Fire Pump Sizing Calculator

Calculator Inputs

Sizing approach

Choose how combined demand is treated for your project basis.

Sprinkler demand (gpm)

Hose stream allowance (gpm)

Standpipe demand (gpm)

If not applicable, enter 0.

Remote outlet pressure requirement (psi)

Elevation gain (ft)

Vertical rise from pump to remote point.

Friction loss (psi)

Backflow/valves/other losses (psi)

Suction pressure at pump (psi)

Use negative for lift / vacuum conditions.

Liquid specific gravity

Water is 1.00 at typical conditions.

Pump efficiency (%)

Motor efficiency (%)

Service factor (x)

Design margin on flow (%)

Design margin on pressure (%)

Available NPSH (ft) (optional)

Required NPSH (ft) (optional)

Reset

Example Data Table

Scenario	Sprinkler (gpm)	Hose (gpm)	Standpipe (gpm)	Remote (psi)	Elevation (ft)	Friction (psi)
Mid-rise office	750	250	500	100	60	25
Warehouse	1000	250	0	90	20	18
High-rise core	900	250	750	125	140	40

Values are illustrative for training and early sizing only.

Formula Used

Combined demand flow (gpm): max(Sprinkler + Hose, Standpipe) or a full sum option.
Elevation pressure (psi): (Elevation ft / 2.31) × Specific Gravity.
Total system pressure (psi): Remote + Friction + Miscellaneous + Elevation pressure.
Pump discharge pressure (psi): Total system pressure − Suction pressure.
Head (ft): (Pressure psi × 2.31) ÷ Specific Gravity.
Brake horsepower: (Flow gpm × Head ft) ÷ (3960 × Pump efficiency).
Motor horsepower: (Brake HP ÷ Motor efficiency) × Service factor.

How to Use This Calculator

Enter sprinkler demand, hose allowance, and standpipe demand as applicable.
Provide remote outlet pressure and estimate friction and valve losses.
Add elevation gain from pump to the remote point.
Enter suction pressure based on your water supply conditions.
Set efficiencies and margins to match your design practice.
Click Calculate to view recommended standard flow, pressure, and power.
Download CSV or PDF for records and reviews.

Practical Notes

This tool supports early sizing for pump flow, discharge pressure, and motor power. Final selection should confirm water supply, controller compatibility, driver type, and acceptance test requirements.

Always verify applicable standards, authority requirements, and detailed hydraulic calculations before procurement.

Technical Article

1) Why fire pump sizing matters

A fire pump is selected to deliver the most demanding combination of flow and pressure for a building’s fire protection system. Under-sizing can reduce sprinkler or standpipe performance, while over-sizing increases cost, driver power, and unnecessary churn in control valves. This calculator supports early sizing by organizing inputs into a consistent flow basis and a clear pressure budget.

2) Defining the design flow

Start with the sprinkler demand (gpm) and add the hose stream allowance (gpm). If standpipes are present, compare that demand to the combined sprinkler plus hose demand and size for the larger value, unless your design basis requires summation. A flow margin (for example 10%) helps cover data uncertainty and future adjustments. The tool then rounds up to common standard ratings such as 500, 750, 1000, 1500, 2000, or 2500 gpm for practical procurement.

3) Building the pressure budget

Pressure at the remote point is not only the outlet requirement. Add piping friction loss, valve and device losses, and elevation gain. Elevation is converted using 2.31 ft per psi for water, adjusted by specific gravity. The calculator subtracts available suction pressure to estimate discharge pressure, then applies a pressure margin (for example 10%) to protect against field variations and conservative loss assumptions.

4) Converting to head and horsepower

Pump curves are typically presented in head (ft), so the tool converts pressure to head using Head(ft) = Pressure(psi) × 2.31 ÷ SG. Estimated brake horsepower is calculated by (Flow × Head) ÷ (3960 × pump efficiency). Motor horsepower then accounts for motor efficiency and service factor, and the result is rounded up to a practical motor size.

5) Using results for review

Treat the recommended standard flow and pressure as a starting point for coordination. Confirm detailed hydraulics, water supply curves, minimum suction pressure, NPSH, and controller/driver selection. Use the CSV and PDF exports to document assumptions, share a review package, and track revisions during design development.

FAQs

1) Should I use “max” or “sum” for combined demand?

Use “max” when sprinkler+hose and standpipe demands are alternatives and you size for the larger case. Use “sum” only when your design basis requires simultaneous demand.

2) Why does elevation affect pressure?

Lifting water to a higher elevation consumes pressure. For water, every 2.31 ft of rise equals about 1 psi of pressure loss (adjusted by specific gravity).

3) What should I enter for suction pressure?

Enter the expected pressure at the pump suction from the water supply at the relevant flow. If the pump is above the water source, suction may be negative (lift conditions).

4) How do I estimate friction loss quickly?

Use preliminary pipe sizing with Hazen-Williams or manufacturer charts, then add fitting losses. Early design often uses a conservative allowance, later replaced by a full hydraulic calculation.

5) Why does the tool round up to standard ratings?

Fire pumps are commonly purchased in standard rated flows and pressures. Rounding up supports availability and acceptance testing, then the selected pump curve is checked against the full system curve.

6) Is the horsepower result final for motor selection?

It is a planning estimate. Final motor sizing should consider the selected pump curve, driver type, service factor, site voltage, ambient conditions, and any specific authority requirements.

7) What else should be verified before ordering?

Confirm water supply test data, minimum suction pressure, NPSH margin, controller and starter type, alarm and supervision points, and field acceptance test requirements. Coordination with electrical and civil teams is essential.