How Do You Calculate Standpipe Demand and Pressure?

Standpipe demand is calculated by fixing the flow at the hydraulically most remote hose outlets, adding 250 gpm for each additional standpipe up to the cap, then starting at the required residual pressure at the most remote outlet and adding elevation and friction loss back to the pump discharge or the fire department connection. For a Class I or Class III system under NFPA 14, the National Fire Protection Association standard for standpipe and hose systems, the starting point is 100 psi residual at the most remote 2.5 in. outlet, 500 gpm on the most remote standpipe and 250 gpm on each additional standpipe, with the combined demand capped at 1,250 gpm, or 1,000 gpm where the building is sprinklered throughout (NFPA 14, Chapter 7).

The pressure side is plain hydraulics: 0.433 psi for every foot of elevation plus Hazen-Williams friction loss through the riser, the feed main, the fittings and the valves. An 8-story office with two standpipes and its most remote outlet 90 ft above the pump discharge needs 750 gpm at about 147 psi at the pump. This article lays out the inputs, the method, that worked example, a second variant for a sprinklered building with three standpipes, the errors reviewers catch most often and the point where the hand method stops.

What Inputs Does a Standpipe Calculation Need?

A standpipe calculation needs fewer inputs than a sprinkler calculation, but each one moves the answer by a large amount. The flow side depends on the class of system and the number of standpipes. The pressure side depends on height, pipe, fittings and the water supply.

The class is the first thing to settle, because it fixes both the outlet size and the design pressure. DGA’s explainer on what a standpipe system is and how NFPA 14 Classes I, II and III differ covers the classes; this article assumes a Class I or Class III system, which is what nearly every high-rise and large commercial building in the region requires. Whether the building needs a standpipe at all is a separate question, answered in when a building needs a standpipe under IBC 905.3.

What Is the Method, Step by Step?

The method runs from the top of the building down to the supply, the opposite direction from the water. Every step adds pressure, so the number only grows.

1. Fix the flow at the most remote outlets. On the hydraulically most remote standpipe, flow 250 gpm from the top outlet and 250 gpm from the next outlet below it, 500 gpm in total (NFPA 14 Chapter 7).
2. Add the other standpipes. Flow 250 gpm from the top outlet of each additional standpipe, until the total reaches 1,250 gpm, or 1,000 gpm where the building is sprinklered throughout. A fourth standpipe in a sprinklered building adds no flow because the cap has been reached.
3. Start at the required pressure. Set 100 psi residual at the most remote 2.5 in. outlet (65 psi at the most remote 1.5 in. outlet for a Class II system). Add the hose valve loss from the manufacturer’s data.
4. Add elevation. Multiply the height from the most remote outlet down to the supply point by 0.433 psi per ft.
5. Add friction. Work down each pipe segment with the Hazen-Williams formula using the flow in that segment and the equivalent length of pipe plus fittings and valves. The flow steps up at each point where another outlet or standpipe joins.
6. Check the lower outlets. Where the static pressure at any hose connection would exceed 175 psi, NFPA 14 Chapter 7 calls for a pressure-regulating valve (PRV) at that outlet, and a 1.5 in. outlet also needs a device where its residual pressure would exceed 100 psi; no part of the system may exceed 350 psi in NFPA 14 (2016), or 400 psi in the 2019 edition.
7. Compare with the supply or size the pump. Plot the demand point against the supply curve. If the supply cannot deliver the flow at the pressure, the gap sets the pump’s net pressure at the demand flow.

Figure 1: Formula Card: Standpipe Demand and Pressure

Q_total = 500 gpm (most remote standpipe) + 250 gpm × (additional standpipes), not more than 1,250 gpm, or 1,000 gpm if sprinklered throughout

P_supply = 100 psi + P_{hose valve} + 0.433 × H + Σ (p_f × L_eq)

p_f = 4.52 × Q^1.85 ÷ (C^1.85 × d^4.87) psi per ft

Legend: H is the height of the most remote outlet above the supply point in ft; Q is the flow in the segment in gpm; C is the Hazen-Williams coefficient (120 for new black steel); d is the internal diameter in in.; L_eq is the pipe length plus fitting equivalent lengths in ft.

The friction term is the only part that needs a formula. DGA’s Hazen-Williams friction loss calculator returns psi per ft for any flow, diameter and C factor, which is enough to run every segment of a standpipe by hand.

Worked Example: an 8-Story Office With Two Standpipes

The building is an 8-story office with two exit stairs, so two Class I standpipes. Floor-to-floor height is 12 ft, the hose valves sit 6 ft above each floor and the pump discharge is at the first floor, so the most remote outlet on the 8th floor is 90 ft above the pump discharge.

Both risers are 6 in. Schedule 40 black steel (6.065 in. internal diameter, C = 120), and the feed main from the pump to the base of the remote riser is 30 ft of 6 in. pipe with a check valve, a gate valve, two elbows and a tee.

The flow is 500 gpm on the remote standpipe (250 gpm at the 8th-floor outlet and 250 gpm at the 7th-floor outlet) plus 250 gpm at the top outlet of the second standpipe: 750 gpm total, well under the 1,250 gpm cap. The pressure build-up from the top outlet to the pump discharge is tabulated below, using a 3 psi hose valve loss from a typical listed 2.5 in. valve and NFPA 13 (2022) Chapter 28 equivalent lengths for 6 in. fittings (30 ft per tee, 14 ft per elbow, 32 ft per swing check, 3 ft per gate valve).

The demand is therefore 750 gpm at about 147 psi at the pump discharge. Elevation accounts for 39 psi of the 47 psi added above the outlet pressure; friction through 6 in. pipe at these flows adds only about 5 psi. That proportion is typical: in a standpipe calculation height dominates, and the pipe size matters far less than it does in a sprinkler system.

Figure 2: Pressure Build-Up From the Most Remote Outlet to the Pump

The second half of the example is the lower-floor check. At 750 gpm the first-floor outlet, 6 ft above the pump, sees about 142 psi residual: the 147 psi at the pump less 2.5 psi through the feed main and 2.6 psi of elevation. That is under 175 psi, but the 175 psi trigger in NFPA 14 Chapter 7 is static pressure, so the flowing number does not settle the question. The static case at churn decides it.

Suppose the pump is rated 750 gpm at 110 psi net, selected because the city main delivers 37 psi residual at 750 gpm (37 psi + 110 psi = 147 psi), and the certified curve shows churn at 120 percent of rated net pressure, or 132 psi. With 40 psi static on the suction, the discharge at churn is 172 psi and the first-floor outlet static is about 169 psi. That clears 175 psi by 6 psi, so no PRVs are needed on this building; a pump with a steeper curve, churning at 140 percent of rated net pressure, would push the first-floor outlet to about 191 psi and PRVs on the lower floors would follow. The pump selection itself, including the churn and 150 percent checks against NFPA 20, the fire pump standard, is covered in how to size a fire pump for rated flow and pressure.

Variant: a Sprinklered Building With Three Standpipes

Now take the same 8-story building, sprinklered throughout, with a larger floor plate that needs three exit stairs and three standpipes. The flow is 500 gpm on the remote standpipe plus 250 gpm on each of the other two, 1,000 gpm, which lands exactly on the sprinklered cap of 1,000 gpm. A fourth standpipe would not raise the demand.

The pressure path is the same as the first example until the feed main, which now carries 1,000 gpm: 0.0352 psi per ft over 123 equivalent ft adds 4.3 psi instead of 2.5 psi, so the pump discharge demand becomes about 149 psi. The sprinkler system shares the risers, which raises the question of how the two demands combine.

Under NFPA 14 Chapter 7, where the building is sprinklered throughout, the combined system is designed for the larger of the standpipe demand and the sprinkler demand including its hose allowance, not the sum. A light hazard office sprinkler demand of roughly 400 gpm with hose at 60 psi is far below 1,000 gpm at 149 psi, so the standpipe demand governs and the sprinkler floor control valves hang off a riser already sized for it. How to plot that demand point against the supply, and what to do when the supply curve falls short, is the subject of how to plot sprinkler demand against the supply curve.

DGA designs and calculates standpipe and combined standpipe and sprinkler systems across New York, New Jersey, Pennsylvania, Maryland and Delaware. Project briefs get confirmation and preliminary code feedback within one business day. See where DGA works →

Which Errors Show Up Most Often?

Standpipe calculations fail review for a short list of reasons, and nearly all of them come from the flow and pressure assumptions rather than the arithmetic. The table pairs each error with what it does to the answer.

Summing the two demands deserves a second warning because it looks conservative. The larger pump it produces churns at a higher pressure and drives the lower outlets over 175 psi, which then forces pressure-regulating valves the building never needed.

Where Does the Hand Method Stop and a Sealed Calculation Begin?

The hand method above is exact for a single-zone, automatic wet standpipe fed by one pump or one public connection. It stops being sufficient the moment the building has more than one pressure zone, an express riser feeding an upper zone, or pressure-regulating valves whose settings have to be computed floor by floor. NFPA 14 Chapter 7 sets the system pressure ceiling (350 psi in NFPA 14 (2016), 400 psi in the 2019 edition), New York City limits each zone to 300 ft, and a building that needs a second zone needs a pump-and-tank arrangement that a licensed Professional Engineer (PE) lays out with the fire department.

A manual standpipe changes the supply end of the calculation. NFPA 14 Chapter 7 allows a manual system to be calculated with the fire department pumper supplying the FDC, so the design point moves to the FDC inlet and the available pressure is the fire department’s pumper capacity set with the authority having jurisdiction (AHJ), less the hose lay from the hydrant. The outlet pressure and elevation terms do not change; only the supply point does.

New York City runs its own version of the standard. The 2022 NYC Building Code Appendix Q names NFPA 14 (2016) in Section Q105.1 and modifies it, among other things replacing the 100 psi minimum in Section 7.8.1 with 65 psi, limiting each standpipe zone to 300 ft, capping hose outlet pressure in the lowest zone at 210 psi and requiring in Section 7.7.2 that the system demand be supplied independently by the attached water supply and by each FDC. A standpipe calculation for a New York City building therefore starts from a different outlet pressure and has to be run twice, once from the pump and once from each FDC.

Pressure-regulating valves are a sizing problem of their own: each valve is set for its floor, the loss through it at 250 gpm comes from the manufacturer’s curve, and NFPA 14 Chapter 7 requires that the outlet pressure below the valve stays within the 175 psi and 100 psi limits across the full range of inlet pressures. Where a project has PRVs, zones or a manual system, the sealed hydraulic calculation goes to the AHJ with the standpipe drawings, and DGA runs it under the firm’s standpipe design service.

How Do the Jurisdictions Differ?

The method is the same in every state DGA works in, because every adopted building code reaches NFPA 14 through its Section 905. What differs is the NFPA 14 edition, the local modifications and who reviews the calculation.

Editions as of November 2026. Local amendments other than New York City’s Appendix Q are not shown.

Frequently Asked Questions

This article is general information, not a hydraulic calculation for any project. Code editions, local amendments and reviewer practice change; confirm the adopted code and the authority having jurisdiction for each project.