Pipe Velocity from Flow and Diameter

This is continuity for a round pipe with the area written out, and the American shorthand version — v (ft/s) = 0.4085 Q (gpm) / d² (in) — is memorised by every pipefitter. Push 100 gpm through 3 in pipe and you get 0.4085 × 100/9 ≈ 4.54 ft/s, right in the sweet spot. Solve it for D instead and it becomes the sizing tool: choose your target velocity, and the pipe size follows.

Velocity limits, not pressure drop, usually govern pipe selection. Below about 2 ft/s (0.6 m/s) closed-loop systems fail to sweep air out of high points and gather sediment; above about 8–10 ft/s (2.5–3 m/s) copper erodes at the elbows, noise becomes audible in occupied spaces, and water hammer forces climb. Hot domestic water is capped lower still — 4–5 ft/s — because erosion-corrosion accelerates sharply with temperature, and this single number is why so many recirculation lines fail at the first fitting downstream of the pump.