Pipe Sizing for Flow Velocity Calculator
Work out the internal pipe diameter needed to carry a given flow at a target velocity, then get the next standard nominal bore up and the actual velocity you would run at in that size.
Enter Values
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 must carry in litres per second.
- Enter the target velocity — around 1–2 m/s is typical for water services.
- Read the exact required diameter, the recommended next standard nominal size, and the true velocity at that bore.
How it works
From continuity Q = A · v, the required flow area is A = Q / v and the diameter is D = √(4A/π). The flow is converted from L/s to m³/s first. Because pipes only come in standard bores, the tool rounds up to the next nominal size from 15 mm to 300 mm and recalculates the velocity that bore actually gives.
Worked example
Worked example. For 10 L/s at a target 1.5 m/s, the area is 0.01/1.5 = 0.006667 m², so D = √(4 × 0.006667/π) = 0.0921 m = 92.1 mm. The next standard bore up is 100 mm, in which the real velocity drops to about 1.27 m/s.
Common mistakes
- Treating the nominal size as the exact internal diameter — wall thickness and pipe schedule change the true bore, so verify it with the manufacturer.
- Designing at too high a velocity, which causes noise, high friction loss and water-hammer/erosion risk.
- Forgetting that the actual velocity in the chosen standard pipe is lower than your target because the real bore is larger than the exact calculated diameter.
Frequently asked questions
What velocity should I target for water?
A common range is about 1–2 m/s for pumped water services, sometimes lower on suction lines and long runs to limit friction. Always check the relevant plumbing/hydraulic standard for the pipe and application.
Why is the velocity at the nominal bore different from my target?
The exact calculated diameter rarely matches a standard size, so the tool picks the next size up. A larger bore means a bigger area, so at the same flow the velocity comes out a little below the target you entered.
Related tools
- Pipe Velocity Calculator
- Pipe Flow Calculator
- Friction Loss Calculator
- Darcy-Weisbach Head Loss Calculator
- Manning Pipe Flow Calculator
- Nozzle Flow Calculator
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Tip: Enter any known values to calculate the remaining results.
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