| Duct Application | Min (fpm) | Max (fpm) |
|---|---|---|
| Main Trunk (low vel.) | 600 | 2,500 |
| Main Trunk (high vel.) | 1,500 | 4,500 |
| Branch Duct | 400 | 1,200 |
| Return Air | 400 | 1,500 |
| Outside Air Intake | 400 | 1,000 |
| Exhaust / Relief | 400 | 2,500 |
| Riser / Vertical | 600 | 2,500 |
| Pipe Application | Min (ft/s) | Max (ft/s) |
|---|---|---|
| Chilled Water | 2 | 4 (≤2″) / 8 |
| Hot Water (heating) | 2 | 4 (≤2″) / 8 |
| Condenser Water | 3 | 4 (≤2″) / 8 |
| Domestic Cold (copper) | — | 8 |
| Domestic Hot (copper, ≤140°F) | — | 5 |
| Steam Condensate | 1 | 7 |
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
Book a Demo →This velocity calculator checks duct airflow and pipe water-flow velocities against the ranges mechanical engineers use in practice. For ducts, enter the airflow in CFM and the duct size; the tool returns velocity in feet per minute (fpm) and flags it against typical design ranges for the selected application. For pipes, enter the flow in GPM and the inside diameter; the tool returns velocity in feet per second (ft/s) and checks it against typical limits for chilled, hot, condenser, and domestic water.
Velocity drives both acoustics and energy. Oversized, low-velocity ducts waste space and first cost; undersized, high-velocity ducts and pipes generate noise, raise fan and pump energy, and — in piping — risk erosion-corrosion. Sizing inside the recommended band keeps systems quiet and efficient.
| Duct velocity | V (fpm) = Q (CFM) ÷ A (ft²) |
| Round area | A = π · (D ÷ 24)² (D in inches) |
| Rect. area | A = (W × H) ÷ 144 (W, H in inches) |
| Pipe velocity | V (ft/s) = 0.4085 · Q (GPM) ÷ d² (in) ≡ Qcfs ÷ A |
The duct ranges are common design rules of thumb for low- and high-velocity systems, not code limits. Hydronic limits follow the pipe-sizing guidance in ASHRAE Fundamentals: about 4 ft/s for 2 in. and smaller pipe (noise), and this tool flags larger hydronic pipe above 8 ft/s (≈8–10 ft/s is the usual noise and erosion ceiling), with ≈2 ft/s as a practical minimum for carrying air and dirt. Domestic copper limits follow Copper Development Association guidance: 8 ft/s cold, 5 ft/s hot up to about 140 °F (2–3 ft/s above that). The constant 0.4085 = (1 ft³/s ÷ 448.83 GPM) expressed for a circular area in square inches.