Pipe Velocity Calculator
Calculate fluid velocity in a pipe from volumetric flow rate and pipe diameter with unit conversion and step-by-step math.
How to Calculate Pipe Velocity
Fluid velocity in a pipe equals volumetric flow rate divided by the cross-sectional area. Only the inside diameter matters because that defines the space available for flow.
Pipe Velocity Formula
For a circular pipe:
$$v = \frac{Q}{A} = \frac{4Q}{\pi d^2}$$where:
- \(v\) — fluid velocity
- \(Q\) — volumetric flow rate
- \(d\) — inside pipe diameter
- \(A = \pi d^2 / 4\) — cross-sectional area
In imperial units with GPM and inches, a handy form is \(v = 0.408 \times Q / D^2\) with velocity in ft/s.
Related tools: Hazen Williams Calculator and Pipe Weight Calculator.
Frequently Asked Questions
What is a typical water velocity in pipes?
Domestic and HVAC systems often target 3 to 8 ft/s. Lower speeds reduce noise and pipe wear.
Why does a smaller pipe increase velocity?
The same flow through a smaller area must move faster. Velocity scales inversely with the square of diameter.
Can I find diameter from velocity and flow?
Yes. Rearrange to \(d = \sqrt{4Q / (\pi v)}\). In imperial units use \(d = \sqrt{0.408 \times Q / v}\).
Do I use inside or outside diameter?
Always the inside diameter. Wall thickness is not part of the flow area.
What pipe size for 120 GPM at 5 ft/s?
About 3.13 inches inside diameter. Higher flow or lower allowed velocity requires a larger bore.
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