Find the velocity, velocity head, and Reynolds number for a flow and pipe, and compare velocity across nominal sizes with recommended/caution/excessive bands by service — steel, stainless, copper, PVC, and HDPE tables included.
US + metric
Formula shown
Shareable results
Pipe Size & Velocity
in
Velocity4.34 ft/sRecommended
Velocity head (v²/2g)
0.293 ft
Reynolds number
91,200
Flow regime
Turbulent
Inside diameter used
3.07 in
Velocity across sizes (for this flow & service)
Size
Inside dia
Velocity
Status
1/2"
0.622 in
106 ft/s
Excessive
3/4"
0.824 in
60.2 ft/s
Excessive
1"
1.05 in
37.1 ft/s
Excessive
1-1/4"
1.38 in
21.5 ft/s
Excessive
1-1/2"
1.61 in
15.8 ft/s
Excessive
2"
2.07 in
9.56 ft/s
Caution
2-1/2"
2.47 in
6.7 ft/s
Recommended
3"
3.07 in
4.34 ft/s
Recommended
4"
4.03 in
2.52 ft/s
Recommended
6"
6.07 in
1.11 ft/s
Recommended
8"
7.98 in
0.641 ft/s
Recommended
10"
10 in
0.407 ft/s
Recommended
12"
11.9 in
0.287 ft/s
Recommended
Formula & method
v (ft/s) = 0.4085 · Q(gpm) / d(in)² · velocity head = v² / 2g · Re = vD/ν
0.4085 = 576 ÷ (448.831 × π), pure unit conversion. Velocity bands (suction lower than discharge) are engineering guidelines from Cameron Hydraulic Data / the Hydraulic Institute — good practice, not code limits.
For preliminary sizing and educational use. Verify against applicable standards and codes; velocity limits vary by fluid, service, and material.
Pipe velocity is just flow divided by area, and it is one of the most useful numbers in a piping system. Too slow and you waste money on oversized pipe; too fast and you get noise, erosion, water hammer, and steep friction losses. In US units the shortcut is v = 0.4085 × gpm ÷ (inside diameter in inches)², and this tool also returns the velocity head, the Reynolds number, and the flow regime.
Where 0.4085 comes from
It is pure unit conversion. Velocity equals flow over area; converting gpm to cubic feet per second (÷ 448.831) and the diameter in inches to a circular area in square feet (π/4 × (d/12)²) collapses to the single factor 576 ÷ (448.831 × π) = 0.4085. Nothing empirical — the same physics as the friction and NPSH calculators.
Recommended velocity depends on service
There is no single “right” velocity — it depends on where the pipe is. Pump suction lines are kept slower (guideline around 5 ft/s and below) to protect NPSH and avoid drawing air or cavitating; discharge and general lines tolerate more (roughly up to 8–10 ft/s before caution). This calculator colours a table of adjacent nominal sizes green, amber, or red for the service you pick, so you can see at a glance which size lands in the comfortable range. These bands are widely used engineering guidelines from sources such as Cameron Hydraulic Data and the Hydraulic Institute — they are guidance for good practice, not code limits, so treat them accordingly.
The pipe tables
Inside diameter varies a lot with material and wall thickness, and velocity depends on the actual bore, not the nominal label. The tool carries steel/PVC Schedule 40 and 80, stainless Schedule 10S, copper types K, L, and M, and HDPE DR11 and DR17, or you can type an exact inside diameter. Take the result into the friction loss calculator to turn velocity into head.
