Pipe velocity calculator
Written by Haimi Jordaan, MEng (Mechanical), University of Pretoria. Seven years in a specialist engineering analysis and design group across CFD, FEA and DEM. This calculator runs the same solver code as the Studio rather than a separate implementation of the equations.Published . Last updated .
Calculator
Result
- Mean velocity
- 1.99 m/s
- Cross-sectional area
- 5.03e-3 m²
Check velocities on every pipe at once, sized to DN/Schedule, in the Studio.
Open the StudioThis free pipe velocity calculator finds the mean velocity of a fluid in a pipe from the flow rate and internal diameter, or works back from a target velocity to the flow it carries. Pick a standard pipe size and it fills the true internal bore for you (Sch 10, 40, 80 and 160, ASME B36.10M), and every input and result reads in metric or imperial units. Velocity is the first sanity check on a pipe size: too fast brings noise, erosion and water hammer risk, too slow lets solids settle.
Method
From continuity, the mean velocity is the volumetric flow divided by the cross-sectional area:
v = Q / A, with A = pi D^2 / 4, so v = 4Q / (pi D^2)
and the flow for a given velocity is Q = v A. Here v is mean velocity (m/s), Q is volumetric flow (m^3/s) and D is the internal diameter (m). This is a geometric result and does not depend on the fluid.
Inputs
- Flow to velocity: the volumetric flow Q and the internal diameter D.
- Velocity to flow: a target velocity v and the diameter D, to read the flow that size carries.
- The internal diameter can be typed, or filled from the standard-size picker (DN and schedule, true bore).
Outputs
- Mean velocity, or the flow rate, depending on the mode.
- The cross-sectional area used.
Water flow capacity of standard pipe sizes
The flow a pipe carries at a given velocity is pure geometry (Q = vA), so this holds for any fluid. Read across at your design velocity to shortlist a size, then check the pressure drop before locking it in.
| Size | Sch 40 bore (mm) | Flow at 1.0 m/s (L/s) | Flow at 1.5 m/s (L/s) | Flow at 2.0 m/s (L/s) | Flow at 3.0 m/s (L/s) |
|---|---|---|---|---|---|
| DN15 (1/2 in) | 15.8 | 0.20 | 0.29 | 0.39 | 0.59 |
| DN20 (3/4 in) | 21.0 | 0.35 | 0.52 | 0.69 | 1.04 |
| DN25 (1 in) | 26.6 | 0.56 | 0.84 | 1.11 | 1.67 |
| DN32 (1 1/4 in) | 35.1 | 0.97 | 1.45 | 1.93 | 2.90 |
| DN40 (1 1/2 in) | 40.9 | 1.32 | 1.97 | 2.63 | 3.95 |
| DN50 (2 in) | 52.5 | 2.16 | 3.24 | 4.33 | 6.49 |
| DN65 (2 1/2 in) | 62.7 | 3.09 | 4.63 | 6.17 | 9.26 |
| DN80 (3 in) | 77.9 | 4.77 | 7.15 | 9.54 | 14.3 |
| DN100 (4 in) | 102.3 | 8.21 | 12.3 | 16.4 | 24.6 |
| DN150 (6 in) | 154.1 | 18.6 | 28.0 | 37.3 | 55.9 |
| DN200 (8 in) | 202.7 | 32.3 | 48.4 | 64.6 | 96.8 |
| DN250 (10 in) | 254.5 | 50.9 | 76.3 | 102 | 153 |
| DN300 (12 in) | 303.3 | 72.2 | 108 | 144 | 217 |
Bores are ASME B36.10M Sch 40 internal diameters, the same table behind the calculator's pipe-size picker. Flows are rounded to three significant figures.
Typical design velocities by service
| Service | Typical velocity (m/s) | Typical velocity (ft/s) | Why it is limited |
|---|---|---|---|
| Pump suction, water | 0.5 - 1.5 | 1.5 - 5 | Protects NPSH and avoids cavitation and air entrainment |
| Pump discharge, water | 1.5 - 3 | 5 - 10 | Balances pipe cost against friction loss |
| General water pipework | 1 - 3 | 3 - 10 | The usual first-pass design band |
| Building services, cold water | up to about 2.4 | up to 8 | Common plumbing-code cap on noise and water hammer |
| Building services, hot water (copper) | up to about 1.5 | up to 5 | Lower cap to limit erosion-corrosion of copper |
| Solids-bearing or drainage lines (minimum) | at least 0.6 | at least 2 | Self-cleansing velocity keeps solids moving |
| Compressed air mains | 6 - 10 | 20 - 33 | Limits the pressure drop across the header |
| Saturated steam mains | 25 - 40 | 80 - 130 | Vendor guidance, superheated lines can run faster |
Rule-of-thumb bands collated from common engineering references, chiefly Crane TP-410 and pump-handbook practice, together with the velocity caps that plumbing codes commonly apply. The caps vary by jurisdiction and we do not tie them to one code here, so read them as indicative. The governing code or specification for your service overrides these, so treat each band as a starting point, not a limit.
Worked example
A flow of 10 L/s in an 80 mm pipe:
A = pi (0.08)^2 / 4 = 0.00503 m^2
v = 0.010 / 0.00503 = 1.99 m/s
which sits in the usual 1 to 3 m/s band for water.
Frequently asked questions
What is a good water velocity in a pipe?
For water, about 1 to 3 m/s is the usual design band. Keep pump suction lines at the low end, roughly 0.5 to 1.5 m/s, and stay above about 0.6 m/s where solids could settle. Plumbing codes commonly cap building water services near 2.4 m/s (8 ft/s), and the governing code or specification always overrides these rules of thumb.
How do I calculate velocity from flow rate and pipe diameter?
Divide the volumetric flow by the pipe's cross-sectional area: v = Q / A with A = pi D squared over 4, so v = 4Q / (pi D squared) using the internal diameter. The calculator does this live in metric or imperial units and shows the area it used.
Should I use the nominal pipe size or the internal diameter?
Always the internal bore. Nominal sizes are labels, not bores: a DN80 (3 inch) Sch 40 pipe has a bore of about 77.9 mm, not 80 mm. The calculator's pipe-size picker fills the true internal diameter for Sch 10, 40, 80 and 160 from ASME B36.10M so the area is right.
Does the fluid change the velocity?
No. Velocity from flow and diameter is pure geometry, so the same flow in the same bore gives the same velocity for water, oil or air. The fluid matters for the pressure drop and the Reynolds number, which the linked calculators handle.
Can I work back from a target velocity to a flow rate?
Yes. Switch to the Velocity to flow mode, enter the target velocity and the bore, and the calculator returns the flow Q = v A. That is the quickest way to read the capacity of a pipe size at a velocity limit.
How do I get velocity from a mass flow rate?
Convert the mass flow to volumetric flow first by dividing by the fluid density, Q = mass flow over rho, then enter that flow. For water at about 1000 kg per cubic metre, 5 kg/s is close to 5 L/s. The calculator itself takes volumetric flow.
References
- ASME B36.10M, Welded and Seamless Wrought Steel Pipe, for the schedule bores in the size picker and the capacity table. asme.org
- Crane Co., Technical Paper No. 410: Flow of Fluids Through Valves, Fittings and Pipe, the published source for the class of liquid design-velocity bands quoted above. They are reasonable-practice ranges, not verified limits. tp410.com
The compressed-air and steam bands, and the plumbing-code caps, are collated practice rather than a single citable source, so we have not attached one to them.
Related
New to the terms? See the glossary and how it works, or browse all calculators.