To size a borehole pump, establish the flow you need, calculate the total dynamic head at that flow, and check that the borehole can support the proposed abstraction rate. Then choose a pump whose verified performance curve meets those requirements and whose size, motor and controls suit the installation.
A useful selection request is “1 m³/h at 47 m total dynamic head, within the borehole test recommendations.” A request for a pump based only on borehole depth or motor kW leaves out the information that determines delivered water.
The steps below cover clean-water borehole supply to a storage tank or directly to a pressure system. Use them to prepare a selection request and compare proposed equipment.
Collect the borehole and supply information
Start with the drilling record, pump-test report and a sketch of the route to the delivery point. Keep water levels, installation depth and total borehole depth as separate measurements.
| Input | What to record | Why it matters |
|---|---|---|
| Water use | Litres per day and any loads that run together | Daily volume and peak flow answer different questions |
| Supply arrangement | Open storage tank or direct pressure supply | Determines the flow and outlet-pressure requirement |
| Pump-test recommendation | Recommended abstraction rate, operating duration and test date | Sets the water-source constraint |
| Pumping water level | Depth below the wellhead at the proposed flow, including expected seasonal changes | Determines the lift while pumping |
| Static water level | Resting level before pumping | Provides context for drawdown; does not replace pumping level |
| Delivery point | Elevation relative to the same wellhead reference and required pressure | Completes the lift and pressure requirement |
| Pipe route | Actual length, internal diameter, material, fittings and valves | Allows friction loss to be calculated at the selected flow |
| Borehole and installation | Actual casing internal diameter, obstructions and proposed pump setting | Determines physical fit and installation conditions |
| Power and equipment | Voltage, phase, frequency, motor type, cable length and proposed controller | Determines electrical and control compatibility |
If the pumping level or test recommendation is missing, obtain that information before accepting a final pump recommendation. Total drilled depth alone cannot fill either gap.
Choose the flow for the way water will be supplied
Filling a storage tank
When the borehole fills a tank, calculate the minimum average refill flow from the daily water requirement and the available pumping window:
Required refill flow in m³/h = daily requirement in m³ ÷ available pumping hours per day.
For example, 3,000 litres per day is 3 m³/day. With four available pumping hours, the minimum average refill flow is 3 ÷ 4 = 0.75 m³/h, or 750 L/h.
The tank separates borehole refill from short periods of high demand. A downstream booster pump, where needed, must still meet the flow and pressure required by the taps or irrigation equipment. Storage does not increase the borehole’s daily water availability.
Supplying taps or irrigation directly
For direct supply, identify the loads that may operate together and their required flow and pressure. A daily consumption figure cannot tell you whether the pump can support simultaneous showers or an irrigation zone.
For irrigation, use the zone layout and emitter requirements rather than a generic number of sprinklers per motor kW. Record the flow and minimum pressure needed by the outlet groups expected to run together; have the domestic demand assessment confirmed for the proposed system. Grundfos’ groundwater sizing guidance likewise starts from required flow and pressure.
Check the proposed flow against the borehole test
Ask for the recommended abstraction rate and operating schedule, together with the water-level behaviour recorded during testing. A short test’s highest measured flow is not automatically the rate available throughout the year.
Pumping level depends on the abstraction rate and conditions in the borehole. The level used for sizing should represent the proposed duty, with allowance for expected seasonal conditions based on the site assessment. South Africa’s Water Research Commission guidance explains why flow, drawdown and recovery observations matter when assessing borehole supply capacity.
If demand exceeds the recommended rate, consider a longer permitted refill window, storage or demand management with the person assessing the borehole. A larger pump cannot resolve an inadequate water source. Confirm that the proposed daily abstraction and operating schedule are acceptable as well as the instantaneous flow.
Calculate total dynamic head at the selected flow
Total dynamic head, or TDH, is the head the pump must provide to move the selected flow from the pumping water level to the delivery point. For a typical borehole supply, combine:
- Lift from the pumping water level to the delivery point.
- Pressure required at that delivery point, expressed as metres of water head.
- Friction losses through the actual pipe route, fittings and valves at the selected flow.
This matches the elevation, pipe-loss and outlet-pressure components in the Grundfos SP-G sizing data booklet.
For water, 1 bar is approximately 10.2 m of pressure head. Add a pressure requirement only where the system needs it. An open tank inlet discharging freely does not need the same residual pressure as a pressurised irrigation connection. The pressure-to-head relationship is explained in Grundfos’ pump-curve guide.
Use one elevation reference throughout. If the pumping level is 35 m below the wellhead and the tank inlet is 5 m above it, the elevation lift is 40 m. Pump setting depth determines installation conditions and contributes to actual pipe length, but it is not substituted for pumping water level in the elevation-lift calculation.
Recheck the pumping water level and recalculate pipe losses when you change the proposed flow. Adding an arbitrary percentage to TDH can hide missing inputs; identify the actual conditions the pump must cover.
Worked example for an open storage tank
The following figures are illustrative assumptions, not measurements from a QDD installation. The chosen refill target is three hours, so the design flow is 3 ÷ 3 = 1.0 m³/h. This is a stated operating choice, not a universal sizing margin.
| Item | Assumed value |
|---|---|
| Daily water requirement | 3,000 L/day, or 3 m³/day |
| Available refill window | 4 hours per day |
| Minimum average refill flow | 0.75 m³/h |
| Design flow to check | 1.0 m³/h; assume the site assessment permits this rate and daily volume |
| Pumping level at 1.0 m³/h | 35 m below the wellhead reference |
| Open tank inlet elevation | 5 m above the same reference |
| Pipe and fitting losses at 1.0 m³/h | 7 m, assumed for the complete actual route |
| Required pressure at the open inlet | 0 m additional pressure head |
TDH = 35 + 5 + 7 + 0 = 47 m.
The design requirement is therefore 1.0 m³/h at 47 m TDH. At that delivered flow, supplying 3 m³ takes approximately three hours, fitting within the four-hour window. Assume the recommended operating schedule also permits this three-hour daily run. This example leaves motor power and pump model to the curve and equipment checks.
The 35 m pumping level and 7 m friction loss must both correspond to 1.0 m³/h. If a different flow is selected, those values must be checked again. Include any inlet device or restriction in the loss assessment if the tank does not discharge freely.
Check a pump curve before choosing motor power
Ask the supplier to show the proposed design flow and TDH on the curve for the exact pump, frequency or speed, and configuration. For the example above, look for performance at 1.0 m³/h and 47 m, within the manufacturer’s permitted operating range.
Maximum head and maximum flow are different ends of a performance curve. A listing that advertises both does not show that the pump delivers maximum flow at maximum head.
The actual operating point is where the pump curve and the system curve meet. The supplier should check the expected operating conditions, including changing water levels or tank conditions, rather than marking a target on a graph without checking the system. Grundfos’ curve-reading lesson explains this intersection.
Once suitable hydraulic performance is established, verify motor loading and power requirements for the proposed assembly. The same motor kW can be paired with pump hydraulics that deliver different combinations of flow and head.
Confirm physical fit and electrical compatibility
Check actual casing internal diameter against the largest outside dimension of the complete pump assembly, including cable guards and connectors. A nominal “4-inch” description is not a complete clearance check.
Confirm installation depth, submergence, incoming-water conditions and motor-cooling requirements using the relevant equipment instructions and borehole record. Keep the pump within the manufacturer’s installation limits; a general sizing article cannot supply a universal distance from the borehole bottom.
For the electrical package, verify supply voltage, phase and frequency, motor type, starting or drive method, cable length and protection. Confirm compatibility for the exact motor and controller, particularly when a permanent-magnet motor or variable-speed drive is proposed. Franklin Electric’s AIM manuals illustrate why cooling, cable and control requirements must be checked against the relevant motor documentation.
For solar pumping, daily water delivery also depends on the available solar resource and the matched pumping system. Rated AC motor power alone does not establish daily solar output.
Send a selection request suppliers can compare
Use the same inputs for every quotation:
- Application: tank filling or direct supply, daily water requirement and simultaneous loads.
- Water source: pump-test report, recommended abstraction schedule and pumping level at the proposed flow.
- Hydraulic duty: required flow, calculated TDH, pipe-route details and delivery pressure.
- Installation: casing internal diameter, proposed pump setting and relevant water-quality information.
- Power package: voltage, phase, frequency, motor type, cable length and controller requirements.
Request the exact model, its performance curve with the proposed duty marked, permitted operating range, motor/controller compatibility and quotation inclusions. If an input is uncertain, identify it so the supplier can explain what must be measured or confirmed.
You can review QDD’s borehole pump range and pump-selection information, then send the available operating details for a selection enquiry.
Common borehole pump sizing questions
What size pump do I need for a 60 m borehole?
A 60 m drilled depth does not establish pump size. You still need the required flow, pumping water level at that flow, delivery elevation and pressure, pipe losses, borehole test recommendation and installation dimensions. Those inputs define the hydraulic duty and the equipment checks.
Is a bigger borehole pump a better choice?
Choose a pump that meets the required duty within the water-source and equipment limits. Extra motor power or flow capacity is useful only if the application and borehole can support it. Check how the proposed pump will actually operate across expected conditions.
Can I size the pump without a pump-test report?
You can estimate demand and map the delivery route, but you cannot reliably confirm the borehole’s available supply or operating water level from drilled depth. Obtain an appropriate water-source assessment before finalising the pump and operating schedule.
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