The formula
Velocity is simply flow divided by the pipe's internal cross-section:
or in the practical metric units this tool uses:
The constant 353.68 collapses the unit conversions (m³/h → m³/s and mm² → m²) into one number worth memorising for field estimates.
Worked example
100 m³/h of water through a DN100 Sch 40 pipe (ID = 102.3 mm):
- v = 353.68 × 100 ÷ 102.3² = 353.68 × 100 ÷ 10465
- v = 3.38 m/s — on the fast side for continuous pumping; DN125 would give 2.1 m/s
Recommended velocity ranges
| Service | Typical target |
|---|---|
| Pump suction (liquid) | 0.5 – 1.5 m/s |
| Pump discharge (liquid) | 1 – 3 m/s |
| Gravity drain lines | 0.5 – 1 m/s |
| Slurry (keep solids moving) | 1.5 – 3 m/s |
| Process gas | 10 – 25 m/s |
| Steam (saturated) | 20 – 40 m/s |
These are rules of thumb, not law — the real constraints are pressure drop, erosion, noise, water hammer risk and NPSH. But a line far outside these bands is usually a design smell worth questioning.
Why instrumentation people care
- Flow meters have velocity windows. Magnetic meters like 1–10 m/s (below ~0.5 m/s accuracy suffers); vortex meters need a minimum Reynolds/velocity to shed vortices at all; ultrasonic meters publish velocity ranges directly.
- Thermowells in fast gas can resonate — velocity feeds directly into wake-frequency calculations.
- Erosion at the meter run: high velocity with entrained solids wears orifice plates' sharp edges, silently degrading accuracy.
Frequently asked questions
What is the formula for velocity in a pipe?
v = Q / A, where Q is volumetric flow and A = π·D²/4 is the pipe cross-sectional area from the internal diameter. In practical units: v (m/s) = 353.68 × Q (m³/h) ÷ D² (mm).
What is a good liquid velocity in a pipe?
Typical design targets: 1–3 m/s for pump discharge liquid lines, 0.5–1.5 m/s for suction lines (to protect NPSH), and up to 4–5 m/s for short clean-water runs. Erosive or slurry services run slower.
What velocity should gas lines use?
Common practice is 10–25 m/s for process gas and up to ~30–40 m/s in utility air or vent lines, limited by pressure drop and noise rather than erosion in clean gas.
Do I use the pipe nominal size or internal diameter?
Always the internal diameter (ID). A DN50 Sch 40 steel pipe has an ID of about 52.5 mm — nominal size and ID differ, and the difference grows with wall schedule.
Provided for reference and education. Verify independently before use in safety-critical work. See our disclaimer.