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
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.
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.
Required pressure is a stack. Lift × 0.433, plus pipe friction, plus device losses, plus residual pressure at the highest fixture.
Incoming pressure does part of the work for you. The pump only has to add the difference between what arrives and what the building needs — a main delivering 45 PSI takes 45 PSI off the job and leaves more of the pump’s curve available for flow. Where the water comes from changes the pump, not just the arithmetic.
Above roughly 80 PSI at the lowest fixture, zoning is the answer — pressure-reducing valves on the lower floors, or a second booster on a mechanical floor serving the upper half.
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.
Your Numbers
Enter the flow and the pressure you already have. Step 1, Step 5, Step 6 and Step 7 still apply — they supply the rise, the water source, the pipe and the power.
Peak simultaneous demand, not every fixture added up
Rise from Step 1 and pipe, backflow and meter losses from Step 6 are added on top.
Step 1 · Building
Type sets the fixture template and which demand method applies.
Floors above the pump
Pump discharge up to the highest fixture. Roughly 10 ft per storey, plus the pump's depth below floor 1 — a 2-storey house is about 20 ft.
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 2 · Rooms
Count rooms, not individual fixtures. A full bathroom means one water closet, one lavatory and one tub or shower — enter 3 for a three-bathroom house, not 9.
WC + lavatory + tub/shower each
WC + lavatory only
Sink and dishwasher
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
Prefilled — leave 12 if unknown. Required on every booster, 8 to 14 typical.
Prefilled — 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. Default is 1% of design peak, 0.2 GPM minimum
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.
One-page duty worksheet on Watermain Supply letterhead: the conditions entered, the arithmetic, and — if you checked a pump — its verdict, check rows and curve.
Your Results
The four numbers a booster is selected on. If you picked a pump to check, its verdict and curve follow.
Peak Flow — Code Method—GPM
Realistic Peak — Low-Flow Fixtures—GPM
Required Boost—PSI
Discharge Pressure / VFD Setpoint—PSI
Detailed Analysis
How each number above was built — the pressure stack, the flow method and the commissioning targets. This is the arithmetic behind the result, not a specification.
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
Why it decides the fit
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
Every controller has a maximum settable pressure. Your required discharge has to sit inside it — and incoming pressure counts toward that, so a strong main widens the range of models that work.
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
Selected near its best efficiency point, a pump runs quietest, coolest and easiest on the seals. That is what turns a pump that merely fits into one that lasts.
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, and it is the number pumps are checked against — the flow the building actually draws. The code number is what a plan reviewer will expect; for a submitted design, size to it.
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.
The Errors That Cost Money
Each of these has put the wrong pump in a basement.
Ignoring suction pressure
Sizing the pump for total discharge instead of boost. The most common error in booster sizing, and it lands you a frame size or two too large every time.
Static pressure instead of residual
Static is what the gauge reads with nothing running. Residual is what is left at peak flow, and it can be 20 PSI lower. Use a flow test.
Adding fixture flows with no diversity
Summing every fixture's rated GPM oversizes the pump two to three times. It then spends its life far left of best efficiency, short cycling and eating seals.
Continuous loads through the fixture curve
The probability math discounts a laundry room to almost nothing. Add it separately, at full flow.
Forgetting the backflow preventer
8 to 14 PSI, mandatory on a booster, and left out more often than any other component.
Matching suction pipe to the pump port
The port size is what the pump is tapped for, not what the pipe should be. Suction is normally a size larger than discharge, because velocity on the suction side costs NPSH and invites cavitation.
Solving a zoning problem with a bigger pump
Past roughly 10 to 12 stories the honest answer is two pressure zones or pressure-reducing valves on the lower floors, not a taller curve.
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. Unless overridden under drive and tank settings, overnight trickle is taken as 1% of design peak flow (0.2 GPM minimum) and permitted starts per hour follow the motor size the duty implies: 15 to 5 HP, 10 to 25 HP, 6 above. 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.
Why are there two flow numbers?
The realistic number is what the building actually draws, and it is what pumps on this page are checked against. The code number is what a plan reviewer will expect; for a submitted design, size to it. Every verdict says which number it used.
Is the setpoint the same as the boost?
They are two different numbers and the difference is worth knowing. 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 whatever arrives at its suction, and it is what pump curves are plotted in. Give the pump more incoming pressure and it holds the same setpoint using less boost, which leaves more of its curve free for flow.
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.
Do I need a tank with a variable speed booster?
Almost always, but a small one. The drive cannot ramp to zero, so a small diaphragm tank lets the pump sleep through overnight trickle demand instead of short cycling. Size it against leakage, not pump capacity — a 500 GPM booster and a 50 GPM booster need a similar tank. The tank calculator works it through.
Do I need an expansion tank at the water heater?
If there are storage water heaters, yes. A booster closes the system, so water expanding as it heats has nowhere to go and pressure climbs until the relief valve weeps. That tank is separate from the booster's pressure tank and is sized against heater volume.
What is a specification sheet telling me when it says max flow and max boost?
Two opposite ends of the curve, not one operating point. Max flow happens at nearly no boost; max boost happens at nearly no flow. The usable duty point is in the middle and is usually about half the headline flow at usable pressure. Always ask for the curve.
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.
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.
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