Fire Pump Horsepower Calculator

Example data table

Use these sample rows to sanity-check your entries.

Formula used

How to use this calculator

1. Enter the demand flow rate and choose its unit.
2. Select total head directly, or compute it from pressures.
3. Set specific gravity for the pumped fluid if needed.
4. Use realistic pump and driver efficiencies for your stage.
5. Press Submit to view results above the form.
6. Download CSV or PDF for submittals and estimates.

Flow and head inputs

Fire pump power depends on required flow and total dynamic head. The calculator accepts flow in gpm, lpm, m³/h, or m³/s, then converts to gpm for horsepower math. Head can be entered directly or derived from suction and discharge pressures with elevation correction. Consistent units reduce bid errors. For metric head, meters convert to feet using 1 m = 3.28084 ft. In pressure mode, 0.433 psi equals 1 ft of water, so gauge errors can shift head at high flows.

Pressure to head conversion

Pressure is converted to head using 2.31 ft per psi for water, then adjusted by specific gravity. For example, 150 psi discharge minus 20 psi suction equals 130 psi, or about 300 ft of head on water. If pumping a glycol blend with SG 1.10, the same pressure requires roughly 330 ft equivalent head.

Efficiency and losses

Hydraulic horsepower is computed with WHP = (Q × H × SG) / 3960. Pump and driver efficiencies translate that hydraulic demand into input horsepower. Typical combined efficiencies for preliminary estimating fall between 55% and 75%, depending on size and speed. The form lets you model conservative values and see how power rises. If pump efficiency is 70% and driver efficiency is 92%, combined is 0.644, so input horsepower is about WHP / 0.644. This helps motor starter and feeder sizing.

Safety factor and sizing

Design teams commonly apply a safety factor to cover uncertainty, aging, and future allowances. A 1.10 factor adds 10% to input horsepower, while 1.25 provides a larger cushion for rough assumptions. Use manufacturer curves for final selection, but the calculator helps screen motor sizes and generator capacity early. for quick estimates.

Using results on site

After you submit, the results display above the form for quick review during takeoffs. Export CSV for schedules, and export PDF for submittals and internal approvals. Compare scenarios in the example table to validate your entries, then document the assumptions: flow basis, head basis, SG, efficiencies, and safety factor.

FAQs

What pump efficiency should I enter?

Use tested efficiency when available. For early estimating, many centrifugal pumps fall around 65–75%. Enter a conservative value when curves are unknown, then refine when the selected model and speed are confirmed.

Why does specific gravity change horsepower?

Horsepower rises in direct proportion to specific gravity. Heavier liquids need more energy to move the same flow against the same head, so the calculator multiplies the head term by the entered SG.

Can I work in metric units?

Yes. You can enter flow in lpm, m³/h, or m³/s, and head in meters. The tool converts to gpm and feet internally, then reports horsepower outputs for your scenario.

What does the safety factor affect?

It multiplies the driver input horsepower to suggest a more robust rating. Use it to cover uncertainty, future allowances, and small data errors. Final sizing should still follow the selected pump curve and equipment ratings.

Why is head from pressure different than my TDH?

Pressure gauges show differential pressure at the taps. TDH also includes elevation change and can include suction conditions, velocity terms, and system losses. Verify gauge locations, unit conversions, and elevation direction.

Is this suitable for final fire pump selection?

Use it for preliminary sizing, budgeting, and comparison of scenarios. Final selection requires manufacturer curves, driver starting conditions, and compliance checks by qualified professionals using project requirements.