanced noise and pressure drop. |
| Branch run | 400 CFM | 700 fpm | ~9.1 in | 10 × 8 in | Good for short runs with registers. |
| Return duct | 900 CFM | 600 fpm | ~16.5 in | 18 × 12 in | Lower velocity helps reduce turbulence. |
Formula used
- Area: A = Q ÷ V
- Unit alignment: Q in ft³/min and V in ft/min gives A in ft²
- Round diameter: D = √(4A/π)
- Rectangular velocity: V = Q ÷ A
- Equivalent diameter (rectangle): Deq ≈ 1.30 × ( (ab)^0.625 ÷ (a+b)^0.25 )
- Pressure loss estimate: ΔP ≈ (Friction Rate) × (Effective Length ÷ 100)
- Effective length: Leff = L × (1 + fittings%)
How to use this calculator
- Enter airflow for the duct section you are sizing.
- Choose a target velocity suitable for noise and space limits.
- Set duct length and friction rate to estimate pressure loss.
- Add a fittings allowance to reflect elbows and transitions.
- Adjust rectangular limits and aspect ratio if space is tight.
- Click Calculate and review round and rectangular outputs.
- Download CSV or PDF to attach to estimates and submittals.
Practical sizing guidance
Velocity selection
Higher velocity reduces duct size but increases noise and loss. Lower velocity improves comfort and efficiency but may require more ceiling space.
Round vs rectangular
Round ducts typically have lower friction and leakage. Rectangular ducts can fit tight shafts but should avoid extreme aspect ratios to limit losses.
Professional duct sizing notes
1) Start with room airflow
Most branch ducts are sized from room loads and diffuser selections. As a practical starting point, many residential supplies land near 75–150 CFM per room, while small commercial branches often range 150–400 CFM. Always verify totals against fan capacity and zoning.
2) Choose a velocity target
Velocity drives both noise and size. Typical guidance is 600–1200 fpm for main trunks and 400–900 fpm for branches, with returns often lower to reduce turbulence. Lower velocity improves acoustic comfort but increases duct area and material cost.
3) Use friction rate for balanced pressure
Friction rate is a convenient way to keep runs “even” across a system. Many designs use 0.05–0.12 in.wg per 100 ft as a planning range. Tight retrofits may accept higher rates, but that can increase fan energy and make balancing harder.
4) Account for fittings as equivalent length
Elbows, wyes, transitions, and dampers add resistance beyond straight duct. A quick estimating method is adding 10–40% to straight length, depending on how “busy” the run is. For final design, use fitting loss data or equivalent-length tables from your standard.
5) Round ducts usually perform better
Round ducts generally have less perimeter per unit area, which can reduce friction and leakage potential. They also resist deformation. When space allows, round sizes often deliver the same airflow at a lower pressure drop than an equivalent rectangular duct.
6) Keep rectangular aspect ratios reasonable
High aspect ratios increase perimeter and can raise friction and breakout noise. Many contractors try to keep aspect ratio at 4:1 or below, and preferably closer to 2:1 where possible. This calculator filters rectangular options to match your chosen limit.
7) Verify velocity at the rounded size
Field sizes are not continuous, so rounding to a standard diameter can change velocity noticeably. For example, a calculated 14.7 inch round may become 15 inches, reducing velocity and pressure loss. Always re-check the “velocity at standard size” before finalizing.
8) Validate with installation realities
Duct sizing is only one part of performance. Confirm insulation, sealing, access, and routing. Leaks at connections can reduce delivered airflow, and poor supports can distort duct shape. After installation, measure static pressure and airflow to confirm the system matches the design intent.
FAQs
1) What velocity should I use for a main supply trunk?
A common target is 700–1200 fpm for trunks. Use lower values when noise is sensitive or runs are long, and higher values when space is limited and the fan can handle extra pressure.
2) Why does the rounded duct size change my velocity?
Because standard diameters come in steps. When you round up, area increases and velocity drops. That can reduce noise and pressure loss, but it also increases material and space requirements.
3) Is round always better than rectangular?
Round often has lower friction for the same airflow, but rectangular can fit tight shafts and soffits. Use rectangular when geometry forces it, and keep aspect ratios moderate to limit added loss.
4) What friction rate should I start with?
Many projects begin around 0.08 in.wg per 100 ft and adjust from there. Lower rates improve efficiency but require larger ducts. Higher rates save space but demand more fan pressure.
5) Does this calculator replace a ductulator?
It follows the same core airflow and velocity logic and adds quick checks, rounding, and reporting. For final design, confirm selections using your preferred standard method and fitting loss data.
6) How should I treat fittings and elbows?
For early estimates, add a fittings allowance like 15–30% to straight length. For detailed design, use equivalent lengths or loss coefficients for each fitting and sum them.
7) What if my result seems too large for the ceiling space?
Increase target velocity carefully, or switch to a rectangular size within a reasonable aspect ratio. Also check routing, reduce run length where possible, and verify fan static pressure can support the new loss.
Plan quieter airflow, lower losses, and balanced comfort everywhere.