Water pressure is the force pushing water through your pipes, while water flow is how much water actually comes out of a tap or shower in a set time. You can have good pressure but poor flow if something narrows the pipe or outlet, and the two are often confused when people describe a weak tap or shower.
Knowing the difference helps you describe the problem clearly and points towards the likely cause.
What Pressure Means
Pressure is often measured in bar. It is the push behind the water, created by the water company’s network and, in some homes, by the height of a storage tank or a pump.
The consumer body CCW explains that the water industry’s Guaranteed Standards Scheme refers to a pressure of 7 metres head, or 0.7 bar, in the pipe, normally measured at the outside stop valve. Pressure inside your home can be quite different, depending on your pipework.
What Flow Means
Flow is usually measured in litres per minute. It is what you notice when you fill a kettle or stand under a shower.
Flow depends on pressure, but also on the size, length and condition of the pipes and fittings in between. A narrow or partly blocked pipe, a partly closed valve, a scaled-up aerator or a long pipe run can all reduce flow even when the pressure is fine.
A simple way to get a rough idea of flow is to time how long it takes to fill a container of known size, such as a bucket marked in litres, from the cold kitchen tap.
How They Show Up at Home
If a tap spurts strongly when first opened but then dwindles to a weak stream, the pressure may be fine but the flow is being limited somewhere. If every outlet in the home is weak from the moment it opens, low pressure on the incoming supply is more likely, though not certain.
Flow can also drop when several outlets are used at once. A shower may weaken when the washing machine fills, simply because the pipes cannot deliver enough water to both.
Gravity-fed systems, where water comes from a tank in the loft, often have lower pressure at upstairs outlets than mains-fed taps downstairs. That can be normal for the system rather than a fault.
Temperature can play a part too. A mixer shower may feel weak on hot because the hot supply has lower pressure than the cold, even though both pipes are working normally.
Why the Difference Matters
Treating a flow problem as a pressure problem can lead to the wrong fix, such as fitting a pump when a partly closed valve was the real cause. A simple check of several outlets, at different times of day, can help show which you are dealing with.
If you are not sure whether you have a pressure or a flow problem, our water pressure problems service in Oxford can test what is reaching your property and what is getting through to each outlet.