Pipe velocity calculator

Find the mean velocity of a fluid in a pipe from the flow rate and the internal diameter, or work back to the flow rate for a target velocity. Velocity is the first sanity check on a pipe size: too fast brings noise, erosion and pressure loss, too slow lets solids settle.

Method

From continuity, the mean velocity is the volumetric flow divided by the cross-sectional area:

v = Q / A,  with  A = pi D^2 / 4,  so  v = 4Q / (pi D^2)

and the flow for a given velocity is Q = v A. Here v is mean velocity (m/s), Q is volumetric flow (m^3/s) and D is internal diameter (m). This is a geometric result and does not depend on the fluid.

Guidance, not a limit. For water, design velocities of roughly 1 to 3 m/s are common: high enough to avoid sediment, low enough to limit noise, water hammer risk and friction loss. Suction lines are usually kept slower than discharge lines. Treat these as rules of thumb, not hard rules.

Inputs

• Volumetric flow Q (L/s) and internal diameter D (mm), to find velocity; or
• velocity v (m/s) and diameter D (mm), to find the flow.

Outputs

• Mean velocity (m/s), or flow (L/s), depending on the mode.
• Cross-sectional area (m^2).

Worked example

A flow of 10 L/s in an 80 mm pipe:

A = pi (0.08)^2 / 4 = 0.00503 m^2
v = 0.010 / 0.00503 = 1.99 m/s

which sits in the usual 1 to 3 m/s band for water.

Frequently asked questions

What velocity should I design for?

For water, about 1 to 3 m/s is typical; keep suction lines slower. Other fluids and services have their own limits. These are guides, not codes.

Does the fluid matter?

No. Velocity from flow and diameter is purely geometric. The fluid matters for the pressure drop, which the pressure drop calculator handles.

Related

• Pipe flow and pressure drop calculator
• Reynolds number calculator
• Open the Studio for full networks.