What size water pump do I need is one of the most common questions we are asked, and the honest starting point is that size is the wrong place to begin. A pump is not a single number you can scale up or down, it is a match between what you have, what you want to do, and the physics that connects them. Start with the size and you will guess.
Start with five simple questions and the right pump almost picks itself. Here is how we work it out, the same way we would on the phone.
Quick answer: Before thinking about size, answer five questions: where is the water, where is it going, how far, how high, and how much do you need. Together these give you the flow and the head, which are the two numbers that define a pump. Only then does the size, the power and the model follow. The job comes first, the size comes last.
Start with what you need it to do
The single most important question comes before all the numbers: what is the pump needed for?
Clearing a flood, watering a garden, filling a pool, draining a site and supplying a borehole are completely different tasks, and each points to a different family of pump.
A flood pump and a borehole pump are not big and small versions of one thing, they are different tools. So the first decision is the purpose, and everything else narrows down from there.
If you are unsure whether the pump should be submersed in the water, or positioned next to it, our guide to surface pump vs submersible pump is the right place to start.
The five questions that size a pump
Answer those five and you have the two numbers that define any pump: the flow, how much water, and the head, how high and how far. Size, power and model all follow from these.
What does the flow actually mean? Think in baths
Flow in litres per minute is hard to picture, so it helps to anchor it to something familiar. A typical bath holds roughly 80 litres of water at a normal fill level, and a brim-full bath is nearer 150 to 200 litres. That gives a simple yardstick for what a flow rate means in practice.
Take two real pumps. The EVAK Residox 400 puddle pump moves about 213 litres a minute, so it would move a bath of water in roughly 22 seconds.
The APP HD-15 site pump moves around 830 litres per minute, meaning it could theoretically empty the same bath in under six seconds. In practice, however, it cannot pump water down to the same low level. While the Residox can remove water to a depth of approximately 1 mm, the HD-15 needs sufficient water depth to keep the pump submerged. This illustrates why flow rate alone, or size does not determine which pump is best for a particular application.
A smaller shallow-water pump like the Stream SPP2-4 is built for clearing thin water off a floor rather than shifting volume at speed, which is a different job again. Seeing flow as baths per minute makes it obvious that the right figure depends entirely on how quickly you need the water gone.
Flow rate compared with a garden tap
To put flow in even more everyday terms, a standard garden hose may only delivers about 12 to 18 litres a minute, and a kitchen tap often just 4 to 6.
We set out the full comparison, including the water company minimum, in our guide to the best borehole pump and typical flow rates, which is the best place to calibrate what a given litres-per-minute figure feels like.
Size is not the same as performance
A bigger pump is not automatically a better one. What a pump delivers depends on its power, its impeller design and how well it is matched to the job, not just its physical size or motor rating.
Two pumps of the same size can perform very differently. It is also common for a single range to include several models of different outputs, so the name alone does not tell you the performance: our comparison of the Tsurumi LB range shows three pumps in one family with quite different flow and head.
Choose on what the pump delivers at your duty, not on how big it looks.
Sizing a swimming pool pump is its own calculation
If your job is a swimming pool, the sizing is driven by turning the pool over in a set time rather than by raw flow, and it has its own method. We built a dedicated tool and guide for it, so head to how to size a swimming pool pump and browse the swimming pool pumps range. The same principle applies as everywhere else: the output depends on the power and the impeller, not just the model name.
Match the physics to the need
Before size, two more things have to line up. The first is whether the pump sits in the water or on the surface, which our surface pump vs submersible pump guide covers.
The second is the power you have. A mains supply, a three phase supply, a generator or petrol all steer the choice, and some pumps are simply not available in every option. Borehole pumps, for example, are not offered in 110V, so the supply can rule a pump in or out before size is even discussed. The physics has to match the need: get the type and the power right, and the size then narrows down cleanly.
Then check the curve, and the hose
With the type, power, flow and head settled, the last step is to make sure the pump is neither over nor underpowered for the duty, which is what the performance curve shows. Our guide to pump curves explained walks through it. And remember that the hose is part of the system: its diameter and length change what actually comes out, as we cover in how hose length and diameter change pump performance. Our Water Pump Performance Calculator ties it together, adding the friction to your lift and checking the pump delivers at your duty.
The shortcut: copy the data plate
There is one situation where you can skip most of this. If you are replacing a pump that worked well, the easiest and most reliable route is to copy the details from its data plate and match them, rather than start from scratch. Our guide to how to find the right replacement pump from an old data plate shows exactly which numbers to read. If the old pump failed early, though, it may have been the wrong size in the first place, in which case the five questions are worth revisiting.
The easiest way to replace an existing pump...
Copy the data plate. If the old pump did its job, reading the flow, head, power, voltage and connection sizes from its data plate and matching them is the quickest, most reliable route to a like-for-like. If it failed early, it may have been the wrong pump originally, so it is worth checking the sizing rather than repeating the mistake.
