Pipe Velocity Calculator

Flow velocity is the average speed of fluid moving along the pipe.

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Enter Values

Flow rate (L/min)

Inside diameter

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

Enter the flow rate the pipe is carrying in litres per minute (L/min).
Enter the pipe's INSIDE diameter in millimetres — use the bore, not the nominal or outside size.
Read the velocity in m/s and ft/s, and check the assessment band against your local plumbing code's velocity limit.

How it works

Flow velocity is the average speed of fluid moving along the pipe. It follows the continuity relationship v = Q ÷ A, where Q is the volumetric flow rate and A is the pipe's internal cross-sectional area. The calculator converts your flow from L/min to m³/s (÷ 60,000) and your inside diameter from mm to a radius in metres, then computes the circular area A = π·r² before dividing.

For a fixed flow, a narrower pipe forces a higher velocity because the same volume must pass through a smaller area each second. Designers keep velocity within a band — high enough to carry sediment and air, low enough to avoid noise, water hammer, and erosion-corrosion of the pipe wall. Many codes cap cold-water velocity near 2.4 m/s (and lower for hot water), so the assessment band is a guide only; always confirm against the limit that applies to your material and jurisdiction.

Worked example

30 L/min through a 25 mm pipe. A 25 mm inside-diameter pipe carrying 30 L/min: 30 L/min = 0.0005 m³/s; area = π × 0.0125² = 0.000491 m². Velocity = 0.0005 ÷ 0.000491 = 1.02 m/s (3.34 ft/s). That sits comfortably inside the typical design range, below the usual ~2.4 m/s cap for cold water.

Common mistakes

Using the outside or nominal diameter instead of the true inside bore — wall thickness can drop the internal area significantly and understate the real velocity.
Mixing units, e.g. entering flow in L/hour or L/s while the field expects L/min. Convert first (1 L/s = 60 L/min).
Assuming the velocity band replaces code — the ~2.4 m/s figure is a common guideline, but the binding limit depends on pipe material, water temperature and your local plumbing/building code.

Frequently asked questions

What pipe velocity is acceptable for plumbing?

A common design range is roughly 0.5 to 2.4 m/s for cold water, with lower limits (often ~1.2–1.5 m/s) for hot water to reduce erosion-corrosion and noise. These are guidelines — the enforceable limit depends on the pipe material and your local code, so treat this tool as a first check and confirm with a licensed professional.

Do I use the inside or outside diameter?

Always the inside (bore) diameter. Velocity depends on the internal cross-sectional area the water actually flows through, so using the outside or nominal size will underestimate the true velocity.

How do I convert my flow rate to L/min?

Multiply L/s by 60, or divide L/hour by 60. For example 0.5 L/s = 30 L/min, and 1,800 L/hour = 30 L/min.

Tip: Enter any known values to calculate the remaining results.

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