Pipe Flow Rate Calculator

Pipe flow rate from diameter and velocity

Volumetric flow rate is Q = A · v = π · (D/2)² · v, with A the cross-section area and v the average fluid velocity. SI gives you Q in m³/s, though plumbing work tends to quote it in L/s or L/min instead. Say you have a 1/2" pipe (about 12.7 mm internal) running water at 1 m/s: A ≈ 1.267 × 10⁻⁴ m² and Q ≈ 1.27 × 10⁻⁴ m³/s, which is roughly 0.127 L/s, or 7.6 L/min. Whether that flow stays laminar (Re < 2300) or turns turbulent comes down to the Reynolds number Re = ρvD/μ.

Applications

It helps you size shower heads, toilet flushes, irrigation lines, hydrants and cold-water columns. For cold water, supply lines usually run 1.5–2.5 m/s. Go below 0.6 m/s and sediment starts to settle; push past 3 m/s and you get noise and water hammer. The same math checks pump suction, swimming-pool returns and industrial process lines.

FAQ

Internal or external diameter? Use the internal one every time. The external diameter counts the pipe wall too, so it would overestimate Q.

What is a safe velocity? Stick to 0.6 to 2.5 m/s for cold water. Anything above that range starts bringing noise and water hammer.

How do I get L/min? Take Q in m³/s and multiply by 60,000. If you already have it in L/s, just multiply by 60.