Well pump sizing calculator

A well pump is bought with three numbers: the flow the house demands, the head the pump must overcome, and the horsepower that can do both at once. Pump sellers publish curves that answer the question precisely, but you need the first two numbers to read a curve at all, and that is what this calculator produces, along with the pressure tank that keeps the whole system from beating itself to death.

Flow: what the house actually asks for

Demand is about simultaneity, not totals. The old rule of one gallon per minute per fixture holds up well for sizing: a modest cabin with eight fixtures wants a pump that can deliver around 8 gpm at pressure, so a shower and a washing machine can coexist. Oversizing flow wastes money and, on low-yield wells, risks pumping the bore dry; a well that recovers slowly needs a smaller pump plus storage, which is a different design and one worth reading about in the Water archive before you buy anything.

Head: depth is only part of the hill

Total dynamic head is everything the pump pushes against, stacked up: the vertical lift from the pumping water level (which sits below the static level when the pump runs, ask your driller for the drawdown), plus the household pressure converted to feet, 2.31 feet per psi, so a 40-60 switch adds about 139 feet, plus pipe friction. That is why a "150 foot well" needs a pump sized for nearly 300 feet of head, and why the pressure setting you choose quietly changes the pump you need.

The horsepower row applies the water-power formula at a typical submersible's wire-to-water efficiency and rounds up to the class sizes pumps are sold in. Use it to know what you are shopping for, then confirm the specific model's curve covers your gpm at your head with margin.

The pressure tank, and the off-grid wrinkle

The pressure tank exists to protect the pump motor: every start is a surge of heat and wear, and the tank's drawdown is what spaces the starts out. Size for a minimum minute and a half of runtime, and remember the label lies, a "44 gallon" tank holds about 14 usable gallons between switch points. When in doubt, buy the bigger tank; it is cheap insurance on a motor that costs a thousand dollars to swap at the bottom of a well.

Off-grid, the start surge is the whole story: a 1 HP submersible can demand several thousand watts for the moment it spins up, which sets the size of your inverter and generator, run those against the generator calculator and the wire gauge calculator for the run to the wellhead. Soft-start pumps and constant-pressure drives shrink the surge dramatically and are usually worth their premium on solar. Many battery-powered homesteads sidestep the problem entirely: pump slowly to a storage tank uphill or in the loft, and let gravity or a small booster deliver pressure, storage you can size with the water storage calculator.

Getting a useful answer out of this

Well pump sizing returns flow you need, total dynamic head, pump class, and pressure tank. Every one of those is a planning figure rather than a specification: it tells you the size of the problem and roughly what it will take to solve, which is what you need before you can shortlist equipment or ask a supplier a sensible question.

The single most useful habit with any calculator on this site is to run it more than once. Change one input at a time and watch which output moves. That tells you where your design is sensitive, and design sensitivity is far more valuable than a single answer, because it identifies the assumption worth spending real effort to pin down. If a modest change in one input swings the result substantially, that input deserves a measurement rather than an estimate.

It is also worth running the pessimistic version. Off-grid systems are sized by their worst case rather than their average, and a design that only works on the numbers you hoped for is a design that works for part of the year. Put in the colder temperature, the longer run, the larger household, the dimmer month, and see whether the answer is still one you can live with.

What a calculator cannot know

Any tool like this works from the inputs you give it and from published averages for everything else. It does not know your particular site, the way your household actually behaves, the corner your building was cut in, or the ten-year-old equipment already installed that does not match its datasheet. Treat the output as a well-informed starting bracket, not as a specification to order against.

Three things in particular sit outside what any of these tools can see. The first is local rules, which decide what you may build regardless of what the arithmetic says: the laws pages cover those by state, county, and town. The second is the condition and quality of what you install, which is why two identical designs perform differently. The third is how the system is wired and protected, which decides whether it is safe rather than whether it is adequate, and which is the subject of lesson six.

Where a number here comes from a constant or a rule of thumb, the method notes above say so and give the source. That is deliberate: a result you cannot check is a result you should not act on, and the arithmetic here is simple enough to verify by hand on paper if you want to.

Where to take the answer next

A single figure rarely settles anything on its own, because off-grid systems constrain each other. The load list decides the array, the array decides the controller, the battery decides what the inverter can deliver, and the climate at your site decides all of it. The system planner takes one set of inputs and produces a whole coherent plan rather than an isolated number, which is usually the better next step once you have a figure you trust.

For location-specific inputs, the site report gives any address its real 30-year sun, climate, elevation, and distance figures, including the worst-month sun hours that size an off-grid system properly. For the equipment that the result points at, the spec tables list what manufacturers publish, normalised so that units of different sizes can be compared, and each product page works its own numbers through. For what things cost this month, the price index.

And for the reasoning rather than the arithmetic, the free course runs through the decisions in the order they actually arrive, from deciding whether the life suits you through to the first year on the land. Most of the mistakes that calculators cannot prevent are ordering mistakes, made before anyone opened a calculator at all.