How to Size a Fire Pump—A Guide for Engineers

December 18, 2025

Learn how to calculate flow rate and pressure to ensure fire pumps are correctly sized for the building height.

By Sahil Mahajan, PE, P.Eng, CPD, LEED Green Associate

Sizing a fire pump requires a precise calculation of two main variables: flow rate (gallons per minute [gpm]) and pressure (pounds per square inch [psi]). These calculations are governed principally by NFPA 13: Standard for the Installation of Sprinkler Systems, NFPA 14: Standard for the Installation of Standpipe and Hose Systems, and local building codes.

1. Determining the Flow Rate (gpm)

In a building protected in accordance with NFPA 13 or NFPA 13R, the water supply for the combined sprinkler and automatic standpipe system shall be based on the sprinkler system demand (including any hose stream demand) or the standpipe demand, whichever is greater.

In most cases, the standpipe demand is greater than the sprinkler demand unless water curtain sprinklers or car stackers are included in the design area. It is the responsibility of the design engineer to ensure that maximum flow is considered in the hydraulic calculations. The sprinkler demand is calculated for a 1,500-sf remote area based on the hazard classification (light, ordinary, or extra hazard) for a wet sprinkler system. A 30 percent increase in remote area is considered in the calculations if a dry sprinkler system is utilized.

Sprinkler system hose demand in fully sprinklered buildings shall not be required to be added to standpipe calculations.

For a combined system in a building equipped with partial automatic sprinkler protection, the standpipe flow rate required shall be increased by an amount equal to the hydraulically calculated sprinkler demand or 150 gpm for light hazard occupancies, or 500 gpm for ordinary hazard occupancies, whichever is less.

Standard Vertical Standpipes (Class I and III)

First riser: The minimum flow rate for the hydraulically most remote standpipe is 500 gpm at the two most remote 2½-inch fire hose connections as per NFPA 14.
Additional risers: For floor areas that do not exceed 80,000 sf, each additional standpipe adds 250 gpm to the demand.
Maximum demand:
- Sprinklered buildings: The maximum flow rate is capped at 1,000 gpm for buildings fully sprinklered in accordance with NFPA 13.
- Partially sprinklered buildings: The calculated demand is generally capped at 1,250 gpm.
- Note on local codes: Be sure to check the local code for any variances. For example, the New Jersey Building Code allows a maximum demand of 1,250 gpm for combined systems.

Horizontal Standpipes

Horizontal standpipes require higher demand calculations to accommodate tactical fire department hose line deployment.

Flow rate: If a horizontal standpipe supplies three or more hose connections on any floor, the minimum flow rate is 750 gpm. This configuration assumes three hose lines are being deployed simultaneously, unlike vertical standpipes, which typically assume two.
Piping: These usually require a minimum of 4-inch pipe to meet the demand.

2. Determining Pressure Requirements (psi)

Once the flow rate is established, the pump must provide enough pressure to overcome gravity (static head) and friction while maintaining residual pressure at the outlet.

Residual Pressure

Minimum requirement: The system must maintain a specific residual pressure at the most remote hose connection. Again, check the local requirements. For instance, this is typically 65 psi for New Jersey and New York City and 100 psi for New York State.
Maximum limit: The maximum pressure at any point in the system cannot exceed 400 psi (per NFPA 14-2019) connected to hose valves. High-pressure zones beyond 400 psi can be in the system as long as they are not connected to hose valves.

The Calculation Formula

To size the pump zone, you must calculate the total pump discharge pressure:

Total pressure = Static head + Residual pressure + Friction loss

Static head: Calculate the vertical distance. For example, a 650-ft zone requires overcoming static pressure calculated as 650 ft × 0.433 psi/ft = 281.45 psi.
Friction loss: Use a Hazen-Williams friction loss calculator to determine loss through piping and fittings.
Safety buffer: It is advised to include a buffer of 20 to 50 psi to account for fluctuations in city water pressure. (Require further confirmation with the water utility company.)

3. Building Height and Zoning Strategies

Building height dictates whether a single fire pump is sufficient or if the building must be divided into zones. A high-rise is defined as a building where an occupiable story is greater than 75 ft above the lowest level of fire department vehicle access.

Zoning for Pressure Management

Because the maximum pressure allowed in the system is 400 psi per NFPA 14-2019 (increased from 350 psi in previous versions), engineers can add more floors to a single fire pump zone.

Optimization: Engineers should try to divide the building into equal zones while keeping the pressure under 400 psi to optimize fire pump size.
Zone height: Based on a 65-psi residual requirement, a single zone can cover approximately 59 to 60 floors (10-ft average floor height) as a rule of thumb. If 100-psi residual is required, the number of floors per zone will decrease.

Very Tall Buildings (>420 ft)

For buildings exceeding 420 feet in height:

Risers: Each sprinkler zone must be supplied by no fewer than two risers.
Water supply: Fire pumps must be supplied by connections to at least two water sources/mains located in different streets to ensure redundancy. If two different streets are not available, two separate connections are allowed to be made from the same street main with an isolation valve in between, pending confirmation from the water utility company and the local fire department.

References

NFPA 13-2019: Standard for the Installation of Sprinkler Systems
NFPA 14-2019: Standard for the Installation of Standpipe and Hose Systems
2021 New Jersey Building Code

About the Author

Sahil Mahajan, PE, P.Eng., CPD, LEED Green Associate, has a Master of Science in Mechanical Engineering and is currently working at KEA Engineers in Iselin, New Jersey, as the Plumbing and Fire Protection Department Head. He has nearly 20 years of experience in HVAC, plumbing, and fire protection design and is a licensed Professional Engineer in the United States and Canada.