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Sprinkler Pipe Friction Loss Calculator (NFPA 13)

Use the Hazen-Williams equation the way NFPA 13 hydraulic calculations do. Enter flow, pipe size and schedule, the pipe length plus the equivalent length of fittings and the starting pressure, and see friction loss, velocity and the pressure at the end of the run.

Your numbers

GPM

120 black steel wet, 100 black steel dry/preaction, 150 CPVC and copper, 140 galvanized wet.

feet
feet
psi
feet

Result

Enter your numbers to see the result.

Take this to the job site

The same tools, and more, in apps that work offline when there is no signal.

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Read the how-to guide

5 min read

How to Use Sprinkler K-Factor to Find Flow and Pressure

Flow from pressure with Q = K√P, the 7 psi floor, and how the demand grows from the remote sprinkler back to the supply.

The calculation

p = 4.52 × Q1.85 ÷ (C1.85 × d4.87) psi per foot, d being the actual inside diameter (for example 2.469 in. for 2-1/2 in. Schedule 40, 2.635 in. for Schedule 10). Total friction = p × (pipe length + fitting equivalent length). Elevation loss or gain is 0.433 psi per foot. End pressure = start − friction − elevation. Velocity pressure is not included, so the result is the normal (static plus friction) pressure the way NFPA 13 reports it.

NFPA 13 limits water velocity in most piping to 32 fps. This is a design aid; hydraulic calculation submittals must be done in a program the AHJ accepts.

Worked example

150 GPM in 100 ft of 2-1/2 in. Schedule 40 at C = 120: velocity is 10.1 fps and friction is 0.084 psi per foot, or 8.4 psi over the run. Starting at 60 psi, the pressure at the end is 51.6 psi.

Frequently asked questions

What C-factor should I use?

NFPA 13 Table 23.4.4.7.1: 120 for black steel (wet), 100 for black steel dry or preaction, 140 for galvanized (wet), 150 for plastic and copper, 140 for cement-lined.

Why do I add equivalent length for fittings?

Each elbow, tee or valve causes extra loss. NFPA 13 gives equivalent pipe lengths for each fitting by size, which you add to the straight length.

Is this a replacement for hydraulic calculation software?

No. It checks a single run. Submittal calculations need the full network, remote area and supply analysis.

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Sample exam questions on this topic

Fire sprinkler (NICET)

NFPA 13 hazard classes, K-factors, Hazen-Williams, spacing rules and NFPA 25 inspection frequencies, explained.

15 questions with explanations

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This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.