Pipe Velocity from Flow and Diameter
Also known as fps in a pipe · flow velocity from GPM
Worked example: 2 m/s in 50 mm pipe → 235.62 L/min — press Try an example to run it live, then adjust anything.
Enter your known values, leave one input blank, and solves for the missing one. Tap a variable’s symbol to see what it means, with a typical value. Try different units for next level excitement!
Continuity →
UniversityFluid Mechanics, HVAC & Refrigeration
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Pipe Velocity from Flow and Diameter explained
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.
Pipe Velocity from Flow and Diameter formula
- = Flow velocity (m/s)
- = Flow rate (L/min)
- = Inside diameter (mm)
Missing one of these? Work it out first, then come back
- Flow velocity — Minor Loss from K Factor, Volumetric Flow Rate (Q = Av)
- Flow rate — Pump Water Horsepower, Pump Brake Horsepower
- Inside diameter — Hazen–Williams Head Loss, Equivalent Length of a Fitting