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Calcimator

Standpipe System Design Calculator

Standpipe class and sizing from building height and use.

About this calculator

This calculator sizes a standpipe system's flow, pressure, and pipe requirements per NFPA 14, the Standard for the Installation of Standpipe and Hose Systems. It starts from the class you select — Class I delivers 500 GPM at 100 psi residual for fire department 2.5-inch hose, Class II delivers 100 GPM at 65 psi for occupant 1.5-inch hose stations, and Class III combines both — then adds 250 GPM for each additional standpipe riser beyond the first, capped at two extra risers. From that total flow, the calculator runs the Hazen-Williams friction-loss equation (P = 4.52 × Q^1.85 / (C^1.85 × d^4.87)) using your chosen pipe roughness (C-factor) and riser diameter, applied over an estimated pipe run of 1.2 times the building height to account for horizontal routing.

It separately computes static head loss at the standard 0.433 psi per foot of elevation, and sums residual pressure, friction loss, and static head into the total pressure a fire pump must deliver at the base of the riser. It also checks water velocity in the riser (0.4085 × Q / d²) against NFPA 14's 15 ft/s guideline and flags if you need a larger pipe. Keep in mind this uses simplified assumptions — a fixed 20% overhead for horizontal pipe length and a flat additional-riser flow — real hydraulic calculations for permit submittals require a full water supply analysis and most-remote-outlet modeling that this tool doesn't attempt.

Inputs

ft
in

Results

Total system flow (GPM)

750

Total pressure needed (PSI)

146.6

Residual pressure (PSI)100
Static head loss (PSI)32.5
Friction loss (PSI)14.1
Pipe velocity (ft/s)19.1
Velocity acceptableNo
Min riser diameter (in)4
Total hose connections10
Friction loss (PSI/ft)0.16

Figures current as of 2026. Source: National Fire Protection Association, NFPA 14, Standard for the Installation of Standpipe and Hose Systems

How to Use This Calculator
  1. Enter Standpipe class, Building height (ft), and Number of floors.
  2. Set Number of standpipes, Hazen-Williams C factor, and Riser pipe diameter (in).
  3. Review Total system flow (GPM) and Total pressure needed (PSI).
  4. Use Residual pressure (PSI) and Static head loss (PSI) to inform your decision.

What each input means

Standpipe class
The standpipe class per NFPA 14, which sets flow and pressure requirements.
Building height (ft)
Height from grade to highest standpipe outlet.
Number of floors
Total number of floors with standpipe connections.
Number of standpipes
Number of standpipe risers in the building.
Hazen-Williams C factor
Pipe roughness coefficient. 120=new steel, 100=aged steel, 150=CPVC/copper.
Riser pipe diameter (in)
Internal diameter of the standpipe riser.

What each result means

Total system flow (GPM)
Total standpipe system flow demand per NFPA 14.
Total pressure needed (PSI)
Required pump discharge pressure: residual + friction + static head.
Residual pressure (PSI)
Minimum pressure at the most remote hose connection.
Static head loss (PSI)
Pressure due to elevation (0.433 PSI per foot).
Friction loss (PSI)
Pipe friction loss calculated by Hazen-Williams formula.
Pipe velocity (ft/s)
Water velocity in the riser. Should not exceed 15 ft/s.
Velocity acceptable
Whether pipe velocity is within the recommended 15 ft/s limit.
Min riser diameter (in)
Minimum pipe diameter per NFPA 14 for the selected class.
Total hose connections
Number of hose connections required across all floors and risers.
Friction loss (PSI/ft)
Friction loss per linear foot of pipe.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Standpipe class = 1, Building height (ft) = 75, Number of floors = 5, Number of standpipes = 2 = 6 input(s) provided
  2. Calculate Total system flow
    Total system flow = flowPerStandpipeGPM + additionalFlow
    750 = 750
  3. Calculate Total pressure needed
    Total pressure needed = residualPressurePSI + frictionLossPSI + staticHeadPSI
    146.6 = 146.6
  4. Calculate Residual pressure
    Residual pressure
    100 = 100
  5. Calculate Static head loss
    Static head loss = buildingHeightFt * 0.433
    32.5 = 32.5

Figures and sources

Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does adding a second or third standpipe only add 250 GPM instead of another full 500 GPM?

NFPA 14 (Standard for the Installation of Standpipe and Hose Systems) treats the first standpipe as needing full flow (500 GPM for Class I/III) because it represents the hydraulically most demanding riser, but additional risers are assumed to operate at a reduced simultaneous demand of 250 GPM each. The calculator caps this addition at two extra risers (500 additional GPM), reflecting that most designs don't assume every standpipe in a large building flows at once.

What does the Hazen-Williams C-factor actually change, and why does pipe material matter?

The C-factor represents pipe roughness in the friction-loss formula P = 4.52 × Q^1.85 / (C^1.85 × d^4.87) — a higher C-factor means a smoother pipe and less friction loss per foot. The calculator lets you enter anywhere from 80 (aged, corroded steel) to 150 (CPVC or copper), and because C is raised to the 1.85 power in the denominator, swapping from aged steel to smooth plastic pipe can noticeably cut the calculated friction loss and the total pump pressure required.

Why is the estimated pipe length 1.2 times the building height instead of just the height itself?

The riser itself only runs vertically for the building height, but real installations also need horizontal pipe runs connecting the riser to the fire department connection, pump room, and floor outlets. The calculator adds a flat 20% overhead to building height as a simplified stand-in for that horizontal routing when computing total friction loss — a rough estimate, not a substitute for an actual pipe route takeoff.

What happens if the calculated pipe velocity exceeds NFPA 14's 15 ft/s guideline?

The calculator computes velocity as 0.4085 × total flow (GPM) / diameter², and if that number comes out above 15 ft/s it reports Velocity Acceptable as No. That's a signal the riser diameter you entered is undersized for the flow — increasing Riser Pipe Diameter in the input will lower velocity (and also reduce friction loss, since both depend on pipe cross-sectional area).

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