Every centrifugal pump comes with a performance curve. It is the single most useful piece of information on a data sheet, and yet it is the part most people skip straight past. That is a shame, because reading it correctly is the difference between a pump that lasts a decade and one that fails inside a year.
This article builds on our earlier guide, Pump Curves Explained: How to Understand Flow Rate and Head, taking it a step further into reading the curve and choosing the right pump.
The good news is that a pump curve is far simpler than it looks. If you can picture driving a car, you already understand the principle. This guide explains what the curve shows, how to read off the numbers that matter, and how to use it to pick a pump that is the right size for the job, not the biggest or most expensive on the shelf.
What a pump curve actually shows
A pump curve is just a graph of two things working against each other.
The vertical axis is usually head, measured in metres. Head is the total height the pump is working against, which is the vertical lift from the water up to the discharge point, plus the friction losses in the pipe, hose and fittings along the way. Added together, that total is called the Total Dynamic Head, and it is the number you read against the curve.
The horizontal axis is usually flow rate, measured in litres per minute. This is how much water the pump can actually deliver at a given head.
The line itself tells the whole story. Where it meets the vertical axis is the shut-off head: maximum pressure, zero flow. Where it meets the horizontal axis is the maximum flow rate: lots of water, but no height to lift it. Every pump trades one off against the other. The more head you ask of it, the less flow you get, and the harder you push it for flow, the less height it can manage.
Head is just another way of saying pressure
It helps to know that head is a measure of pressure. When a pump is rated for 20 metres of head, it can hold up a 20 metre column of water above it, and that column of water exerts a specific pressure at the bottom. Expressing pressure as a height rather than a force is simply more convenient for pump work, because it maps directly onto the lift you can measure on site.
If you prefer to think in bar, the conversion is easy. As a rule of thumb, 10 metres of head is roughly 1 bar. More precisely, 1 bar equals about 10.2 metres of water column (and about 14.5 psi, or 100 kPa).
- To convert head into bar, divide the metres by 10.2. So 20 metres of head is roughly 2 bar.
- To convert bar into head, multiply by 10.2. So 3 bar is roughly 30 metres of head.
These conversions assume water, which covers almost everything you are likely to pump. A pump's head rating in metres stays the same whatever the liquid, so metres of head is the figure to read against the curve, and bar is just a handy translation when a spec or a fitting is quoted that way.
Reading off your duty point
The point on the curve that matters for your installation is the duty point. Finding it takes four steps:
- Work out your total head: the vertical lift plus an allowance for friction losses.
- Find that head on the vertical axis.
- Move straight across until you meet the curve.
- Drop straight down to the horizontal axis. The reading there is the flow you will get.
For example, if a pump's curve passes through 4 metres of head at 120 litres per minute, then on an installation with 4 metres of total head it will deliver 120 litres per minute. Lengthen the pipe run so your total head rises to 6 metres, read across again, and the same pump will deliver less. Nothing has changed about the pump. You have simply asked more of it.
If you are not sure what your friction losses add up to, our Water Pump Performance Calculator will estimate them for your specific hose and pipe run, so you can work out your real total head before you choose a pump.
Think of it like a car engine
Here is where our founder, civil engineer Simon Crowther, has an analogy that makes everything click. On a standard pump curve, with head running up the side and flow along the bottom, the line starts high at shut-off and slopes down towards maximum flow. Wherever your duty point lands on it, you are really in one of three driving scenarios, each defined by how head and flow are balanced.
High head, low flow: towing a caravan uphill in a 1.0 litre. This is the steep end of the curve, near shut-off. The engine is straining against resistance, progress is slow, fuel economy drops and everything is under stress. In pump terms, that means too much vertical lift, a long pipe run, undersized hose or restrictive fittings. The pump runs hot and recirculates water inside itself, which wears the seals and bearings.
Balanced head and flow: cruising on the motorway in the right gear. This is the middle of the curve. The engine is relaxed, economy is good, wear is low, and it will happily run like that for hours. This is exactly where you want your pump to sit: sensible energy use and good long-term reliability. Think motorway miles.
Low head, high flow: stuck in first gear at maximum revs. This is the flat end of the curve, often called run-out. Very little head and a huge amount of flow, so the engine is screaming with almost nothing to push against. A pump run here can overload its motor, pull excess current and cavitate, all of which shorten its life.
A well-sized pump is the cruising-in-the-right-gear pump. That is the whole idea behind the rule we use to specify them.
The middle-third rule
Specify a pump so its normal duty point falls within the middle third of its curve. That is where the pump is most efficient and where the stress on its components is lowest. Push it too far towards high head and low flow, near shut-off, and you get heat build-up and accelerated seal and bearing wear. Push it too far the other way, towards low head and high flow at run-out, and you risk motor overload and cavitation.
Why oversizing is not the safe option
It is tempting to buy the biggest pump you can afford and assume you are covered. You are not. Oversizing wastes energy, increases wear and shortens pump life, because the pump ends up running at the low-head, high-flow end of its curve. Undersizing is just as bad: the pump cannot keep up, runs hot and may fail prematurely. Right-sizing is the goal, and the curve is how you get there.
This is also why a pump's flow figure can be misleading on its own. A headline "delivers 335 litres per minute" is the maximum flow at minimum head. As a quick illustration, clearing a flooded cellar of 5,000 litres in one hour needs roughly 85 litres per minute, while clearing the same cellar in 15 minutes needs around 335 litres per minute. Once you add real lift and friction, the flow you actually get will be lower. Always read the curve, not just the top-line number.
From curve to choosing a pump
Reading the curve is half the job. The other half is matching it to the right type of pump for your situation, whether that is a submersible for drainage and flood removal, a surface pump, or an engine-driven unit for sites with no mains power.
Frequently asked questions
What is a duty point on a pump curve?
The duty point is where your installation's total head meets the pump's curve. It tells you the flow rate that particular pump will actually deliver in your system, as opposed to its maximum figures.
What is the difference between head and flow?
Flow is the volume of water moved, measured in litres per minute. Head is the height the pump works against, measured in metres, including both vertical lift and friction losses. The two trade off against each other along the curve.
Is pump head the same as pressure?
Yes. Head is a way of expressing pressure as a height of water. To convert, use roughly 10 metres of head to 1 bar, or more precisely 1 bar to about 10.2 metres. So a pump rated for 30 metres of head produces about 3 bar, and 1 bar is also about 14.5 psi.
What is the middle-third rule?
It means specifying a pump so its normal operating point sits in the middle third of its performance curve. This is the most efficient region and the one that puts the least stress on the pump, giving the longest service life.
Is a bigger pump always better?
No. An oversized pump wastes energy, wears faster and can run off the end of its curve. An undersized pump cannot keep up and runs hot. The right pump is the one whose duty point lands in the middle third for your specific job.
How do I work out my total head?
Add the vertical lift, the height from the water level to the discharge point, to the friction losses in your pipe, hose and fittings. Our friction-loss calculator estimates the friction part for you, and the total is the figure to read against the curve.
Can you help me read a curve for my project?
Yes. Call us on 0115 987 0358 with your flow, head, distance and what you are pumping, and we will size a pump for you from its curve.
