Pipe Velocity Calculator Formula
Understand the math behind the pipe velocity calculator. Each variable explained with a worked example.
Formulas Used
Average Velocity
velocity = q_m3s / areaVariables
| Variable | Description | Default |
|---|---|---|
flow_rate | Volumetric Flow Rate (Q)(L/s) | 15 |
diameter | Pipe Inner Diameter(mm) | 100 |
q_m3s | Derived value= flow_rate / 1000 | calculated |
d_m | Derived value= diameter / 1000 | calculated |
area | Derived value= pi * pow(d_m, 2) / 4 | calculated |
How It Works
Pipe Flow Velocity
Velocity is derived from the continuity equation by dividing flow rate by cross-sectional area.
Formula
V = Q / A = 4 Q / (pi D^2)
where Q is the volumetric flow rate and D is the pipe inner diameter. Maintaining velocities within recommended ranges (1-3 m/s for water) avoids excessive pressure loss and erosion.
Worked Example
15 L/s flowing through a 100 mm diameter pipe.
- 01Q = 15 L/s = 0.015 m^3/s
- 02A = pi x 0.1^2 / 4 = 0.007854 m^2
- 03V = 0.015 / 0.007854 = 1.91 m/s
Frequently Asked Questions
What is a safe maximum velocity for water pipes?
For potable water systems, 1.5-2.5 m/s is typical. Fire protection systems may allow up to 3-4 m/s. Exceeding 3 m/s risks water hammer, noise, and accelerated pipe erosion.
Is this the velocity at the centre of the pipe?
No, this is the average (bulk) velocity. In turbulent flow, the centreline velocity is about 1.2 times the average. In laminar flow, the centreline velocity is exactly 2 times the average.
How does pipe size affect velocity?
For a given flow rate, halving the diameter quadruples the velocity (since area scales with D^2). This dramatically increases friction losses, which scale with V^2.
Learn More
Guide
How to Size a Pump: Flow Rate, Head, and Efficiency Calculations
Learn the step-by-step process for sizing centrifugal and positive displacement pumps. Covers flow rate requirements, total dynamic head, system curves, pump curves, NPSH, and efficiency considerations.
Ready to run the numbers?
Open Pipe Velocity Calculator