Does NFPA 13 limit pipe velocity?

No. There is no velocity limit for sprinkler piping in NFPA 13. Some authorities having jurisdiction and some insurers impose one; check locally. The threshold in this tool is a designer convention.

Why does the nominal size not match the diameter used?

Nominal size names the pipe, it does not measure the bore. A 2 in schedule 40 pipe has an internal diameter of 2.067 in and a 2 in schedule 10 pipe has more. The calculation uses the internal diameter from the pipe schedule you select.

What does the 20 ft/s flow figure mean?

It is the flow that would put this pipe exactly at 20 ft/s, so you can see how much headroom the pipe has before upsizing becomes worth considering.

Does velocity affect the friction loss calculation?

Not directly. Hazen-Williams works from flow and diameter, not velocity. Velocity is reported because it is the quantity that tells you whether a pipe is working too hard.

Pipe velocity calculator

Velocity in a pipe from flow and internal diameter, in feet per second.

Preliminary. The reference tables behind this calculation are pending a second check against their printed source. Verify against your adopted edition before using a result in a submitted calculation.

14.3ft/s

at 150.0 gpm

Internal diameter: 2.067in
Flow at 20.0 ft/s: 209.2gpm
20.0 ft/s: advisorynot a code limit

The formula

v = 0.4085 Q / d^2

v: velocity in the pipeft/s
Q: flow through the pipegpm
d: internal diameter, not the nominal sizein
0.4085: the US customary constant

Velocity is advisory, not a code limit

NFPA 13 does not set a maximum velocity for sprinkler piping. The 20 ft/s figure many designers work to is a practical convention: above it the pipe is noisy, the friction loss is a large share of the total, and small errors in length or fitting count move the answer more than they should.

The full app flags a pipe over the threshold in the notes list and says plainly that it is not a code limit, so an advisory is never mistaken for a compliance failure.

Run the whole system, not one pipe. Same solver, pass or fail against your flow test, NFPA 13 report as a PDF.

Try the full calc

Questions about this calculation

Does NFPA 13 limit pipe velocity?: No. There is no velocity limit for sprinkler piping in NFPA 13. Some authorities having jurisdiction and some insurers impose one; check locally. The threshold in this tool is a designer convention.
Why does the nominal size not match the diameter used?: Nominal size names the pipe, it does not measure the bore. A 2 in schedule 40 pipe has an internal diameter of 2.067 in and a 2 in schedule 10 pipe has more. The calculation uses the internal diameter from the pipe schedule you select.
What does the 20 ft/s flow figure mean?: It is the flow that would put this pipe exactly at 20 ft/s, so you can see how much headroom the pipe has before upsizing becomes worth considering.
Does velocity affect the friction loss calculation?: Not directly. Hazen-Williams works from flow and diameter, not velocity. Velocity is reported because it is the quantity that tells you whether a pipe is working too hard.

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Updated 2026-09-07