What pump diameter do you need – 2" or 3" borehole

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Why the diameter of the bore determines the pump selection from scratch

When a customer comes to atria.sk with a request for a well or borehole pump, the first question we ask is not "what power do you need" or "from what depth do you pump water", but simply: "what is the diameter of your borehole or well?" It may sound trivial, but this single piece of information determines whether the customer will receive a pump that physically fits into it at all. We have seen dozens of cases where a customer ordered a pump based on power or price and during installation realized that it simply would not fit into the borehole pipe, or that it did fit, but with such minimal clearance that jamming, overheating and rapid wear were imminent.

The diameter of the bore determines the maximum outer diameter of the pump that can be installed in it. In wells and boreholes in Slovakia, we most commonly encounter two sizes - 2 inches (2") and 3 inches (3"), with the less common 4" size being covered in a separate article "What bore diameter do I need: the difference between 2", 3" and 4" pumps." In this article, we will focus specifically on the decision between 2" and 3" bores, as these two sizes account for the vast majority of implementations in family homes, cottages and gardens in Slovakia.

What exactly does the 2" and 3" designation mean for a bore

The inch designation refers to the internal diameter of the casing pipe (also known as the liner), which the bore is equipped with. This pipe protects the wall of the bore from collapse and also serves as a guiding path for the pump. The conversion to millimeters is as follows:

2" bore - internal diameter of the casing approximately 48-51 mm (more precisely 2 inches = 50.8 mm, but the actual internal diameter varies according to the wall thickness of the pipe and the manufacturer, typically 48-51 mm)
3" bore - internal diameter of the casing approximately 76-80 mm
4" bore - internal diameter of the casing approximately 100-105 mm

It is important to understand that this data refers to the internal diameter of the pipe, not the diameter of the actual borehole drilled into the ground. The borehole is usually slightly larger, as the casing is inserted into it and the space between the casing and the bore wall is then backfilled or grouted. However, for pump selection, only the internal diameter of the casing is relevant - this is the "tunnel" through which the pump is lowered and in which it will operate.

How to determine the diameter of your own bore if you don't have documentation

With older wells and boreholes, it is common for the owner to not have access to the drilling documentation or to not know what diameter of casing was used during the implementation. In practice, we recommend several methods:

Measuring the diameter from the outside - if the head of the bore (the top end of the casing) is accessible above ground or in a shaft, the diameter can be easily measured with a sliding caliper or a tailor's tape measure around the circumference.
Removing the old pump - if there is an old non-functional pump in the bore, its diameter (usually indicated on the label or measured) will indicate the minimum internal diameter of the casing.
Checking the project documentation for the bore - for legally implemented bores, there should be a drilling log or protocol where the diameter of the casing pipe is indicated.
Test insertion of a measuring tube or template - in case of doubts, we can advise you on how to verify the diameter yourself using a simple test rod with a known diameter.

If you are unsure and are planning to order a pump, it is better to verify the diameter twice. Returning or exchanging a pump after you discover at the installation site that it does not fit means lost time, unnecessary work for the installer and in the worst case, damage to the thread or seal due to forced insertion.

Pumps for 2" bore: narrow, performance limited, but irreplaceable

Only a very limited range of pumps can fit into a 2" bore, as the internal space of approximately 48-51 mm limits the size of the motor and hydraulics. Practically the only type of construction that can reliably fit into such a diameter and at the same time be able to pump water to a reasonable height are screw (helical or spiral) pumps.

A typical representative of this category is the Submersible screw pump for narrow bore STING 2". A screw pump works on the principle of a rotating screw (eccentric screw rotor) in a flexible stator, which literally "pushes" the water in front of it in closed chambers. This design allows for relatively high pumping height with a small body diameter, which is exactly what is needed for narrow bores.

Advantages of screw pumps for 2" bore

They can fit into bores where other constructions physically have no chance
They can operate with slightly contaminated water containing sand, as the rotor in the rubber stator is more tolerant to abrasive particles than the impellers of centrifugal pumps
They have a stable, uniform flow without pulsations
They are suitable for deeper bores with a lower diameter, where you need to maintain pumping height

Limitations you need to be aware of

Lower maximum flow compared to 3" pumps of the same power - the screw construction simply cannot "pump" as much water as a larger centrifugal pump
The stator (the rubber part in which the rotor turns) is a wear part that must be replaced over time, especially when operating dry or with higher sand content in the water
They must not run dry or for a short time, as the rubber stator overheats and is irreversibly damaged when running without water

Therefore, screw pumps are practically used exclusively where the physical diameter of the bore does not offer any other option. If you have a choice between drilling a 2" or 3" bore (for example when planning a new bore), we recommend 3" in most cases, as it provides much greater flexibility in pump selection for the future.

Pumps for 3" bore: wider selection, higher performance, more options

A diameter of 3" (76-80 mm internal diameter of the casing) opens up a much wider range of constructions. Pumps of this diameter can usually accommodate multi-stage submersible centrifugal pumps, which are the most common type for wells and boreholes in family homes today.

An example is the Submersible pump IBO 3" STM 20, which uses a multi-stage centrifugal hydraulics - several impellers are arranged in sequence inside the body, each adding its share to the total pumping height. This design allows for significantly higher flows than screw pumps while maintaining sufficient pumping height for typical family homes.

Why 3" bore is a more practical choice if you have the option

Greater pump selection - from more affordable basic models up to powerful multi-stage pumps for supplying larger households or irrigation
Higher achievable flow rate - at the same power consumption, a 3" centrifugal pump can deliver more water per minute than a 2" submersible pump
Easier maintenance - centrifugal pumps generally have a more robust construction and longer service life under normal operation
Space for future expansion - if you are planning an irrigation system, a swimming pool or a larger house, a 3" borehole will allow you to switch to a more powerful pump at any time without the need for re-drilling

The flow values in the graph are approximate and are provided only for illustration of the relative difference between types - the actual flow always depends on the exact model, the head and hydraulic losses in the piping. More precise recommendations according to the well depth can be found in the article "What performance and head to choose for a pump according to the well depth."

Practically: how to proceed with selection according to the measured diameter

When you know the actual internal diameter of your borehole, proceed according to the following logic:

Step 1 - measure or verify the internal diameter of the casing

As mentioned above, either by direct measurement, according to the documentation, or according to the original pump.

Step 2 - add a safety margin

The pump should not tightly "scrape" against the walls of the casing. The recommended minimum clearance between the outer diameter of the pump and the internal diameter of the casing is at least 3-5 mm (i.e., a pump with an outer diameter of 48 mm in a casing with an internal diameter of at least 50-51 mm is borderline acceptable, but ideally you should have a larger margin). This clearance is also important for motor cooling - most submersible pumps are cooled by water flow around the motor housing upwards to the suction inlets, and too tight an installation limits this flow.

Step 3 - consider the straightness and alignment of the borehole

Older or poorly drilled boreholes may have slight waviness or deviation from the vertical. With borderline dimensions (e.g., a pump with a diameter of 47-48 mm in a borehole with an internal diameter of exactly 50 mm), it may happen that the pump gets stuck in a place where the casing is slightly deformed or where the joints of the pipe segments are not properly aligned.

Step 4 - choose the construction type according to the diameter and required performance

For a 2" borehole, the choice practically falls on a 2" submersible pump type STING 2". For 3" and wider boreholes, you have a choice between submersible and centrifugal types, and for the usual water supply to a family house, multi-stage centrifugal pumps such as IBO 3" STM 20 are recommended in the vast majority of cases. The difference between these two construction principles is discussed in detail in the article "Submersible pump - screw vs. centrifugal - differences and use."

What if the borehole is narrowed only on part of its length (so-called neck or reduction)

It is not uncommon for a borehole to have a diameter of 3" along its entire length, but in the upper part (for example, due to the installation of another type of coupling or due to a transition to another casing), it is narrowed to 2" on a short section. In such a case, the narrowest point through which the pump must pass during lowering is decisive. We recommend always measuring the diameter at several points in such boreholes, ideally using a test template or a caliper lowered on a rope to the bottom, to ensure that the pump can pass the entire length without getting stuck.

Mounting specifications according to the borehole diameter

The diameter of the borehole not only affects the choice of the pump itself, but also the way of its suspension and the guidance of the discharge pipe. In narrow 2" boreholes, the space is so limited that it is worth using a thinner steel safety rope for suspending the pump and high-quality pressure PE pipe with the smallest possible outer diameter of the fittings, so that you can use every available millimeter of space. In 3" boreholes, the handling space is more comfortable, which also facilitates any future maintenance or pump replacement without the risk of getting stuck.

A detailed step-by-step installation procedure, including recommendations for suspension, electrical connection and installation of a check valve, can be found in the separate article "Installation of a submersible pump in a well or narrow borehole."

Sand, impurities and their impact on selection according to borehole diameter

Regardless of whether it is a 2" or 3" borehole, the content of sand and mechanical impurities in the water significantly affects the pump's lifespan. In narrow boreholes, the situation is even more sensitive, as the space for water circulation around the pump is smaller, which can lead to faster sedimentation near the suction inlets. If your well or borehole produces water with a higher sand content, we recommend also installing an 1" sand filter for suction in the system, which captures mechanical impurities before they enter the distribution system and thus protects not only the pump, but also the subsequent fittings, pressure tank and household appliances.

With shaft pumps in up to 2" bores, the sensitivity to sand is slightly lower than with conventional centrifugal pumps that have tight clearances between the impeller and the body, but it still holds true that a high content of abrasive particles shortens the life of the stator. In the case of 3" centrifugal pumps, on the other hand, clogging and wear of the impellers and their blockage are a risk, which is manifested by a gradual drop in performance.

Practical case scenarios

Scenario 1: Old well at a cottage with unknown diameter

The customer has a dug well at the cottage, into which a bore with a casing was added some years ago. They do not know the diameter of the casing, as the documentation has been lost. We recommend removing the original, already non-functional pump (if present) and measuring its outer diameter - this will provide reliable information about the minimum inner diameter of the casing. If the pump is not present, the diameter must be verified by measuring from the top or using a test probe.

Scenario 2: New bore planned for a family house

If the customer is still planning the realization of the bore and is asking which diameter to choose, we clearly recommend 3" (or even 4", if geological conditions and the budget allow), because this choice will preserve maximum flexibility in pump selection in the future - from simple household water supply to potential garden irrigation connection.

Scenario 3: Narrow bore with deep groundwater level

In a 2" bore with a water level depth of around 40-50 m, the shaft pump of the STING 2" type is practically the only realistic option, as it combines the necessary lift height with a small diameter. In such a case, the pump should not be assessed only by price, but mainly by whether the declared lift height at a given flow rate corresponds to the actual depth and the household's needs.

How the bore diameter relates to other selection criteria

The bore diameter is only one, albeit limiting, criterion for pump selection. Other criteria - required flow rate, lift height, water quality, control method (pressure switch, frequency converter, float switch) - are equally important and together form a comprehensive picture of which pump is optimal for a given application. We therefore recommend following a systematic approach according to the guide in the article How to choose a pump for a well: key selection criteria, where the bore diameter is listed as the first filter that narrows the selection to physically usable models, and then the decision is made based on performance parameters.

If you are hesitating between specific models from different manufacturers, the comparison in the article Comparison of STING and IBO pumps - which one to choose may also help, where we discuss the differences in construction, materials, and recommended use of both brands available in our range.

Maintenance and lifespan depending on pump diameter

Smaller 2" pumps generally have more compact components and smaller tolerances, which means they are more sensitive to dry running, overheating, and mechanical impurities. We recommend always installing dry-running protection (a float switch or electronic protection based on pressure/drop in current) with them, as repairing or replacing the stator of a shaft pump after overheating is a common, but unnecessary expense that can be easily avoided.

With 3" centrifugal pumps, the construction is more robust, but even here, regular pressure, flow rate, and seal condition checks extend the lifespan. Common faults and their solutions, including signs of impeller wear, capacitor damage, or shaft seal problems, are thoroughly discussed in the article Most common faults of submersible pumps and how to solve them.

Why the clearance between the pump and the casing wall is also important, not just the pump diameter itself

When comparing the outer diameter of the pump with the inner diameter of the casing, it is worth also considering what happens in the space between them during operation. This annular space is not just "extra free space" - it is also the only path through which water flows toward the pump's suction inlets, and at the same time, it is the space through which heat is dissipated from the motor. In most submersible pumps (this applies to both shaft and centrifugal designs), water flows upward along the motor housing during suction, cooling the motor, and only then enters the suction basket. If this gap is too tight, the flow speed in it decreases, the water remains around the motor for a longer time, and the cooling is less effective - especially during longer continuous operation, for example, when filling a large tank or during irrigation.

For this reason, in borderline combinations - for example, a pump with an outer diameter of 46-48 mm in a casing with an actual inner diameter just over 50 mm - we recommend also considering the operating mode. If you plan to run the pump only for short periods (for example, an automatic system with a pressure tank for normal household consumption), a tighter clearance is not such a problem as with a pump that will run continuously for long periods, for example, when filling a larger storage tank or during an irrigation cycle in dry summer months. For such applications, it is better to choose a pump with as much clearance as possible relative to the casing's inner diameter, or even reconsider whether a 3" bore with a centrifugal pump would not be more operationally advantageous in the long run.

Frequently asked questions

I have a bore with a diameter of exactly 50 mm - will a 3" pump fit in there?

No, 3" pumps have an outer diameter of around 74-76 mm and will not physically fit into a bore with an inner diameter of 50 mm. In such a case, it is necessary to use a shaft pump specifically designed for 2" bores, such as the STING 2".

Is it possible to enlarge an existing 2" bore to 3" so that I can use a more powerful pump?

Technically, this is an intervention comparable to drilling a new bore, as a new, wider casing must be drilled and installed. In practice, it is usually more costly and time-consuming than investing in a high-quality shaft pump for the existing 2" diameter. Enlargement is worth considering mainly if you plan a significant increase in water consumption (for example, an irrigation system) and the existing bore would not theoretically be able to provide sufficient flow rate even in theory.

Can a submersible pump also be used in a 3" borehole, or only a centrifugal pump?

Yes, submersible pumps are also available in versions for 3" boreholes, where they offer a higher head at a lower flow rate compared to centrifugal pumps of the same diameter. However, for standard water supply to a family house, multi-stage centrifugal pumps are preferred in the vast majority of cases for 3" boreholes, as they provide a better flow-to-reliability ratio during regular operation.

How exactly can I determine the outer diameter of a specific pump before purchasing?

The outer diameter is always listed in the product's technical specifications, usually in millimeters and also with an inch designation (for example, "diameter 74 mm - suitable for a 3" borehole"). When ordering via atria.sk, we recommend comparing this value with your measured borehole diameter and, in case of any uncertainties, contacting us before placing your order. We will gladly assist you based on the specific dimensions.

What happens if I accidentally order a pump with a larger diameter than my borehole?

The pump will simply not fit into the casing or will get stuck at the top. We do not recommend trying to force it in under any circumstances, as this may damage the pump housing, cable, or the casing itself. In such a case, the pump must be replaced with a model of the correct diameter.

Does the pump diameter also affect noise or vibrations during operation?

Indirectly, yes - with a tight fit and minimal clearance between the pump and the casing wall, motor vibrations can be transferred to the casing and amplified, which is noticeable during operation, especially in shallow wells. Maintaining the recommended clearance of 3-5 mm helps minimize this effect and also ensures the necessary motor cooling.

Summary in conclusion

Deciding between a 2" and a 3" pump is not a matter of preference, but a physical limitation of your borehole - the inner diameter of the casing simply determines which types of pumps you can even install. For 2" boreholes, submersible pumps such as the STING 2" remain practically the only realistic option, while 3" boreholes open up a wider range of possibilities, including powerful multi-stage centrifugal pumps such as the IBO 3" STM 20. We always recommend measuring or verifying the diameter of your borehole before placing any order, adding a safety margin, and then selecting a specific model according to the required performance and head. If you are unsure, we will gladly recommend a specific model from our range based on the dimensions and parameters of your well or borehole that you provide.

Do you have a question on this topic?

Struggling to decide or dealing with a specific situation in your household? Write to us - we will be happy to help.