Pipe Sizing Calculator (Flow and Velocity)
Size a pipe from the flow rate and the velocity you want. The calculator gives the minimum inside diameter and the smallest standard Sch 40 and Sch 80 pipes that meet it, with their actual velocity.
- D = √(4Q ÷ πv) = √(4 × 0.01389 ÷ (π × 2)) = 94.03 mm
- Pick the smallest standard pipe whose inside diameter is at least this value.
Velocity in nearby Sch 40 sizes
| NPS | ID mm | Velocity m/s |
|---|---|---|
| 2" | 52.51 | 6.41 |
| 2 1/2" | 62.71 | 4.5 |
| 3" | 77.92 | 2.91 |
| 4" | 102.26 | 1.69 |
| 5" | 128.2 | 1.08 |
| 6" | 154.06 | 0.745 |
Pipe Sizing Formula
v = Q ÷ (π/4 × D²)
Q is the volume flow in m³/s, v the design velocity in m/s and D the inside diameter in m. Pipe inside diameters are calculated from ASME B36.10M outside diameters and wall thicknesses.
Worked example
50 m³/h of water at 2 m/s needs D = √(4 × 0.01389 ÷ (π × 2)) = 94.0 mm. The smallest Sch 40 pipe is NPS 4" (ID 102.26 mm), giving 1.69 m/s.
Frequently Asked Questions
What velocity should I use to size a pipe?
It depends on the fluid and the service and is usually set by company standards. Pumped water is often sized for about 1 to 3 m/s, with lower velocity on pump suction lines.
Why does the actual velocity differ from my target?
Standard pipes come in fixed sizes, so the chosen pipe is a little larger than the minimum diameter and the velocity is a little lower.
Should I also check pressure drop?
Yes. Use the pipe pressure drop calculator for the chosen size and length, especially for long lines or viscous fluids.
Learn more on the blog
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