How fast is the water moving in your pipe?

Common questions

How do you calculate water velocity in a pipe?

Velocity = flow rate ÷ the pipe's inside cross-sectional area (v = Q ÷ πr²). In US units the shortcut is velocity (ft/s) = 0.4085 × GPM ÷ diameter² (inches); in metric, velocity (m/s) = 21.22 × (L/min) ÷ diameter² (mm). Enter the flow and inside diameter above and the calculator does it for you.

What is a good water velocity for a pipe?

A common plumbing design guideline keeps water below about 8 ft/s (2.4 m/s) for cold water and about 5 ft/s (1.5 m/s) for hot water, to limit noise and erosion-corrosion. Most supply runs sit comfortably in the 2–5 ft/s (0.6–1.5 m/s) range. These are rules of thumb, not code minimums.

Why does high water velocity matter?

Fast water is noisy, and over years it can erode pipe walls and fittings (erosion-corrosion), especially in hot copper lines. If your figure comes out above the guideline, stepping up one pipe size drops the velocity quickly — velocity falls with the square of the diameter.

Should I use the inside or outside diameter?

The inside diameter (the bore) — that is the space the water actually flows through. Nominal pipe sizes and outside diameters differ from the true inside bore, so use the real inside diameter for an accurate velocity.