Booster Pump Sizing Calculator

Engineering-Led Sizing · Watermain Supply · Houston, TX

Booster Pump
Sizing Calculator

Answer questions about the building and get what a booster is actually selected on: peak flow in GPM, required boost in PSI, and the VFD setpoint to commission it at.

4Numbers You Need
0.433PSI Per Foot of Lift
3Building Types
1-888.510.4036Talk to a Person

The Short Version

  1. Peak flow is probabilistic. Fixtures are counted as fixture units and run through a probability curve, because they are never all open at once. Adding every fixture's full flow oversizes the pump 2 to 3 times.
  2. Continuous loads are not. Laundry, irrigation, pool fill and cooling tower makeup run steadily. They are added on top of the probabilistic number, never counted as fixture units.
  3. Required pressure is a stack. Lift × 0.433, plus pipe friction, plus device losses, plus residual pressure at the highest fixture.
  4. Boost is not discharge. Subtract suction pressure. A pump fed 45 PSI only has to add the difference. Where the water comes from changes the pump, not just the arithmetic.
  5. Above roughly 80 PSI at the lowest fixture you have a zoning problem, not a bigger-pump problem.

The Calculator

Estimating tool. It produces a design target to check a selection against and to start a conversation with a mechanical contractor. It is not a stamped design and does not replace the engineer of record.

Step 1 · Building

Type sets the fixture template and which demand method applies.

Floors above the pump
Pump discharge to highest flow outlet
Pump to that same outlet

Step 2 · Dwelling Units or Guest Rooms

Count rooms, not individual fixtures. A bathroom is counted as a group — water closet, lavatory and tub or shower together.

Average across the unit mix. Each = WC + lavatory + tub or shower
Sink and dishwasher
1 = every unit, 0 = none

Step 3 · Common Areas

Lobby restrooms, pool bath, fitness, leasing office, banquet. Public toilets are usually flushometer valves, which carry several times the load of a tank toilet — this is the input that moves the answer most.

No tank, chrome valve on the wall
Pool, fitness, locker room

Step 4 · Equipment That Runs Continuously

These do not stop and start like a faucet, so they are added on top at full flow rather than through the probability curve. Fold a 30 GPM laundry into fixture units and the math discounts it to almost nothing.

Step 5 · Where the Water Comes From

This decides the kind of pump as much as the size of it. A pump that has to lift water up out of a well or an underground tank is a different machine from one fed by a pressurised main.

Residual, not static — from a flow test, not a hose bibb
Vertical distance the pump must pull up

Step 6 · The Pipe

Leave both sizes on Recommend a size and the calculator works out what the flow needs and tells you. Change them if the pipe is already in the ground. Suction and discharge are sized to different rules and are often not the same size — the suction side is usually one size larger.

Source to pump inlet. Sized slow to protect the pump.
Pump outlet up into the building
Elbows and valves add friction like extra pipe length
Leave 12 if unknown. Required on every booster, 8 to 14 typical.
Leave 5 if unknown. Add each device.

Step 7 · Available Power

Does not change the sizing numbers. It decides which machines can be used at all, and it is the most common reason a correctly sized quote has to be redone.

Drive and tank

How far the drive overshoots before shutting down
How far pressure drops before it wakes
Leakage and dribble fixtures — a design estimate

Optional · Check a Specific Pump

Pick a model and the calculator will say whether it covers this building, where it fails if it does not, and how close it runs to its best efficiency point.

Peak Flow — Code Method—GPM
Realistic Peak — Low-Flow Fixtures—GPM
Required Boost—PSI
Discharge Pressure / VFD Setpoint—PSI
Pressure term PSI Where it comes from
Flow term Value Method
Commissioning target Value What it is

Checking a Specific Pump

A pump is a fit only if it clears every constraint. The curve is one of them, and not usually the one that fails.

Check What fails if it is missed
Boost at design flow Read the curve at your GPM, not at the corner of the spec sheet. Max flow and max boost are opposite ends and never happen together.
Setpoint ceiling A controller has a maximum settable pressure. Above it the pump cannot hold your discharge pressure no matter what the curve says.
Suction condition Most boosters cannot pull a lift at all. If the water is below the pump, the machine has to be self-priming and within its rated suction depth.
Voltage and phase Decides which machines can be used at all, and is the most common reason a correctly sized quote has to be redone.
Distance from best efficiency A pump that clears every hard limit but runs at 40% of its best efficiency point will be noisy, inefficient and back under warranty.

Curve data is taken from the manufacturer's current published duty points. Where a duty falls beyond the last tabulated point, the calculator says so rather than extrapolating.

Peak Flow, Step by Step

Flow is how much water the building can draw at once, in gallons per minute.

1. Count fixture units. Every fixture gets a number. A private lavatory is small; a public toilet on a flushometer valve is roughly fourteen times larger, because it draws a large slug of water on every actuation. Tank against flushometer moves the answer more than any other input on the form.

2. Run the total through the probability curve. Fixture units are not gallons. The curve converts them, and it flattens hard as the building grows: doubling the units does not double the flow. That flattening is the entire reason a 200-unit building does not need a 400 GPM pump.

3. Add continuous loads at full flow. A laundry, an irrigation zone, a cooling tower and a pool fill are not intermittent. They run. Putting them through the fixture-unit curve discounts them to nearly nothing and undersizes the pump — a worse failure than being expensive.

Worked exampleA 60-unit apartment building: 1.4 baths per unit, a kitchen and washer in each unit, and a common-area group of six flushometer restrooms, two urinals, eight lavatories, two pool showers, three mop sinks and four hose bibbs.
Units contribute 470 fixture units, common areas 107. Total 577. Through the tank-toilet probability curve that returns 82 GPM. A 30 GPM laundry room and a 20 GPM pool fill are added on top at full flow. Design peak: 132 GPM.
Where the code method overestimatesThe probability curve in general use dates to the 1930s and predates low-flow fixtures. Measured hot water flow in multifamily buildings has come in far below what designs based on it predict, and IAPMO built the Water Demand Calculator — free, and in the 2021 and 2024 Uniform Plumbing Code as Appendix M — to correct it for single- and multifamily dwellings. The second number on this page estimates that correction. Treat it as the number your pump should run near, and the code number as the number it must be able to reach.

Required Pressure, Step by Step

Head is how high a pump can push water, in feet. Boost is the pressure it adds, in PSI. One foot of water is 0.433 PSI.

Worked exampleThe same building: 85 ft of rise, 3 in copper riser, 120 ft horizontal run, 132 GPM, normal fixtures at the top, city pressure 42 PSI at peak.
Developed length with a 50% fitting allowance is 308 ft. Hazen-Williams at C = 140 gives 4.05 ft per 100 ft, so friction is 5.4 PSI.
Static: 85 × 0.433 = 36.8. Friction: 5.4. Backflow preventer: 12. Meter and strainer: 5. Residual: 20.
Discharge, and therefore setpoint = 79.2 PSI. Boost = 79.2 − 42 = 37.2 PSI.
Leave the suction credit out and the same building appears to need a 79 PSI boost — more than twice the machine, and a frame size or two of wasted capital.
Setpoint against boostThe drive holds discharge pressure, so the setpoint is the number you program into the controller and it does not change. Boost is what varies: if city pressure sags, the pump works harder to hold the same setpoint, up to its capability. Pump curves are plotted in boost, controllers are set in discharge, and confusing the two is how a pump gets selected a full size wrong.

Where the Water Comes From

The suction side decides what kind of machine this is, not just how big it needs to be.

Source What it does to the pressure What it does to the pump
City main Credit — subtract residual pressure at peak flow Standard multistage booster. Needs a low-suction cutout so it never pulls the main down.
Tank at or above pump No credit at pump level; the static head of the water above it if elevated Standard booster, flooded suction. The most forgiving arrangement. Confirm the tank cannot run empty.
Well, cistern, buried tank Penalty — the lift is added to required head Self-priming. Most multistage boosters cannot pull a suction lift at all.
Suction lift is a different problemA pump pulling water up cannot pull harder than the atmosphere pushes — about 34 ft at sea level, before losses. Subtract the lift, suction line friction and the water's vapour pressure, and what is left is net positive suction head available. Below what the pump requires, water flashes to vapour inside the impeller and collapses again. That is cavitation: it sounds like gravel in the casing and destroys impellers and seals. Hot water makes it far worse. The calculator estimates NPSH available and flags a thin margin.

Assumptions and Sources

Everything the calculator assumes, so the arithmetic can be audited rather than trusted.

Item What is used
Pressure conversion 0.433 PSI per foot of water, equivalently 2.31 ft per PSI.
Friction Hazen-Williams. C = 150 plastic, 140 new copper, 120 new steel, 100 aged steel. Schedule 40 internal diameters. Developed length is rise plus horizontal run, multiplied by the fitting allowance.
Recommended pipe sizes Smallest standard Schedule 40 size holding velocity at or below 6 ft per second on the discharge and 5 ft per second on the suction, at design flow. Suction is held slower because velocity there costs NPSH.
Fixture unit values Estimating figures in the pattern of published plumbing-code tables. The table adopted by the authority having jurisdiction governs and should be used for any submitted design.
Fixture unit to GPM A fitted approximation of the standard probability curve, with separate curves for tank and flushometer-dominated systems. Not a transcription of a code table.
Realistic peak A reduction applied to the code number for dwelling-unit buildings only, bracketing published comparisons that show reductions from roughly half down to about one third. Shown as a range. For a submitted residential design, run IAPMO's free Water Demand Calculator, which is in the 2021 and 2024 Uniform Plumbing Code as Appendix M.
Hotels No validated low-flow method exists for hotel occupancy, so no reduction is applied and only the code number is shown.
NPSH available Atmospheric pressure at sea level, water at 70°F, less the lift, the vapour pressure and estimated suction line friction. Altitude and hot water both reduce it and are not modelled.
Tank drawdown Boyle's law between the stop and restart pressures, with precharge set 5 PSI below restart. Nominal tank volume before any acceptance factor — real usable volume runs below nominal, so confirm against the tank data sheet.
Estimated pump power Flow × head ÷ 3960 at 60% wire-to-water. An order-of-magnitude check, not a motor selection. Discharge velocity is flagged above 8 ft per second.
Not included Fire protection, hot water recirculation return, water treatment backwash, and any equipment not entered as a continuous load.

This tool estimates a design target. It is not a stamped design, does not replace the engineer of record, and does not substitute for the plumbing code as adopted locally.

Questions Buyers Ask

If the answer you need is not here, call. Sizing conversations are the part of this we are actually good at.

Is the setpoint the same as the boost?

No. The setpoint is discharge pressure, what the controller holds and what you program into the drive. Boost is what the pump adds on top of suction pressure, and it is what pump curves are plotted in. If suction pressure drops, the setpoint stays put and the boost rises.

What should the tank precharge be?

About 5 PSI below the restart pressure, set with the system drained and checked with a tire gauge at the valve. Precharge set to a residential default in a commercial system leaves a large tank behaving like a small one, and the pump short cycles anyway.

Can this size a fire pump?

No. Fire sprinkler and standpipe systems are governed by NFPA 20 and require a listed fire pump, usually on a separate service. This calculator covers domestic water only.

Get It Sized by a Person

Send the numbers this page produced, or just the building details, and we will come back with specific models and the reasoning behind them. If a booster is the wrong answer for your building, we will tell you that too.

Call 1-888.510.4036 Email a Sizing Request
All products are genuine, factory-new equipment in original manufacturer packaging, sold and supported by Watermain Supply, an independent industrial supplier. Engineering-led sizing, based in Houston and serving the Gulf Coast. 1-888.510.4036
Estimating figures on this page are subject to change and are not a substitute for the plumbing code as adopted by the authority having jurisdiction. Confirm all sizing against current manufacturer literature and the installation manual before specifying.
Watermain Supply is a DBA of E4 Industrial LLC, an independent industrial supplier based in Houston, Texas.