What is the difference between water pressure and flow rate?
Two different measurements that people use interchangeably and shouldn't — and why almost every disappointing shower is a flow problem being treated as a pressure problem.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 3 min
Short answer
Pressure is how hard the water pushes, measured in bar. Flow rate is how much arrives, measured in litres per minute. A house can have high pressure and poor flow, which is what a narrow old supply pipe gives you. Most shower and boiler disappointments are flow problems, and no pump or booster fixes a flow problem.
The distinction sounds academic until you spend money on the wrong solution. Shower manufacturers quote a minimum pressure, combi boilers quote a flow rate, plumbers ask about both, and a householder describing a 'low pressure' problem is usually describing something else. Getting the two straight takes two minutes and saves a great deal.
Step by step
- Understand pressure.The force the water pushes with, measured in bar. It is set by the water company's mains and by the height of the reservoir or tower serving your area — or on a tank-fed system, by the height of the loft tank above the tap. It is roughly the same whether one tap is open or ten.
- Understand flow rate.The volume actually delivered in a given time, measured in litres per minute. It is set by pressure and by every restriction between the main and the tap: pipe diameter, pipe length, bends, valves, filters, scale, and how many outlets are running.
- See how one can be high and the other low.A house on a good main can have plenty of pressure but a long, narrow, part-blocked lead or old copper supply pipe. Pressure at the tap when nothing is running looks fine; open the tap and the flow is poor because the pipe cannot deliver volume. That is the classic UK case.
- Know which one your appliances care about.An electric shower cares about pressure to operate its valve, but its performance is flow-limited. A combi boiler's hot water performance is entirely a flow-rate question. A mixer shower needs both. A thermostatic shower valve needs balanced pressure between hot and cold more than it needs a lot of either.
- Test them separately.Flow rate is a jug and a stopwatch at the kitchen cold tap. Pressure needs a gauge on an outside tap or washing machine valve, though most householders never need this — if the flow is adequate, the pressure is.
- Diagnose from which one is short.Adequate flow but weak feel usually means low pressure — common on tank-fed upstairs outlets. Good pressure at rest but disappointing volume means a flow restriction. Both poor at every tap suggests the incoming supply or a partly closed stopcock.
- Match the fix to the fault.A pump helps a low-pressure tank-fed system. Nothing pumps a mains supply that cannot deliver volume — the answer there is an accumulator or a break tank and pump, or upgrading the supply pipe. Spending on the wrong one is the expensive mistake.
Tips
- Water companies have a duty to supply a minimum pressure at the boundary, not at your tap. If pressure at the boundary is fine, everything after that is your pipework.
- A 15mm supply pipe from the street was normal in older houses and is a real bottleneck today. Modern practice is larger, and upgrading the buried supply pipe is often the only thing that transforms a house.
- Simultaneous demand exposes flow limits, not pressure limits — which is why the shower drops when someone flushes.
- In a hard water area, decades of scale narrows the bore of old pipework and reduces flow while pressure stays the same.
Questions people ask
Which figure does a shower manufacturer mean?
Both, and they are given separately. The minimum pressure is what the valve needs to operate; the flow rate at that pressure is what you will actually get out of the head. Reading only the pressure figure is how people buy showers that disappoint.
Can I increase my mains pressure?
Not directly and not lawfully — pumping straight off the main is prohibited because it draws down the supply to your neighbours. The lawful routes are an accumulator, or a break tank with a pump.
Why is my pressure fine but the bath takes forever?
Classic flow restriction. Look at the supply pipe size, the bath tap size, and whether the stopcock or an isolating valve is only part open.