BOOSTER PUMP SIZE
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. Booster pump sizing follows IPC Section 606 and manufacturer head-vs-flow curves.
How to use this calculator
Measure the incoming static pressure at your meter or well tank with a $10 hose-bib gauge during no-flow conditions. Set the target delivered pressure (40-60 PSI is comfortable; code minimum is 20 PSI at the most remote fixture; above 80 needs a pressure-reducing valve).
Enter the peak demand in GPM (use the water supply pipe sizing calculator to compute from WSFU). Enter the vertical lift to the highest fixture — each foot of lift costs 0.433 PSI. The calculator returns required PSI boost, approximate motor HP, and pump duty class (single-stage, multi-stage, or high-pressure).
Worked example
For a typical low-pressure home: incoming 35 PSI static, target 60 PSI, 15 GPM peak demand, 25 ft vertical lift to the second-floor master bath:
Lift loss: 25 × 0.433 = 10.8 PSI. Required boost: (60 - 35) + 10.8 = 35.8 PSI.
Motor HP: (15 × 35.8) ÷ (3960 × 0.65) = 537 ÷ 2,574 = 0.21 HP → ½ HP pump (smallest standard).
Duty class: medium duty (35.8 PSI boost). Spec a multi-stage Grundfos CRE or Goulds Aquavar VFD pump in the 1/2-3/4 HP range.
For a top-floor apartment in a 4-story building: incoming 50 PSI, target 60 PSI, 20 GPM, 40 ft lift:
Lift: 40 × 0.433 = 17.3 PSI. Boost: (60 - 50) + 17.3 = 27.3 PSI. Motor: (20 × 27.3) ÷ 2,574 = 0.21 HP → ½ HP. Duty: medium.
For a well system feeding an irrigation main 200 ft from the well: incoming 40 PSI well tank, target 60 PSI, 30 GPM peak: boost only (no lift) = 20 PSI. Motor: (30 × 20) ÷ 2,574 = 0.23 HP → 1/2 HP. Light duty single-stage.
Common mistakes & waste factors
Forgetting vertical lift. Each foot of lift adds 0.433 PSI to the boost requirement. A 30-ft lift adds 13 PSI on top of horizontal pressure loss — easy to miss when sizing for "low pressure."
Under-sizing GPM. A pump rated for 10 GPM at 30 PSI boost might fail at 15 GPM (pumps lose head as flow increases). Always size GPM for peak demand AND check the pump curve.
Ignoring the pump curve. Datasheet pumps show GPM × PSI capability across a curve — peak GPM is at minimum boost, peak boost at minimum GPM. Spec the pump for your design point on the curve, not the headline numbers.
Skipping the bypass tank. Constant-pressure pumps need a small (2-5 gal) hydropneumatic tank on the discharge to prevent rapid cycling on small flows (toilet flapper drips, ice maker fill). Without it, the pump cycles 100+ times per day and burns out fast.
Rules of thumb
Boost PSI = (Target − Supply) + (Lift × 0.433).
Motor HP = (GPM × Boost PSI) ÷ (3960 × 0.65 efficiency).
Residential pump duty: ½ HP for ≤25 PSI boost, ¾ HP for 25-50 PSI, 1+ HP for 50+ PSI.
VFD (variable-frequency drive) pumps maintain constant pressure across all flow rates — preferred for residential. Constant-speed pumps require pressure tank for stability.
Always install a 2-5 gallon hydropneumatic tank on the discharge to absorb pressure variations.
Add 10-20% to GPM for VFD pump capacity headroom (allows the pump to run below max speed).
Well systems with submersible pumps: oversizing wastes electricity but prevents rapid cycling. Match pump performance to actual demand curve.
Common questions
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