Well pump sizing in two numbers: flow and head
Every well pump sizing guide eventually says the same thing, and it is worth saying plainly: a pump is not chosen by horsepower. It is chosen by two numbers — how much water you need (flow, in gallons per minute) and how hard the pump must work to deliver it (total dynamic head, in feet). Horsepower falls out of those two once you know the efficiency. Pick the horsepower first and you are guessing; pick flow and head first and the horsepower is arithmetic. That is the logic this water well pump sizing calculator follows, and it is why a good well pump size calculator works from your conditions rather than handing you a one-line chart that ignores your pressure and pipe.
So the order is: settle the flow your home needs, build the total dynamic head, then convert the pair into water horsepower, brake horsepower and a real motor size.
Total dynamic head: the part people get wrong
Total dynamic head, or TDH, is the total resistance the pump must overcome. Calculating TDH for a pump is a single addition once you have the parts, and the public formula is short:
That TDH calculation — the pump dynamic head calculation that every total dynamic head calculator is really doing — has one trap, the first term. The pump does not have to lift water the full depth of the borehole — it only lifts from the level the water actually stands at while you are pumping. That is the pumping water level: the static water level (where the water rests with the pump off) plus the drawdown (how far it falls as the well is pumped). Size to the total well depth instead and you can overstate the head by a hundred feet or more, which is exactly how people end up with an oversized pump that costs more, pumps the well down and short-cycles. Sizing to the pumping water level rather than total depth is the single correction that fixes most over-sized quotes.
Pressure head: don't forget it, or the pump never reaches cut-out
The pump also has to develop the pressure your home runs at. Because every 1 psi of water pressure equals 2.31 ft of head, a 50 psi cut-out adds 50 × 2.31 = 115.5 ft of head, and a 60 psi cut-out adds 138.6 ft. Leave that out and the pump may move plenty of water at zero pressure but never build enough to reach the switch's cut-out, so it runs continuously and your pressure stays weak. This is the “pump head pressure” that the discharge-pressure question is really about — to calculate pump discharge pressure you simply express the total head back in psi — and the built-in head ↔ pressure converter handles the psi-to-feet step for you.
Friction head: this tool won't fake it for you
Friction head — the loss as water rubs through the drop pipe and the run to the house — is a real part of TDH, and a long or undersized pipe can add tens of feet. This calculator deliberately does not invent a friction number for you; a guessed friction figure buried inside a pump result is worse than no figure. Instead, work it out from your real pipe inside diameter, length and flow in the pipe flow calculator, then paste the feet of head back into the friction field above. If you only want a ballpark, a modest residential run is often a handful to a few tens of feet, but check it rather than trust it.
Worked example, every step shown
Take a well with the water standing 100 ft down, falling another 20 ft of drawdown while pumping, a 50 psi cut-out, and 20 ft of pipe friction, feeding a home at 10 gpm:
So this well wants a pump that delivers 10 gpm at about 255 ft of head, met by roughly a 1½ HP submersible. Load that scenario straight into the tool with ?static=100&drawdown=20&cutout=50&friction=20&gpm=10&eff=0.5.
From head and flow to horsepower
Once flow and head are fixed, the standard water-horsepower relationship gives the rest:
Water horsepower is the useful work done on the water. Brake horsepower is what the motor must actually produce, and it is always larger because no pump is perfect. Wire-to-water efficiency for a small submersible is modest — roughly 0.35 to 0.55 — so a brake figure two to three times the water figure is normal, not a mistake. The tool rounds the brake horsepower up to the next standard motor; rounding down leaves the pump unable to hold its duty point.
Submersible well pump sizing
Submersible pump sizing follows the same two numbers, with one wrinkle: submersibles are multistage, so more head is reached by stacking stages rather than by raising horsepower outright. A lower-horsepower multistage pump can push water higher than a higher-horsepower pump with fewer stages, which is why any submersible well pump sizing chart or submersible pump size chart is really a flow-versus-head chart, not a horsepower chart. Use the submersible mode (it applies the 0.35–0.55 efficiency band), read off water hp, brake hp and the next motor, and then take your operating point to any submersible pump's published curve to confirm the model. For the cable feeding it, well pump wire size is a separate calculation — long 240 V runs need their own check in the wire size calculator.
Never size past the well's yield
A bigger pump cannot make a well produce more water than it holds. If you fit a pump that draws faster than the aquifer refills, it pumps the water level down to the intake, loses prime or runs dry, and short-cycles — the fastest way to kill a motor. So the well's sustained yield is a hard ceiling on flow. When you enter a yield, this tool caps the demand at it and warns you. If your household's peak demand genuinely exceeds the yield, the right fix is not a larger pump but a storage system: a cistern or holding tank that a modest pump fills slowly over the day, with a separate pressure pump serving the house.
| Fixture | Typical flow |
|---|---|
| Shower | ≈ 2.0 gpm |
| Bathroom faucet | ≈ 1.0 gpm |
| Kitchen faucet | ≈ 1.5 gpm |
| Washing machine | ≈ 2.0 gpm |
| Dishwasher | ≈ 1.5 gpm |
| Small home (typical peak) | ≈ 8–10 gpm |
| Large home (typical peak) | ≈ 12–15 gpm |
Finding your duty point on a pump curve
People search for a “pump curve calculator” expecting a chart of a specific pump. What you actually need is your own operating point — your required GPM at your total dynamic head — which you then lay over whatever pump's curve you are comparing. A good match has the curve passing through or just above your point, ideally near the middle of the curve where the pump runs most efficiently. This tool gives you that point from public physics, so you can compare any pump's curve yourself without depending on one maker's reproduced table.
Well pump sizing FAQ
What size well pump do I need?
Size to two numbers, not to horsepower: the flow your home needs in gallons per minute, and the total dynamic head the pump must push against. Most homes want roughly 8–12 gpm. TDH is the pumping water level (static level plus drawdown) plus the pressure head from your cut-out (psi × 2.31 ft) plus pipe friction, plus any lift to a raised tank. Then water horsepower is GPM × TDH ÷ 3960, and a real motor is that divided by efficiency, rounded up to a standard size.
Do I size a well pump to the depth of the well?
No — that is the single most common sizing error. The pump only lifts water from the level it actually sits at while pumping, which is the static water level plus the drawdown, not the full borehole depth. Sizing to total depth can overstate the head by 50–150 ft and push you to an oversized pump. Enter your static level and drawdown and the calculator uses the pumping water level, which is what the physics cares about.
How do I calculate total dynamic head (TDH) for a well pump?
TDH (ft) = pumping water level + pressure head + friction head + surface lift. The pumping water level is your static level plus drawdown. Pressure head is your cut-out pressure × 2.31 ft per psi, so 50 psi is 115.5 ft. Friction head is the loss in the drop pipe and the run to the house. Example: 100 + 20 + 115.5 + 20 = 255.5 ft TDH. The calculator shows each part so you can check it by hand.
What horsepower well pump do I need for a 300-foot well?
It depends on the pumping water level and your pressure setting, not the borehole depth alone. As a feel: at about 10 gpm and 255 ft of head, water horsepower is 10 × 255 ÷ 3960 ≈ 0.65 hp; at about 50% efficiency the brake horsepower is ≈ 1.3 hp, rounding up to a 1½ HP motor. Deeper pumping levels or higher cut-out pressures raise both. Enter your own numbers rather than relying on a depth rule of thumb.
What size submersible pump do I need?
Submersible sizing uses the same two numbers — flow and TDH — but submersibles are multistage, so a given duty can be met by stacking stages rather than raising horsepower. Wire-to-water efficiency is modest, roughly 35–55%, so brake horsepower comes out well above water horsepower. The submersible mode uses that band and reports water hp, brake hp and the next standard motor; match those numbers to any pump's published curve.
How many GPM does a house need from a well pump?
Add up the fixtures likely to run at once: about 2 gpm a shower, 1 gpm a bathroom faucet, 1.5 gpm a kitchen faucet, 2 gpm a washing machine, 1.5 gpm a dishwasher. A small home often lands around 8–10 gpm and a larger home around 12–15 gpm. You can never draw more than the well sustains — if demand exceeds yield, the answer is a storage tank or cistern, not a bigger pump.
Does this calculator work out pipe friction loss for me?
Not on this page, and that is deliberate. Friction head is one input to TDH, and getting it right needs your real pipe inside diameter, length and flow. This tool lets you paste a friction-head figure or type a rough estimate, and links to the pipe flow calculator, which is built for friction loss. Keeping the jobs separate means each is done properly rather than hiding a guessed friction number inside the pump result.
What is a pump curve and how do I use my TDH and GPM with it?
A pump curve is the manufacturer's chart of flow versus head for a given pump. Find your operating point — your required GPM at your TDH — and pick a pump whose curve passes through or just above it, ideally near the middle where it runs most efficiently. This tool gives you that duty point from public physics; you then match it to whatever pump's curve you are comparing, with no brand's table reproduced here.
Is this well pump sizing calculator free and private?
Yes. It is free, needs no signup or download, and runs entirely in your browser, so nothing you enter leaves your device. You can copy a shareable link that reopens the calculator with your static level, drawdown, cut-out pressure, friction, flow and efficiency already filled in.