Head and pressure describe the same thing in two different units of measurement.
Pump engineers talk in metres of head, where as taps, hoses and appliances are rated in bar or psi.
Being able to translate between them is one of the most useful skills in choosing and understanding a pump, and it is simpler than it looks once you have the key number. This guide gives you that number, the conversion tables in every common unit, and the part most explanations skip: why the pressure a pump gives you changes depending on how much water is flowing.
10 metres of head equals roughly 1 bar, which is about 14.5 psi or 100 kPa.
Head and pressure are the same physical thing measured differently: head is the height of a column of water, and pressure is the force that column exerts. The conversion holds for water at any point in a system.
Why height and pressure are the same thing
Imagine a tall pipe full of water. The water at the bottom is under pressure simply because of the weight of all the water stacked above it.
The taller the column, the greater that pressure. That is all head is: the height of a column of water, used as a way of expressing pressure. A pump that can raise water 30 metres is producing enough pressure to support a 30 metre column, whether or not the water actually goes up a pipe. This is why a pump's ability is quoted in metres, and why that figure converts cleanly into bar or psi. If you want the companion piece on what head is made of, the lift plus the friction, see our guide to what pump head, total head and friction loss are.
The conversions, in every common unit
| Head (metres) | Head (feet) | Bar | psi | kPa |
|---|---|---|---|---|
| 1 m | 3.3 ft | 0.098 bar | 1.42 psi | 9.8 kPa |
| 5 m | 16.4 ft | 0.49 bar | 7.1 psi | 49 kPa |
| 10 m | 32.8 ft | 0.98 bar | 14.2 psi | 98 kPa |
| 20 m | 65.6 ft | 1.96 bar | 28.5 psi | 196 kPa |
| 30 m | 98.4 ft | 2.94 bar | 42.7 psi | 294 kPa |
| 50 m | 164 ft | 4.9 bar | 71.1 psi | 490 kPa |
And the other way round, starting from pressure:
| Pressure | Equivalent head | Also equals |
|---|---|---|
| 1 bar | 10.2 m (33.5 ft) | 14.5 psi, 100 kPa |
| 1 psi | 0.70 m (2.31 ft) | 0.069 bar, 6.9 kPa |
| 1 kPa | 0.102 m (0.33 ft) | 0.01 bar, 0.145 psi |
For everyday use, the rounded rule is all you need: 10 metres equals 1 bar equals about 14.5 psi equals 100 kPa.
The precise factor is that 1 bar supports 10.2 metres of water, but 10 metres to the bar is close enough for sizing a pump.
Static pressure: the reading with no flow
Here is where most explanations stop and where the interesting part begins. The pressure a pump or supply gives you is not a single fixed number, it depends on whether the water is moving.
Static pressure is the pressure when nothing is flowing, when every tap is shut. With no water moving, there is no friction, so you see the full pressure the system can hold. It is the highest pressure reading you will get, and it is what a gauge shows on a closed system.
Dynamic pressure: what you get when water moves
Dynamic pressure is the pressure while water is actually flowing, and it is always lower than the static pressure.
The moment water moves, friction in the pipes and fittings starts using up pressure, and some of the pressure is also converted into the energy of the moving water itself. The faster the flow, the more pressure is lost this way. This is why a tap that reads a healthy static pressure can still feel weak when several outlets are open at once: the flow has pulled the dynamic pressure down.
Why pressure changes with flow: the pump curve
A pump behaves the same way, and its performance curve is really a map of this relationship. At zero flow the pump produces its maximum pressure. As you open up and let more water through, the pressure it can hold falls, because more of its energy goes into moving the water and overcoming friction. Draw that out and you get the familiar curve of pressure, or head, falling as flow rises. It is the single most important thing to understand about any pump, and we cover reading it in pump curves explained and how to read a pump curve.
Shut-off head: the top of the curve
The very top of that curve has a name: the shut-off head.
It is the maximum head, and therefore the maximum pressure, a pump can produce, and it happens at zero flow.
At shut-off the pump is holding water as high as it possibly can but moving none of it. It is a useful number because it tells you the absolute ceiling of what a pump can push against, but it is not a working figure: you never run a pump at shut-off, because no water is moving.
The useful performance is always somewhere down the curve, at a real flow, which is why a pump quoted with an impressive maximum head still has to be checked at the flow you actually need. We explain why that headline figure flatters in why max flow is not your real flow.
Pump efficiency and longevity
Where a pump operates on its performance curve is known as its duty point. For the best efficiency and longest service life, the duty point should ideally be near the middle of the pump curve and close to the pump’s best efficiency point. Here, the pump moves water smoothly and efficiently, with less vibration, heat and mechanical stress on its internal components.
It is similar to a car cruising steadily on a motorway at comfortable revs, rather than labouring in too high a gear or being driven continuously near its limit. A healthy flow of water through the pump also helps carry heat away and keep it operating at a suitable temperature.
What real appliances and fittings need
Converting head to pressure becomes concrete when you look at what everyday things actually require. These are typical figures, and they show why the same supply feels strong for one job and weak for another.
| Use | Typical pressure | Roughly, in head |
|---|---|---|
| UK mains supply (typical) | 1 to 4 bar | 10 to 40 m |
| Water company minimum at the boundary | about 0.7 bar | 7 m |
| Garden hose and nozzle | around 1 to 2 bar | 10 to 20 m |
| Garden sprinkler (typical) | around 2 to 3 bar | 20 to 30 m |
| Domestic pressure washer | 100 to 150 bar | over 1,000 m equivalent |
The pressure washer line shows why they are a different class of machine: they produce pressure many times that of a mains supply, which is why you cannot get a pressure-washer jet simply by connecting a hose to the tap.
A hose nozzle, by contrast, does not add pressure at all, it just restricts the flow so what pressure you have shows up as a faster jet.
The real world
The important point is not simply how much pressure a pump can produce at shut-off. It is how much head and flow it can deliver at the same time, at the duty point your system requires.
Our Water Pump Performance Calculator works this out for you. Enter the pump, pipe or hose size, pipe length and vertical lift, and it will estimate the friction loss and show where the pump is likely to operate on its curve.
That lets you compare pumps using the performance you will actually receive, rather than relying on the maximum flow or maximum head printed at opposite ends of the curve.
You can also browse our full range of water pumps. Each product page includes performance information to help you assess the available flow and pressure,
