Deep Well Pump Sizing | Match Flow And Head, Not Horsepower

Get deep well pump sizing wrong and you’ll either run the house dry mid-shower or short-cycle a pump into an early grave. The fix isn’t a bigger motor — it’s matching the pump’s flow rate and total dynamic head (TDH) to the well’s actual water level, yield, and the home’s peak demand.

The rule that governs every decision: choose the pump from its performance curve at the required GPM and TDH, not from horsepower alone. Read the curve correctly and the rest of the job is arithmetic.

What Does Total Dynamic Head Actually Measure?

Total dynamic head is the full vertical workload the pump must overcome, expressed in feet: the static water level, drawdown while pumping, vertical lift to the highest tap, friction loss through pipe and fittings, and the pressure head required at the delivery point.

Start from the water level, not the borehole depth. The static water level comes from the well log; the pumping water level (static level minus drawdown) is the figure the pump actually works against — drawdown is the drop in water level while the pump runs, and a slow-recovering well shows a deeper one. A 200-foot well with a 60-foot static water level behaves like a shallow well until drought drops it.

Pressure converts to head at a handy constant: 1 PSI equals 2.31 feet of head. Add friction loss through the pipe run and you have the TDH to plot.

How Many GPM Does Your Home Need?

Most residential wells are sized for 8-12 GPM at peak demand — the worst realistic moment, like a morning rush with a shower, washer, and outdoor tap running together, not average daily use. Small homes with one or two bathrooms run comfortably at 5-8 GPM, while larger households with irrigation often want 12-16 or even 16-20 GPM.

Estimate the number from the fixtures people might genuinely run at once — showers, washing machine, outdoor taps — and leave water treatment appliances out of the fixture total, since the standard sizing method ignores them.

How To Size A Deep Well Pump

Once you have these numbers, the selection process is a straight sequence.

1. Confirm the static water level and drawdown from the well log or a field measurement — the Water Systems Council’s well pump sizing sheet is the standard worksheet for recording both.
2. Estimate peak simultaneous GPM from the fixture list, keeping the total at or below well yield.
3. Calculate TDH: vertical lift plus friction loss plus required pressure head (PSI multiplied by 2.31).
4. Plot that GPM against that TDH on the manufacturer’s pump curve, and pick the model whose operating point lands in the 60-80% efficiency band.
5. Round up to the next standard horsepower, then verify voltage, phase, and ampacity against your electrical service — 115V/230V single-phase is the residential norm, with three-phase service common only on farm and industrial sites.
6. Match the pump diameter to the borehole, per WDMPumps’ technical book, which ties pump body size to casing size — standard systems fit 4- to 8-inch casings, with larger sizes on special order.

A high-demand home at 400 feet can easily need more than the rule suggests. When the pump is running, you should see steady pressure at the highest tap with no short-cycling at the tank.

Two mistakes cause most failed installs: sizing by well depth alone and picking horsepower before looking at the curve. Both ignore the actual conditions the pump works under, which is exactly what TDH and yield capture. If you’d rather compare top candidates side by side, the best deep well pumps on the market are ranked in our tested roundup with sizing notes for each.