BOOSTER PUMP SIZE
BOOSTER PUMP SIZE
pump = peak GPM @ (target − supply) PSI
PSI
PSI
GPM
ft
RESULT
—
FILL IN ABOVE
Sizes the boost stage. Variable-speed (VFD) pumps modulate to maintain target; constant-speed sizes for the peak design point. Estimate only — verify with a licensed plumber and local plumbing code/inspector before purchase or installation. Not a substitute for engineered drawings.
About this calculator
A booster pump adds pressure to a water supply that doesn't arrive at adequate PSI for the building — common with low municipal pressure, well systems on long supply lines, or top-floor fixtures in tall buildings. This booster pump sizing calculator returns the required pump PSI boost and GPM rating based on incoming static pressure, target delivered pressure, and peak flow demand. Use the result to spec a constant-pressure or VFD pump from any major manufacturer. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.
Common questions
When do I need a booster pump?
Three common scenarios. (1) Low municipal pressure: incoming static under 30 PSI consistently delivers under 20 PSI at far fixtures, which is below code minimum. (2) Tall buildings: any structure where elevation lift to the top floor exceeds the available pressure margin (every story adds about 4 PSI of static loss). (3) Long well runs or rural service lines where friction loss eats most of the available pressure before it reaches the building. Test incoming pressure with a hose-bib gauge before assuming you need one.
What's the difference between constant-speed and VFD booster pumps?
Constant-speed pumps cycle on/off when pressure drops below the lower set point and run until pressure hits the upper set point. They're cheaper and simpler but cause noticeable pressure fluctuations and pump short-cycling. VFD (variable-frequency drive) pumps modulate motor speed continuously to hold pressure at exactly the set point — much smoother, much quieter, and usually quieter long-term running cost. VFD is the modern default for residential and small commercial.
How do I prevent the pump from short-cycling?
Two ways. Add a pressure tank downstream of the pump (typical 4–14 gal bladder tank) to give the pump time between starts — sized for at least one minute of run time at peak demand. Or use a VFD pump that ramps speed instead of cycling on/off. Short-cycling kills pump motors fast — the start current is 6–8× running current and the bearings/seals see the thermal shock at every start.