Well & Irrigation Pump Sizing Calculator

Engineering-Led Sizing · Watermain Supply · Houston, TX

Well & Irrigation Pump
Sizing Calculator

Answer questions about the well, the lawn and the house, and get what a jet or irrigation pump is actually selected on: a flow window in GPM, the pressure it must hold, the pressure switch setting, and which kind of pump and ejector fits the well you have. No brands, no models — just the duty point.

25 ftShallow Well Limit
0.433PSI Per Foot of Lift
1/3 – 3 HPRange This Page Covers
1-888.510.4036Talk to a Person

The Short Version

  1. Depth to water decides the pump type. Water within 25 ft of the pump: a shallow well jet or a self-priming sprinkler pump. Deeper than 25 ft: the ejector has to go down the well. Deeper than about 120 ft: submersible territory.
  2. Casing size decides the ejector. Deep well ejectors come in exactly two shapes — a single-pipe packer for 2 in casing and a two-pipe assembly for 4 in and larger. Past 4 in, a submersible is usually the better machine.
  3. Flow is a window, not a number. Household demand is probabilistic. Irrigation demand is arithmetic — inches per week, square feet, and the hours you are willing to run. Both produce a range, and the pump should land inside it.
  4. Pressure is a stack. Depth to water, plus rise to the highest outlet, plus pipe friction, plus what the fixture or sprinkler head needs. The switch setting follows from it.
  5. Jet pumps lose flow fast with depth and pressure. If the required flow sits above what a jet can do at your depth, the honest answer is a submersible, and the calculator will say so.
Pump Curve 101How to read head, flow and efficiency on a published curve. Jet Pump GuideInstallation, priming, ejector selection and parts, series by series. Pressure Tank CalculatorSize the tank so the pump does not short-cycle. Booster Pump CalculatorFor city water and multi-story buildings.

The Calculator

Estimating tool. It produces a duty window and a pump class to check any manufacturer's published table against. It is not a stamped design and does not replace a well driller's yield test.

Step 1 · What the Pump Does

Sets which demand method runs and what pressure the outlet needs.

Step 2 · Where the Water Is

This decides the kind of pump more than anything else on the form. Depth to water is measured from the pump, not from the ground.

Measure inside at the wellhead. 4 in steel is 4.5 in outside.
From the driller's log or a weighted tape
Depth to water with the pump off for an hour or more
How far the level drops under load. Leave 10 if unknown — then test it.
Lowest expected water level, not today's
Source or wellhead to the pump inlet
Zero if the pump is at ground level beside the well

Step 3a · The House

Count rooms, not fixtures. A full bathroom is one water closet, one lavatory and one tub or shower.

Step 3b · The Lawn

Irrigation is arithmetic, not probability. Inches per week times area gives gallons; the hours you will run gives GPM.

Turf and beds actually watered. 1/4 acre = 10,890 sq ft, 1/2 acre = 21,780.
Gulf Coast turf in July: 1.0 to 1.5. Cooler climates: 0.75 to 1.0.
Total pump run time, all zones. Most systems water at night.
Usually zero. Each adds 5 GPM on top.

Step 3 · Transfer Duty

Transfer pumps are sized on the flow you want and the height you push it to. There is usually no pressure requirement at the outlet.

A 20,000 gal pool at 30 GPM takes about 11 hours

Step 4 · Where the Water Goes

The pressure stack on the discharge side. Rise is measured from the pump to the highest outlet or the farthest sprinkler head, whichever is worse.

Second-floor shower: about 15. Flat lawn at pump level: 0.
Pump to the farthest outlet or zone valve
Sediment filter 3 to 5, softener 5 to 8, backflow preventer 8 to 12
Flow Window—GPM
Pressure at Pump Discharge—PSI at design flow
Pressure Switch—PSI cut-in / cut-out
Motor Size, Estimating—HP window
—
Pressure term Value Where it comes from
Flow term Value Method
What to look for on the published table Value Note

Take the flow window and the discharge pressure to any manufacturer's capacity table for the pump class above, at your depth to water. A pump that lands inside the window at that pressure is a candidate. Then read the pressure tank calculator for the tank.

The Six Answers This Page Can Give

Every well and irrigation job resolves to one of these. The calculator picks by depth to water, casing size, and how much flow and pressure you need.

Pump class Water is Casing Pipes to the well Typical duty
Shallow well jet Within 25 ft of the pump Any One suction pipe; ejector bolted to the pump Household supply, 1/2 to 1 HP. Flow falls off quickly past 15 ft of lift.
Self-priming sprinkler / centrifugal Within 25 ft — lake, pond, cistern, shallow well Any One suction pipe, no ejector Irrigation and transfer, 1 to 3 HP. High flow at modest pressure. Not for household supply.
Convertible jet + 2 in single-pipe packer 25 to 120 ft 2 in only One drop pipe; the casing itself is the drive pipe, sealed by a packer Household supply from a 2 in well, 1/2 to 1 HP. Capacity limited by the small casing.
Convertible jet + 4 in two-pipe ejector 25 to 120 ft 4 in and larger Drive pipe and suction pipe, ejector hung between them Household supply, 1/2 to 1-1/2 HP. Better flow than a packer at the same depth.
Multi-stage deep well jet 25 to 160 ft 2 in or 4 in+ Same as above, with a regulating valve on the pump Deep water and real pressure at the same time — irrigation from a deep well, second-floor showers from 80 ft. 3/4 to 1-1/2 HP.
Submersible Past 120 ft, or anywhere a jet cannot make the flow 4 in and larger One drop pipe; pump and motor are in the well Not sized on this page. When the calculator sends you here, it is because a jet would be the wrong buy, not a bigger one.

Ranges are approximate and overlap at the edges. The 25 ft shallow-well limit is at sea level; deduct about 1 ft for every 1,000 ft of elevation. For install detail on any of these, see the jet pump guide.

The Most Efficient Point, and the Window Around It

A pump has one flow where it converts the most motor power into water. Select near it and the pump runs cool and quiet for twenty years. Select at either end of the curve and it does not.

FLOW — GPM HEAD — FT (PRESSURE) PREFERRED WINDOW · 70% TO 120% OF BEP FLOW 50–70% 120–130% BEP FLOW = 100% Head-capacity curve Shut-off head at the left: max pressure, zero flow. Never live here. Efficiency curve Peaks once, at BEP. BEP — best efficiency point Select so the duty lands near here. Power curve Load rises with flow Run-out Far left: recirculation, heat, vibration

A typical centrifugal curve. The best efficiency point (BEP) is where the efficiency curve peaks. The green band is the preferred operating window: 70% to 120% of the flow at BEP. The orange bands, 50–70% and 120–130%, are allowable for short periods. Outside them, seals and bearings pay for it.

The recommended window, in numbers

Select so the design duty falls between 70% and 120% of BEP flow — the Hydraulic Institute's preferred operating region — and aim for 85% to 105% if the choice is free. Half of BEP flow or 130% of it is the outer limit for occasional operation, not a place to design to. On the head axis, a pump running inside that window sits between roughly 60% and 85% of its shut-off head.

Jet pumps do not publish an efficiency curve — use this instead

A jet pump table gives GPM by depth to water at a few pressures. The equivalent of BEP is the middle of the table: not the shallowest depth, not the highest pressure column, and never the last non-blank cell. Select so your depth sits inside the ejector's rated band with room on both sides, and your pressure sits one column below the maximum the table shows. A jet running at the edge of its table is the same thing as a centrifugal running at run-out.

Why the ends of the curve are worse than they look

Far left, flow is low but the pump spins at full speed: water recirculates in the impeller and heats, radial thrust peaks, seals fail early. That is where an oversized household pump lives, because a tank and switch can stop a pump but cannot slow it. Far right, power draw is highest and the motor runs hot, and suction requirement climbs so a pump on a lift loses prime — an irrigation pump on a wide-open zone with no back-pressure is the classic case.

Published pump head-capacity curve example

A real published curve. Read it the same way: find where efficiency peaks, take 70% and 120% of that flow, and make sure your duty point sits between them. For a fuller walk-through — head, NPSH, and how to read a family of curves — see Pump Curve 101.

Flow, Step by Step

Flow is how much water the pump moves, in gallons per minute. The house and the lawn are sized by different methods.

Household. Fixtures are never all open at once. Each is counted as fixture units and the total is run through a probability curve, the same method a plumbing code uses. A three-bath house with a kitchen, a washer and two hose bibbs lands around 12 to 14 GPM by the code method; the pump should be able to reach it and will spend most of its life well below it.

Irrigation. No probability. Inches per week × area gives gallons per week. Divide by the minutes you are willing to run and that is the GPM. The design choice is the run time: a pump half the size runs twice as long, every night, all summer.

Worked example — irrigationA half-acre lot, 17,000 sq ft of turf, rotors, 1.25 in per week, watering 3 nights at 4 hours.
17,000 sq ft × 1.25 in ÷ 12 × 7.48 gal/cu ft = 13,250 gal per week.
3 nights × 4 hours × 60 = 720 minutes. 13,250 ÷ 720 = 18.4 GPM.
At 4 hours a night that lawn wants 18 GPM. At 6 hours a night it wants 12. The calculator shows a window around the number you gave it, and the flag tells you when the well is unlikely to keep up.
Where irrigation demand collides with the wellA jet pump 60 ft to water makes roughly 8 to 12 GPM at modest pressure and a good deal less at rotor pressure. A half-acre lawn at four hours a night wants 18. The choices are longer run time, more zones with fewer heads each, or a submersible. There is no jet pump that closes that gap by being bigger.

Pressure, Step by Step

Head is how high a pump can push water, in feet. One foot of water is 0.433 PSI; one PSI is 2.31 ft. Everything on this page is stacked in PSI so it can be compared to a switch setting.

Worked example — deep well, householdA 4 in well, 120 ft deep, static water at 40 ft, pump beside the wellhead. Three full baths, a half bath, kitchen, washer and two hose bibbs. Second-floor shower 15 ft above the pump, 60 ft of 1-1/4 in plastic, 30 PSI wanted at the fixtures.
Depth to water: 40 static + 10 drawdown allowance = 50 ft = 21.7 PSI of lift.
Rise: 15 × 0.433 = 6.5 PSI. Friction in 60 ft of 1-1/4 in plastic at 14 GPM: about 1.5 PSI. Fixtures: 30 PSI.
Pressure the pump must hold: 6.5 + 1.5 + 30 = 38 PSI. Total head on the pump: 50 + 15 + 3 + 88 = about 156 ft.
The switch: 38 PSI required means cut-in has to be 40, so 40/60. The casing is 4 in and the water is at 50 ft, so the answer is a convertible jet with a 4 in two-pipe ejector in the 30–50 ft band, set about 60 ft down, and the flow window is 8 to 14 GPM read at the 40 PSI column. The pump must also reach 65 PSI shut-off at 50 ft or it will never turn off.
The switch follows the pressure, not the other way roundA 30/50 switch means the pump starts at 30 PSI and stops at 50. The lowest pressure the house ever sees is the cut-in. If a shower needs 40 PSI, cut-in has to be at or above 40 — so the switch is 40/60 and the pump must be able to reach 60 PSI at the well's depth or it will never shut off. Select the pump at the cut-out pressure, then check the flow window at cut-in.

Ejectors, and Where to Set One

Past 25 ft of lift the ejector goes down the well. Its shape is decided by the casing; its depth setting is decided by the water.

2 in casing — single-pipe packer

One drop pipe carries water up. The casing itself is the drive pipe, sealed off by a packer above the ejector. Simple, but the annulus between a 1-1/4 in pipe and a 2 in casing is small, which is why packer capacities are lower than two-pipe at the same depth.

4 in and larger — two-pipe

A drive pipe down and a suction pipe up, ejector between them, nothing touching the casing. Better flow, easier to pull. In a 5 or 6 in casing it still fits, but at that diameter a submersible usually wins on efficiency and should be priced alongside.

The pressure control valve

Every deep-well jet needs one on the discharge, as close to the pump as possible. It holds back-pressure so part of the flow keeps driving the ejector; without it the pump delivers everything to the house on a big draw, the ejector starves, and the pump loses prime.

Setting depth

At least 5 ft below the static level, ideally 10 ft below the pumping level so drawdown never uncovers it, and at least 5 ft off the bottom so it does not draw sediment. On a low-yield well add a tail pipe below the ejector: the pump then keeps drawing down the tail pipe instead of breaking prime.

Ejector kits are rated by depth-to-water band — typically 30–50, 50–80 and 80–120 ft — and by pump horsepower. Select the band your pumping level sits inside with margin on both sides. The jet pump guide covers the physical install.

What Goes In the Well

The pump is the part with a nameplate. These are the parts that decide whether it primes, holds prime, and keeps running.

Casing Where you see it What fits
1-1/4 and 2 in Driven wells and sand points; older shallow drilled wells Shallow well jet if water is within 25 ft. Past that, 2 in casing only — single-pipe packer ejector.
4 in The standard domestic drilled well, steel or PVC Two-pipe deep-well jet, or a 4 in submersible. Both fit; the submersible wins past about 80 ft or on flow.
5 in Common in PVC-cased wells in some regions Same as 4 in, with more room to work.
6 in and up Newer domestic, irrigation and small ag wells A two-pipe jet fits but rarely makes sense. Submersible territory.

The pipes, by name

Suction pipe — well or lake to the pump inlet on a shallow well or sprinkler pump; also the pipe water comes up on a two-pipe deep well, typically 1-1/4 in. Drive pipe — the second pipe on a two-pipe deep well, carrying water down to run the ejector, typically 1 in. On a packer system the casing is the drive pipe. Tail pipe — the length below the ejector to the foot valve, used on low-yield wells. Drop pipe — any pipe hung in the well; correct, but say which one. Service line — pump to tank to house.

Foot valve — required on every suction pump

A check valve with a strainer on the bottom of the suction pipe. It keeps the pipe full of water after the pump stops; without it the water drains back and the pump has to re-prime on every cycle, which a jet pump cannot do on its own. Set it at least 5 ft off the bottom. Driven wells have no room for one — use an in-line check valve at the wellhead, close to the pump. Lake and pond intakes get a foot valve with a strainer, off the bottom and away from weeds.

Priming

Fill the pump and the suction pipe completely with water through the priming plug before the first start. Shallow well: pump plus suction pipe. Deep well: pump plus both pipes — far easier if the pipes are filled before the pump goes on. Self-priming sprinkler pump: fill the pump casing only; it clears the suction line itself, within its rated lift and after a minute or two. Never run any of them dry, even briefly — the mechanical seal is water-lubricated and fails in seconds. After the first fill, the foot valve is what keeps the prime.

At the top of the casing

A well seal closes the casing and carries the pipes through it: one hole for a shallow well or a submersible, two for a two-pipe jet. A packer system uses a casing adapter instead, because the casing is pressurised. Where the line has to exit below frost, a pitless adapter takes the discharge out through the side of the casing. Every deep-well jet also needs a pressure control valve on the pump discharge, and every pressure system needs a tank and a switch.

The Errors That Cost Money

Each of these has put the wrong pump on a well.

Using the static level as the depth to water

Static is with the pump off. Under load the level drops, sometimes by 20 ft or more. Size on the pumping level, and if nobody knows it, run a two-hour drawdown test before ordering.

Guessing the casing size

Pipe is named by nominal size, not by what a tape reads on the outside — 4 in steel casing measures 4.5 in around. Measure the inside at the wellhead, because the ejector either fits it or does not.

Reading the table at the wrong pressure

Deep-well jet tables are usually published at 20 or 30 PSI discharge. A household on a 40/60 switch, or rotors at 45 PSI, will see far less than the table number. Find the column that matches your cut-out pressure, or derate.

Putting a shallow-well pump on a 30 ft lift

Atmospheric pressure limits suction to about 25 ft at sea level after friction. At 30 ft the pump will prime on a good day and lose it on a hot one. Past 25 ft, the ejector goes in the well.

Fixing low pressure with more horsepower

On a jet pump, horsepower buys flow more than pressure. If the need is 50 PSI from 70 ft of water, the answer is a multi-stage jet or a submersible, not a 1-1/2 HP single-stage.

Forgetting the pump has to reach cut-out

A pump that makes 40 PSI at your depth on a 40/60 switch runs forever and never stops. Check the shut-off pressure in the table against the cut-out, at your depth, before anything else.

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; 2.31 ft per PSI.
Depth to water Static level plus drawdown plus the height of the pump above the wellhead. Surface sources use the lift you enter plus suction friction.
Shallow-well limit 25 ft of total suction lift, vertical plus friction, at sea level. Elevation is not modelled; deduct 1 ft per 1,000 ft.
Friction Hazen-Williams, C = 150 plastic, 140 copper, 100 aged steel, Schedule 40 inside diameters, fitting allowance of 30%. Suction friction uses 1-1/4 in pipe.
Household demand Fixture units in the pattern of published code tables, run through a fitted probability curve. The window shown is 60% to 100% of that figure. Estimating figures, not a code transcription.
Irrigation demand Area × inches per week ÷ 12 × 7.48, divided by run minutes. Window is ±15% around that, with a floor of 6 GPM so a zone can run at least two heads.
Outlet pressure Rotors 45 PSI, fixed sprays and rotary nozzles 30 PSI, drip 25 PSI, transfer 5 PSI; household per your selection, plus device losses you enter.
Pressure switch Smallest standard setting whose cut-in meets the required discharge pressure: 20/40, 30/50 or 40/60. Above 40 PSI required, 40/60 is shown and flagged. Capacity tables are read at the cut-in, which is how manufacturers publish them.
Jet capability envelope A generic ceiling for 1 to 1-1/2 HP jets, falling with depth: about 17 GPM at 30 ft to about 5 GPM at 110 ft at 20–30 PSI, reduced further at higher pressure. Estimating envelope drawn from published tables; any manufacturer's table governs.
Motor size Mid-window flow × total head ÷ 3960 at an assumed wire-to-water efficiency: 45% sprinkler centrifugal, 35% shallow jet, 32% deep jet to 60 ft, 22% deeper. Rounded to the standard sizes either side. An estimating band, not a selection.
Not included Well yield, water quality, sand, elevation above sea level, hot water, frost, and any code requirement of the authority having jurisdiction.

This tool estimates a duty window. It is not a stamped design and does not replace a driller's yield test or 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.

How do I find my depth to water?

The driller's log has the static level and usually a tested yield. Failing that, lower a weighted tape or a chalked string down the casing until it comes up wet. Measure with the pump off for an hour. Then run the pump for two hours and measure again — the difference is drawdown, and the second number is the one to size on.

Can a 1 HP jet pump run my sprinklers?

From shallow water, comfortably. From 50 ft or more, it will deliver roughly 8 to 10 GPM at 20 PSI and less at rotor pressure — enough for two or three rotors per zone and long run times. If the lawn wants more than that, see the flow section: the fix is zones and hours, or a submersible.

Why does the calculator say submersible when I asked about jet pumps?

Because past about 120 ft, or when the flow you need is more than a jet can make at your depth, a jet pump is the wrong machine rather than the wrong size. Submersibles are not sized here, but the duty window this page produced is exactly what a submersible is selected on.

What pressure switch should I use?

The one whose cut-in meets the pressure your outlet needs. Household at 40 PSI means 40/60. Then confirm the pump can reach the cut-out at your depth, or it will never shut off. See the pressure section.

How big a pressure tank do I need?

Enough drawdown that the pump runs at least a minute per cycle. That depends on pump flow and switch setting, and the pressure tank calculator works it out. Precharge goes 2 PSI below cut-in.

Do I need a foot valve?

On any pump that sits above the water, yes. It is a check valve with a strainer on the bottom of the suction pipe, and it is what keeps the pipe full of water after the pump stops. Without it the pump has to re-prime on every cycle, which a jet pump cannot do on its own. The one exception is a driven well, where there is no room for one — fit an in-line check valve at the wellhead instead.

Can I use an irrigation pump for the house?

No. Self-priming sprinkler and centrifugal pumps are built for flow at modest pressure and are commonly rated for non-potable use. Household supply needs a jet or submersible with a tank and switch. For both jobs off one well, size for the house and check the lawn fits, or run two pumps.

Get It Sized by a Person

Send the numbers this page produced, or just the well details, and we will come back with the pump class, the ejector and the reasoning. If a jet pump is the wrong answer for your well, we will tell you that too.

Call 1-888.510.4036 Email a Sizing Request Read the Jet Pump Guide
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 a well driller's yield test or 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.