In short: a pump's flow rate can be estimated from Q (GPM) = 3,960 × BHP × η ÷ (H × SG). A 1.5 HP pump running at 65% efficiency against 80 ft of head delivers about 48.3 GPM of fresh water. Lower head, more power, or higher efficiency all raise the flow rate.
This is a planning-aid estimate based on published hydraulic relationships, not a substitute for the manufacturer's pump curve. For sizing a system, confirm against the actual curve or a licensed engineer.
How to calculate pump flow rate
A pump converts shaft power into hydraulic power — the product of flow rate and head. Only part of the shaft power actually reaches the water; the rest is lost to friction and turbulence inside the pump, which is what efficiency accounts for. Rearranging the standard water-horsepower relationship for flow gives:
Worked example. A 1.5 HP pump at 65% efficiency, lifting fresh 20°C water (SG ≈ 0.998) to 80 ft of total dynamic head:
If you don't already know the total dynamic head, use the Total Dynamic Head Calculator first — it accounts for static lift, friction and velocity losses.
Why head, power and efficiency change the answer
Head is the main resistance the pump works against — every extra foot of lift or friction loss claims more of the available power, leaving less for flow. Doubling head roughly halves flow rate for the same power and efficiency.
Power sets the total energy budget. More shaft horsepower means more hydraulic power is available, so flow rate rises proportionally at a fixed head.
Efficiency determines how much of that shaft power actually reaches the water rather than being lost as heat and turbulence inside the pump casing. A pump running well off its best-efficiency point can lose 15-20 percentage points of efficiency, which shows up directly as lower flow for the same power draw — see the Pump Efficiency Calculator.
Reference table
Estimated flow rate at 65% efficiency, fresh water at 20°C (SG ≈ 0.998) — a typical small centrifugal pump.
| Power | Head | Flow rate |
|---|---|---|
| 0.5 HP | 40 ft | 32.2 GPM |
| 1 HP | 40 ft | 64.5 GPM |
| 1.5 HP | 80 ft | 48.3 GPM |
| 2 HP | 100 ft | 51.6 GPM |
| 3 HP | 60 ft | 128.9 GPM |
| 5 HP | 150 ft | 86.0 GPM |
Worked examples
Small submersible well pump
≈8.3 GPM
A 0.5 HP submersible pump at 50% efficiency, lifting water 120 ft, delivers about 8.3 GPM — typical for a shallow household well.
Home booster pump
≈138.8 GPM
A 3 HP booster pump at 70% efficiency against 60 ft of head can move roughly 139 GPM, well above typical residential demand — most homes would use a smaller unit.
Irrigation pump, warm supply
≈372 GPM
A 5 HP irrigation pump at 75% efficiency against 40 ft of head, drawing 25°C water (SG ≈ 0.997), delivers about 372 GPM — the low head and high power favor volume over pressure.
What else affects the result
Where you are on the pump curve. Efficiency isn't fixed — it changes with flow and head, peaking at the pump's "best efficiency point." A single efficiency value is an estimate; the manufacturer's curve gives the real number at your operating point.
Suction conditions. If available suction head is too low, a pump can cavitate and deliver far less than this formula predicts, regardless of power. Check available net positive suction head with the NPSH Calculator.
Pump speed. Flow rate scales linearly with impeller speed under the pump affinity laws — a variable-speed drive changes the answer without changing power or head. See the Pump Affinity Laws Calculator.
Fluid density. Denser fluids — seawater, brine, glycol mixes — need more power for the same flow and head. Use the water type selector above, or the custom density option for a specific fluid.
Frequently asked questions
How do I calculate pump flow rate from horsepower?
Use Q (GPM) = 3,960 × BHP × η ÷ (H × SG), where BHP is the pump's brake (shaft) horsepower, η is efficiency as a decimal, H is total dynamic head in feet, and SG is the specific gravity of the fluid. For example, a 1.5 HP pump running at 65% efficiency against 80 ft of head delivers about 48.3 GPM of fresh water.
What pump efficiency should I use?
Small centrifugal pumps typically run 60-75% efficient, jet pumps 40-50%, and submersible well pumps 50-65%. Check the manufacturer's pump curve for the actual value at your operating point if you have it; efficiency changes with flow and head, so a single number is only an estimate.
Does water temperature affect pump flow rate?
Only slightly. Warmer water is less dense, which lowers its specific gravity, and the flow-rate formula is inversely proportional to specific gravity. Between 4°C and 60°C the effect is under 2%, so temperature rarely changes the practical answer unless you're pumping a much denser fluid like seawater or brine.
What's the difference between brake horsepower and water horsepower?
Brake horsepower (BHP) is the mechanical power delivered to the pump shaft, usually close to the motor's rated output. Water horsepower (WHP), also called hydraulic power, is the smaller portion of that power actually transferred to the water. The ratio WHP ÷ BHP is the pump's efficiency.
Why does higher head reduce flow rate for the same pump?
A pump has a fixed amount of hydraulic power available at a given horsepower and efficiency. Power equals flow rate multiplied by head, so if head goes up, flow rate must come down to keep the power the same. This is why every pump has a performance curve that trades flow for head rather than a single fixed GPM rating.
Sources & references
- Hydraulic Institute. ANSI/HI 1.1-1.5 & 9.6.1 — Rotodynamic (Centrifugal) Pumps for Nomenclature, Definitions, Application and Operation.
- Kell, G. S. (1975). Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C. Journal of Chemical & Engineering Data, 20(1), 97–105.
- National Institute of Standards and Technology (NIST). Thermophysical Properties of Fluid Systems.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)