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Free HVAC Duct Calculator
A browser-based ductulator for residential and light-commercial HVAC. Enter airflow in CFM, pick a target velocity or friction rate, and get round and rectangular duct sizes in seconds.
Quick answer
To size an HVAC duct, divide airflow (CFM) by target velocity (FPM) to get the required cross-sectional area, then convert to a round diameter with D (in) = √(4·A/π) × 12. For a rectangular equivalent, use the ASHRAE equal-friction formula De = 1.30 · (a·b)0.625 / (a+b)0.25. Typical residential design velocities run 600–900 FPM at a friction rate of 0.08–0.10 in. w.g. per 100 ft.
Inputs
Keep aspect ratio ≤ 4:1 to control friction and leakage.
Recommended velocities
| Application | Velocity (FPM) | Notes |
|---|---|---|
| Residential trunk | 700–900 | Balance size vs. noise |
| Residential branch | 600–700 | Quiet delivery to registers |
| Residential return | 500–700 | Low velocity = low noise |
| Commercial main | 1200–1800 | Low-velocity systems |
| Commercial branch | 800–1200 | Diffuser noise controls upper limit |
How the math works
Cross-sectional area A (sqft) = CFM ÷ velocity (FPM). Round diameter (in) = √(4·A/π) × 12. Rectangular equivalent uses the ASHRAE equal-friction formula De = 1.30 · (a·b)0.625 / (a+b)0.25, solved for the selected aspect ratio. Friction rate is estimated with 0.109136 · Q1.9 / D5.02 (in. w.g. per 100 ft, galvanized round duct).
When you need Manual D
- New construction or permitted retrofits requiring stamped duct design.
- Complex layouts with many fittings — equivalent length matters.
- High-static or zoned systems where ESP budget is tight.
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