Home How to Size a Borehole Submersible Pump: Depth, Flow and Total Head
Short answer: Size a borehole pump on four numbers, in this order — the pumping water level of the borehole (not its total depth), the total head the pump must develop (vertical lift plus friction losses plus discharge pressure), the flow rate you need in litres or gallons per minute, and the inside diameter of the casing so the pump and motor physically fit. Get the first of those wrong — using total depth instead of pumping level — and every subsequent number is wrong with it. That single mistake causes more oversized, over-cycling borehole pumps than any other.
The borehole driller records a number that matters far more than the depth on the drilling report: the static water level — how far below ground the water sits when the well is resting.
Then measure or estimate the pumping (dynamic) water level — how far the level drops while the pump is running at your target flow rate. This is the number the pump actually has to lift from, and it is always lower than the static level.
| Depth measurement | What it means | Do you size on it? |
|---|---|---|
| Borehole total depth | How deep the hole was drilled | No |
| Static water level | Resting level below ground | As a starting point |
| Pumping water level | Level while pumping at target flow | Yes — size on this |
| Pump setting depth | Where the pump intake sits | Set above the well bottom |
A common and expensive error: sizing on the static level, or worse, on total depth. If the static level is 30 m but it draws down to 55 m at your required flow, a pump sized for 30 m will run, then progressively lose output as the level falls, then run dry.
Total head is the total work the pump does, expressed in metres (or feet) of water column. It is not just the depth of the hole.
Total head = vertical lift + friction loss + discharge pressure
Vertical lift is the distance from the pumping water level up to the highest point the water must reach — the top of the tank, the top of the building, or the highest sprinkler on the field.
Friction loss is the pressure consumed by pushing water through pipe, bends, valves and fittings. It rises sharply with flow velocity — roughly with the square of the flow rate — which is why undersized discharge pipe is such a common cause of disappointing performance. Long horizontal runs and narrow pipe hurt badly here; a large-diameter run to a distant field can add more head than the well’s own depth.
Discharge pressure is any additional pressure you need at the outlet: about 1 bar (roughly 10 m of head) for each 1 bar of working pressure you want at the tap, or the pressure required at a sprinkler nozzle.
Worked example:
| Component | Value |
|---|---|
| Pumping water level below ground | 40 m |
| Height to tank / highest outlet | 8 m |
| Vertical lift | 48 m |
| Friction loss (pipe run, fittings, valve) | 12 m |
| Required outlet pressure (2 bar) | 20 m |
| Total head required | 80 m |
The pump must therefore deliver your target flow rate at 80 m of head, not at the 40 m the hole happens to be deep. Choosing the pump off the “40 m” figure would produce a unit that cannot reach the tank at full flow.
Flow rate (litres per minute, cubic metres per hour, or gallons per minute) should come from demand, and from what the borehole can actually sustain.
For domestic supply, add up the fixtures that genuinely run at the same time — typically two to three at peak. A single shower is roughly 8–12 L/min depending on the fitting; a garden tap or two can easily double that.
For irrigation, work from the area, the application rate the crop needs and the hours per day you can run. If a field needs 20 m³/day and you can irrigate for 5 hours, you need at least 4 m³/h — plus margin.
For the well, compare your required flow against the borehole’s safe yield. Pumping beyond the sustainable yield lowers the water level further, increases drawdown, and starts the pump towards dry running. If the yield is marginal, a smaller pump running longer hours is almost always the better engineering choice than a bigger pump running the well dry.
This step gets skipped and then discovered at installation.
Submersible pumps and motors have a nominal outside diameter — commonly 3″, 3.5″, 4″, 5″, 6″ and 8″ — and boreholes are cased in a range of diameters. You need clearance around the pump and motor for the water to flow past and cool the motor. A pump that fits with millimetres to spare will overheat and fail; the motor depends on the flow of water past its housing to shed heat.
Check three things: the casing inside diameter (not the nominal outside diameter), the pump-plus-motor diameter, and the cable thickness at the pump (a heavy drop cable can be the widest part of the assembly). If the clearance is tight, a smaller-diameter pump or a different motor may be the only workable option.
Once you have total head and flow, choose the pump from its performance curve. Every pump has a curve showing flow against head, and the correct choice is a model whose curve passes through your operating point comfortably — ideally at the efficient middle of the curve, not at its extreme edge.
Two practical points:
Running a pump far to the right of its curve (much more flow than design) causes cavitation and rapid wear. Running it far to the left (much less flow than design, e.g. a throttled valve) keeps the pump hot. Both shorten life.
On paper, two pumps can look identical — same head, same flow, same horsepower. In a sand-laden or slightly corrosive borehole, they will not last the same time.
| Component | Budget build | Durable build |
|---|---|---|
| Pump stages / impellers | Engineering plastic | Stainless steel |
| Pump shaft | Carbon steel | Stainless steel |
| Motor housing | Painted steel | Stainless steel |
| Bearings | Standard | Sand-resistant / carbide |
| Motor protection | None | Built-in thermal + dry-run protection |
If your water is clean and the pump is in a comfortable operating window, a lighter build can be perfectly adequate. If the water carries sand, the well draws down hard in summer, or the pump is hard to service (deep setting, remote site), the stainless-steel stage stack and dry-run protection pay for themselves the first time a pump does not have to be pulled.
| Mistake | What you see |
|---|---|
| Sized on borehole depth instead of pumping level | Adequate flow at first, then fading badly as the season progresses |
| Discharge pipe too narrow | Pump runs at high amperage, flow far below the curve |
| Pump oversized for the well’s yield | Frequent dry running, thermal trips, motor burnout |
| Pump too tight in the casing | Motor overheats, windings fail early |
| No dry-run protection | A single dry period destroys the pump |
| Oversized for a small pressure tank | Short cycling day and night |
What is the difference between borehole depth and pumping water level? Borehole depth is how deep the hole was drilled. Pumping water level is how far below ground the water sits while the pump is running at your target flow. The pump only has to lift from the pumping level — but that level, not the depth, is what you must size on.
How much head does 1 bar equal? Approximately 10 metres of water column (about 33 feet). So if you want 2 bar of usable pressure at the outlet, budget roughly 20 m of head for it.
Can the same pump work for both household supply and irrigation? Occasionally, but the duty points differ. Household supply is usually high head with modest flow; irrigation is often lower head with much higher flow. Matching one pump to both normally means running it inefficiently for at least one of them.
How do I know if my borehole can sustain the flow I need? From the driller’s yield test if you have it, otherwise a pump test that records the water level at different flow rates. If the level falls steadily as you increase flow, you are approaching the sustainable limit.
Will a bigger pump give me more pressure? Not necessarily. Total head determines pressure, and head is consumed by lift, friction and outlet requirement. Fitting a bigger pump without checking head can simply move more water at the wrong pressure, at higher energy cost.
What size discharge pipe should I use? Size it to keep friction loss small relative to your total head. As a working rule, keep the pipe one or two sizes larger than the pump outlet where the run is long — the extra pipe costs far less than the energy wasted pushing water through an undersized line.
Sizing is arithmetic, not guesswork — and the arithmetic is unforgiving. An oversized pump destroys itself on dry running; an undersized one never reaches the tank. The four steps above resolve both.
Elestar builds deep well and borehole submersible pumps with stainless-steel stage assemblies and published performance curves, along with farm and agriculture irrigation pumps and household water supply sets. If you send us your pumping level, required flow, casing diameter and total head, our engineers will plot the operating point against the curve and confirm the fit before you commit to a model. Start at our product range or contact us directly.
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