SizingKitPipe & pumps
Well pump size calculator
A well pump is chosen on two numbers and a tank: the gallons per minute the house draws at once, and the total dynamic head that flow has to be pushed against. This page builds the flow from the fixtures at their IPC Table 604.4 ceilings, builds the head from static water level, drawdown, drop-pipe friction and the pressure switch's cut-out, and then returns brake horsepower at an efficiency you can change and the tank volume that keeps the pump running for a full minute per cycle. It is free, needs no account, and it prints every table and constant it used beside the answer.
- 100% free
- No signup
- Flow, head, hp and tank
- 3 pressure-switch settings
- Drawdown by Boyle's law
- Printable tank table
How much water at once
Count the outlets that could plausibly run together in the busiest few minutes. This is a simultaneous-use sum at the flow ceilings of IPC Table 604.4 — it is not Hunter's curve. The water supply fixture unit method in IPC Appendix E converts fixture units to gallons per minute on a probability curve, and that table is not reproduced on this site: where a designer needs it, it has to come from the code book.
Simultaneous demand 6.9 gpm · 26.1 L/min. water closet and urinal are left out on purpose: they are rated in gallons per cycle rather than gallons per minute, and a tank toilet draws its 1.6 gallons through a ballcock at whatever rate that valve passes, not at a rate the fixture standard sets.
Leave blank and the fixture total above is used. Fill it in when the well's own sustained yield is the limit — a pump cannot deliver more than the aquifer does, and pumping a well below the pump inlet is how pumps are destroyed.
The well, the pipe and the switch
Total dynamic head for a well pump is built from four measurements, and only one of them is on the well log. Static water level is the depth to water with the pump off. Drawdown is how much further the level falls while the pump runs, and it is a property of the well's yield and the pumping rate, not of the pump — the sum of the two is the pumping water level and is the true static lift. To that add the rise from the wellhead to the pressure tank, the friction in the drop pipe and the service line, and the pressure-switch cut-out expressed as head (about 138 ft for a 60 psi cut-out), because the pump has to be able to reach cut-out at the design flow or it will never shut off. Pump setting depth is deeper still than the pumping water level and matters for the pump's submergence, not for the head.
Depth to water with the pump off. It is on the well log.
How much further the level falls at this pumping rate. A property of the well's yield, not of the pump, and it is the term a driller measures.
Pumping level 145 ft plus this is the static term.
Bore 0.875 in against a name of 1 in — ASTM F876. It holds 0.0312 gal per foot.
Pump setting depth plus the buried run to the house. The pipe holds 7.5 gallons.
Cut-out 60 psi is 138.5 ft of head the pump has to reach at the design flow or it never shuts off.
Default 38 psi — the switch table's own instruction is to set it 2 psi below cut-in with the system drained, and to re-check it whenever the switch setting changes.
Default 55 °F. A representative mains water temperature for the continental United States. Shallow groundwater sits within a degree or two of the mean annual air temperature at the location, which is the fastest way to estimate it for a given place.
The duty point and the motor
DUTY POINT
6.9 gpm at 314 ft
26.1 L/min at 95.6 m
Read a submersible curve at that point. It is 135.9 psi (9.37 bar) at the pump discharge with the tank sitting at cut-out.
| Term | Feet | Meters |
|---|---|---|
| Pumping water level and the rise to the tank | 157 | 47.9 |
| Friction in 240 ft of 1 in PEX | 17.9 | 5.5 |
| Cut-out pressure, 60 psi | 138.5 | 42.2 |
| Velocity head at 3.68 ft/s | 0.211 | 0.064 |
Velocity in the 1 in PEX is 3.68 ft/s · 1.12 m/s. The guidance figure for this material is 8 ft/s on noise grounds: PEX is not subject to erosion-corrosion and is flexible enough to absorb surge, so the practical limit is noise. Manufacturer listings commonly permit up to 8 ft/s continuous.
POWER AT THAT POINT
Water horsepower
0.546 hp
Brake horsepower at the shaft
0.84 hp
Electrical input
0.74 kW · 738 W
Ranges from about 35–55% for a small residential submersible well pump, 55–75% for a typical end-suction centrifugal, and 75–88% for a large well-matched unit near its best efficiency point. It falls away sharply on both sides of that point, so a pump run far off its curve can be at half its rated efficiency. Take this figure from the pump curve at the actual duty point before believing any horsepower answer.
Runs from roughly 75% on a fractional-horsepower single-phase motor to 95% on a NEMA Premium motor of 10 hp or more. Efficiency also falls when a motor is lightly loaded, which is another cost of oversizing.
Round the shaft figure up to a motor the manufacturer actually publishes at this flow and head — this page will not offer a horsepower ladder, because which sizes exist is a property of the maker's range and not of the arithmetic. Check the published curve at 314 ft before believing any of it: the efficiency above is the single input that moves the horsepower most.
Friction by Darcy-Weisbach over 240 ft of PEX 1 in at ε 0.0015 mm, with the water at 55 °F weighing 62.39 lb/ft³ — the density used to turn the 60 psi cut-out into 138.5 ft. Horsepower from Q × H × SG ÷ 3960, where 3960 is 33,000 ft·lbf/min per horsepower divided by 8.3333 lb per gallon. Switch settings from The standard factory differentials of residential well pressure switches (Square D FSG2 / Pumptrol and equivalents), which are supplied set at 30/50 or 40/60 psi with a 20 psi differential.
The pressure tank, and why it is bigger than it looks
A tank's drawdown is the water it gives back between cut-out and cut-in, and it is a fraction of the tank rather than the tank. The fraction comes from Boyle's law on the air charge, so it depends on the ratio of the absolute pressures and not on the 20 psi difference the switch is set to.
The figure on the label, not the drawdown printed beside it.
Manufacturers commonly require one minute for pumps up to about 1 hp and two minutes or more above that; three-phase and larger motors need longer still. Short-cycling is the dominant cause of premature well-pump failure, and it is a tank problem — either the tank is too small for the pump's flow, or the precharge has bled off. Check the pump manufacturer's own figure before sizing the tank.
Drawdown from your tank
8.3 gal
31.2 liters · 25.8% of the tank
Needed for a 1 min cycle
6.9 gal
a tank of about 27 gal at this switch and precharge
Run per cycle as fitted
1.2 min
72 seconds at the design flow
| Switch | Prechargepsi | Drawdown% of tank | From your tankgal · L | Tank neededgal | Cut-out as headft · m |
|---|---|---|---|---|---|
| 30/50 psi | 28 | 29.5% | 9.4 · 35.8 | 23 | 115.4 · 35.2 |
| 40/60 psi | 38 | 25.8% | 8.3 · 31.2 | 27 | 138.5 · 42.2 |
| 50/70 psi | 48 | 22.9% | 7.3 · 27.7 | 30 | 161.6 · 49.2 |
Where the head and its four terms come from is set out on the pump head calculator, and the reason a submersible sidesteps the suction problem entirely — it is under the water, not above it — is on the NPSH calculator. Friction on its own, per 100 ft of drop pipe, is on the pipe pressure loss calculator, and the hot side of the same house is on the water heater size calculator.
How to size a well pump and its tank
Flow first, because head depends on it; then the well; then the motor and the tank that stops it short-cycling.
Count the outlets that overlap
Add up the fixtures that could plausibly be running in the same minute at their rated flows — a shower and two faucets is 6.9 gpm at the federal ceilings and 5.7 gpm with a WaterSense shower and lavatory, since the kitchen sink faucet has no WaterSense figure and stays at 2.2. Then check that figure against the well itself: a pump cannot deliver what the aquifer will not yield, and the driller's sustained-yield test is the number that wins if it is lower.
Add the well up: static level, drawdown, rise, pipe and switch
Static water level is the depth to water with the pump off and comes off the well log. Drawdown is how much further the level falls while pumping at your flow, and it belongs to the well rather than the pump. The two together are the pumping water level, which is the real lift. Add the rise to the tank, the friction in the drop pipe and service line, and the switch's cut-out expressed as head — 138.5 ft for a 60 psi cut-out on 55 °F water.
Take the horsepower to the curve and size the tank from the cycle
Brake horsepower is water horsepower divided by the pump's efficiency at that duty point, and a small submersible is nowhere near the 65% a general centrifugal manages — the field is editable for that reason. Then size the tank backwards from the cycle: the drawdown in gallons has to be at least the pump's flow in gallons per minute, or the pump starts more often than it should and the motor is what pays for it.
Technical specifications
| Demand method | Simultaneous-use sum at IPC Table 604.4 maximum flow rates — showerhead 2.5 gpm, private lavatory and sink faucets 2.2 gpm, public lavatory 0.5 gpm — with the EPA WaterSense figures of 2.0 and 1.5 gpm offered as the stricter alternative. |
|---|---|
| What is deliberately not here | Hunter's curve. The water supply fixture unit to gallons-per-minute conversion in IPC Appendix E is not reproduced on this site, because a wrong figure sizes the pump wrong in both directions at once. Where a design needs it, take it from the code book and type the result into the override field. |
| Static term | Pumping water level — static level plus drawdown — plus the rise from wellhead to tank. Drawdown is measured on the driller's yield test at the intended pumping rate and is a property of the well, not of the pump. Pump setting depth is deeper again and matters for submergence rather than for head. |
| Pressure term | The switch cut-out converted to head at the water's own density. On 55 °F water the three standard settings give 115.4 ft, 138.5 ft and 161.6 ft of head at 50, 60 and 70 psi. The pump has to reach cut-out at the design flow or it never shuts off. |
| Tank drawdown | Boyle's law on the air charge, with the precharge two psi below cut-in as the switch table instructs: 29.5% of the tank at 30/50, 25.8% at 40/60 and 22.9% at 50/70. A 32 gallon tank on a 40/60 switch therefore returns about 8.3 gallons, not 32. |
| Power | Water horsepower is Q × H × SG ÷ 3960, where 3960 is 33,000 ft·lbf/min per horsepower divided by 8.3333 lb per gallon. Divide by pump efficiency for shaft horsepower and again by motor efficiency for the kilowatts the meter sees. No horsepower ladder is offered, because which sizes exist belongs to the manufacturer's range. |
| Friction on the true bore | 1 in PEX has a 0.875 in bore against 1 in Schedule 40 steel's 1.049 in, so the same 240 ft run in PEX loses about twice the head at the same flow even though steel's wall is thirty times rougher. Nominal size is a name and this page never squares it. |
| Where the arithmetic runs | On this device, in the page. Well logs, yields and the numbers off your own pressure switch are not transmitted or stored anywhere, which also means the page answers at the wellhead with no service. |
Frequently asked questions
Is the static water level the same as how deep the pump is set?
No, and mixing them is the most common way a well pump is oversized. Static water level is the depth to the surface of the water with the pump off, which on a 400 ft well might be 110 ft. The pump is set far deeper — typically some distance above the bottom, below the pumping level, so that it stays submerged — but that extra depth is not lift, because the pump is inside the water column and the column above it is doing nothing to resist. The lift is to the pumping water level: static level plus drawdown, and nothing further.
Why does a 32 gallon tank only give back about eight gallons?
Because a diaphragm tank stores water by compressing air, and the air only compresses over the 20 psi between cut-in and cut-out. The usable fraction follows the ratio of the absolute pressures rather than their difference, which works out to roughly a quarter of the tank on ordinary residential settings. It is the single most surprising figure in a well system and it is why the tank in a crawlspace looks so much larger than the water it delivers.
What should the tank precharge be, and how do I check it?
Two psi below the switch's cut-in, set with the system drained so there is no water pressure on the diaphragm — a 40/60 switch wants 38 psi. Check it at the Schrader valve on the tank with an ordinary tire gauge after opening a faucet and letting the pressure fall to zero. A tank that has bled its charge has almost no drawdown left, and it short-cycles the pump without any other symptom. Re-check it whenever the switch setting is changed: a tank precharged for 30/50 and then run on 40/60 loses most of its usable volume.
Should the switch be set at 30/50 or 40/60?
40/60 gives noticeably better pressure at upstairs fixtures and costs about 13% of the tank's drawdown to do it, because the drawdown depends on the pressure ratio rather than the differential. On a generously sized tank that trade is easy; on a marginal one it is the difference between a pump that starts once a minute and one that starts four times. Check what is downstream before going higher — a 70 psi cut-out is above the 60 psi at which IPC 604.8 calls for a pressure-reducing valve on a water distribution system.
How many gallons per minute does a house actually need?
It depends entirely on how many outlets can run at once, which is why this page asks you to count them rather than asking how many bathrooms there are. The honest limitation is that a simultaneous-use sum at rated flows is conservative: real fixtures rarely all run at their ceiling at the same instant, which is exactly what Hunter's probability method was invented to account for. That method, as tabulated in IPC Appendix E, is not carried on this site — so treat the fixture total here as an upper bound, and use the override field for a fixture-unit figure worked out from the code book or for the well's own tested yield if that is lower.
The answer came out at 0.84 hp. What do I actually buy?
The next motor the manufacturer publishes for that flow and head, which this page will not guess at. Read the submersible's own curve at the duty point rather than at its headline rating: pumps are sold by nominal horsepower and stage count, and two units badged the same size can deliver quite different flows at 300 ft of head. Note also that oversizing has a running cost beyond the extra draw — a motor loaded well below its rating loses efficiency, and a pump run far from its best efficiency point can be at half the efficiency the catalog headline suggests.
What makes a well pump short-cycle, and does it matter?
It matters more than anything else on this page: rapid starting is the dominant cause of premature submersible failure, and the cause is almost always the tank rather than the pump. Either the tank is too small for the pump's flow, or its precharge has bled away. The check is arithmetic — the tank's drawdown in gallons must be at least the pump's flow in gallons per minute for a one-minute cycle, and manufacturers commonly ask for two minutes or more above about 1 hp. Confirm the required run time with the pump maker before settling the tank size.
About sizing a well pump, and the four measurements it rests on
Only one of the numbers a well pump is chosen on is written down anywhere, and it is the static water level on the well log. The other three have to be established. Drawdown is how far the level falls while pumping and belongs to the aquifer's ability to feed the borehole at that rate — the drawdown at the same 10 gpm can differ several times over between two wells on the same street, and the same pump in both is doing two very different amounts of work. The pressure switch's cut-out is a design decision that shows up as head. And the friction in the drop pipe is set by a bore, not by a label: the 1 in PEX that is easy to lower down a casing has a 0.875 in bore where 1 in Schedule 40 steel has 1.049 in, and that difference alone is worth more than doubling the friction on a 240 ft run.
The tank is where most residential well systems are actually wrong. Its job is not storage in any meaningful sense — it is to keep the pump from starting every time somebody washes their hands. A diaphragm tank gives back only the water the compressed air can push out over the switch's differential, which is roughly a quarter of its nominal volume on ordinary settings, and the whole of that fraction disappears if the precharge has leaked away. Sizing it is a cycle calculation run backwards: decide the minimum run time the pump manufacturer wants, multiply by the pump's flow to get the gallons the tank has to release, and divide by the drawdown fraction for the switch setting in use. The printed table on this page does that for all three standard settings at once, which is the comparison a supply house counter rarely shows.
A submersible sidesteps the suction problem that dominates surface pumping — it sits under the water rather than above it, so what matters there is submergence and the well's recovery rather than the atmosphere's ability to lift a column, which is set out on the NPSH calculator. A jet pump in a basement does not get that concession and is limited by the same physics as any other surface pump. The head arithmetic here, and what to do when one term dominates the total, is broken out on the pump head calculator; friction alone, per 100 ft of pipe, is on the pipe pressure loss calculator; and the peak-hour demand on the hot side of the same house is on the water heater size calculator. None of this is a permit submission: well construction, pump setting and the electrical supply to it are all subject to local rules, and a licensed well contractor and electrician sign the work off.
Where the well figures are worked out
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.