Does this use the nominal size or the internal diameter?

The internal diameter. The nominal size you pick selects a row in the pipe dimension table, and the calculation uses that row's actual bore. Diameter enters the equation to the 4.87 power, so the difference is not small.

Which C-factor should I use?

It depends on the pipe material and whether the system is wet, dry or preaction. Black steel in a wet system is normally 120 and in a dry or preaction system 100; CPVC is 150 and copper 140. Confirm the values against the edition of NFPA 13 your authority having jurisdiction has adopted.

Does this include fittings?

No. This calculates friction loss over a straight run. Fittings are handled by adding their equivalent length to the pipe length, which the equivalent length of fittings table on this site gives you.

Is this the same calculation the full app runs?

Yes. This page loads the same WebAssembly solver the paid product runs, compiled from the same Rust crate the server links natively. All exponentiation goes through libm, so the browser and the server produce bit-identical results.

What velocity is too high?

NFPA 13 sets no velocity limit for sprinkler piping. Many designers treat 20 ft/s as a practical ceiling because higher velocities are noisy and make the calculation sensitive to small errors. The velocity shown here is advisory, not a compliance check.

Hazen-Williams friction loss calculator

Pressure loss in a pipe from flow, size, material and length. The NFPA 13 form of the equation.

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.

19.9psi

total friction loss over 100.0 ft

Loss per foot: 0.1989psi/ft
Velocity: 14.3ft/s
Internal diameter: 2.067in

The formula

p = 4.52 Q^1.85 / (C^1.85 d^4.87)

p: friction loss per foot of pipepsi/ft
Q: flow through the pipegpm
C: roughness coefficient for the pipe and system
d: internal diameter, not the nominal sizein

Why this form and not the general one

NFPA 13 states Hazen-Williams in US customary units with the constant already folded in, so the result is pressure loss per foot of pipe in psi rather than head loss in feet. That is the form used here, and it is the form a plan reviewer expects to see on the calculation sheet.

The internal diameter is looked up from the reference tables in the solver, not entered, because using the nominal size in place of the actual bore is one of the most common ways a hand calculation ends up optimistic. A 2 in schedule 40 pipe has a 2.067 in bore, and the diameter is raised to the 4.87 power.

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 this use the nominal size or the internal diameter?: The internal diameter. The nominal size you pick selects a row in the pipe dimension table, and the calculation uses that row's actual bore. Diameter enters the equation to the 4.87 power, so the difference is not small.
Which C-factor should I use?: It depends on the pipe material and whether the system is wet, dry or preaction. Black steel in a wet system is normally 120 and in a dry or preaction system 100; CPVC is 150 and copper 140. Confirm the values against the edition of NFPA 13 your authority having jurisdiction has adopted.
Does this include fittings?: No. This calculates friction loss over a straight run. Fittings are handled by adding their equivalent length to the pipe length, which the equivalent length of fittings table on this site gives you.
Is this the same calculation the full app runs?: Yes. This page loads the same WebAssembly solver the paid product runs, compiled from the same Rust crate the server links natively. All exponentiation goes through libm, so the browser and the server produce bit-identical results.
What velocity is too high?: NFPA 13 sets no velocity limit for sprinkler piping. Many designers treat 20 ft/s as a practical ceiling because higher velocities are noisy and make the calculation sensitive to small errors. The velocity shown here is advisory, not a compliance check.

The other calculators

How these results are checked against worked examples · Pricing for the full app

Updated 2026-09-07