Water pressure is the force pushing water through the pipes, while flow is the amount of water that comes out of a tap or shower over a period of time. They are linked, but a home can have reasonable pressure and still have poor flow if something is restricting the pipes or fittings.
Understanding the difference can help explain why a tap seems weak and what may need checking. The two terms are often used as if they mean the same thing, which can make problems harder to describe.
Pressure: The Push Behind the Water
Pressure is the force in the water supply. It is affected by the water company’s network, the height of your property compared with the source, and in some homes by storage tanks or pumps.
WaterSafe says water companies are committed to a minimum standard of 7 metres static head, which it gives as 0.7 bar. This is a minimum standard rather than a typical figure, and what you see at your taps can depend on other factors.
Pressure can also change during the day. At busy times, when many homes are using water, it may be a little lower than at quieter times.
Flow: How Much Water Comes Out
Flow is about the amount of water you get. It can be reduced by things that restrict the water on its way to the tap, such as narrow or partly blocked pipes, a partly closed stopcock or valve, a blocked aerator or shower head, or long pipe runs.
This is why a home with good pressure can still have a weak shower or a tap that fills a kettle slowly. It is also why turning on several taps at once may make each one weaker.
Why the Difference Matters
If pressure is low across the whole property and neighbours are affected too, the cause may be in the public supply. If pressure is fine but flow is poor at certain taps, the cause is more likely to be within the property, such as a valve, fitting or section of pipework.
Some showers and appliances need a minimum pressure or flow to work properly, so the maker’s guidance may mention both. A problem with one does not always mean a problem with the other.
Getting It Checked
Note which taps are affected, whether hot, cold or both are weak, and whether it changes at certain times of day. Your water supplier may be able to check the pressure at the boundary if you think the public supply is the problem.
Our page on diagnosing water pressure problems explains how pressure and flow problems within a property are assessed.
If you are not sure whether your home has a pressure or a flow problem, a plumber can check the system and help narrow down the possible cause.