Flow Rate and Water Pressure,

What's The Difference?

Having the great privilege of working with a team of talented and very knowledgeable engineers, I can safely say I have absolutely no idea how toilets work, why your bath is leaking or why radiators need to be bled. I still giggle when a customer tells me they need one of our engineers to come and give them a new ballcock, in short, I know a shamefully small amount for someone who works for a heating, plumbing and electrical company.

That’s why I’ve decided to ask and answer questions that stump me when they come up, putting them into layman’s terms so people like me, who have no plumbing/heating/electrical knowledge whatsoever, can understand them.

‘Flow Rate’ and ‘Water/Mains Pressure’ are terms that you might have heard used by someone you know with plumbing expertise; you might even be guilty of using them interchangeably under the assumption that they pretty much mean the same thing. There is a difference however and I, the woman who kept flushing cistern blocks for a full 3 months, am about to explain it to you 😉

Flow Rate vs. Water Pressure

Flow Rate = how much water is leaving your tap, in the UK we measure this in Litres Per Minute (l/min), the ideal flow rate for an average UK home is 10-15 litres per minute. The flow of the water is determined by the size of your pipes, the wider your pipes are the more water can run through them, narrower pipes will supply water at a lower flow rate. If you have a low flow rate then it could be caused by low water/mains pressure or it might mean you need bigger pipes.

Water/Mains Pressure = the pressure that pushes the water through your pipes, we measure this in units of pressure called bars. For residential plumbing it is best to be within a range of 3-4 bars, anything over 5.5 bars can damage your plumbing. Low flow rate doesn’t necessarily mean low water pressure, but low water pressure always causes a low flow rate.

Measuring Flow Rate

There are gadgets you can buy online to do this, but if you’re old school or just can’t justify spending the money, you can use a bucket and a timer to measure your flow rate by doing the following:

  • Find a water container/jug/bucket  (preferably 1 or 2 litres in size)
  • Find a stopwatch (or stopwatch setting on your phone)
  • Place your container under your bathroom tap or shower and turn it on
  • Time 6 seconds on your stopwatch & turn your tap or shower off.
  • Take the amount of water in the jug in litres (e.g. 0.8 litres) and multiply this by 10. This will give you your flow rate in litres per minute (e.g. 0.8 litres x 10 = 8 litres per minute). If your flow rate is less than 10 litres per minute then its time to ring a plumber.


Measuring Water Pressure

or this you’re going to need a water pressure gauge:

  • Turn off all taps inside and outside of your home
  • Fasten the pressure gauge to your outside tap (or the tap at your kitchen sink if you don’t have an outdoor tap)
  • Turn the tap on full (it has to be turned all the way up for an accurate reading)
  • Check the gauge, you want between 3-4 bars.

Anything above 5.5 bars can damage your plumbing and should be checked by a plumber, especially if accompanied by banging pipes, dripping taps, running toilets or if you’re running out of hot water quickly.

Anything below 3 bars means you have low water pressure and this could be a symptom of a bigger issue like a leak or an obstruction and should be checked by a plumber.

Common Reasons for Low Water Flow & Pressure

Have you’ve ever wondered why your shower which once had a steady stream is now a disappointing lukewarm drizzle? Or have you turned a tap on all the way to find that it couldn’t wet your toothbrush? Chances are that your low pressure/flow rate issues are due to one of the following issues:

Narrow Pipes

This will affect your flow rate more so than your water pressure. The UK is home to a plethora of old buildings, many of them date back long before indoor plumbing was a thing, some aren’t as ancient as that; take post war houses for example. In the post war years the government built houses fast and cheap to cope with a growing population, these houses are around 70 years old now and the pipes from that era are much narrower than in modern homes. Pipes need to be changed every 80-100 years depending on the material used so if you replace yours, you’ll only ever have to do it once. Wider pipes = more water in your pipes = a higher flow rate.



How many showers are there in your home?

Are you using them both at once?

Is someone washing the dishes while you shower?

Did someone flush the toilet?

These are the questions you should ask yourself before calling a plumber, water pressure can be affected by the number of people using it at one time and it should pass. 

If it doesn’t then you can start to worry.


Blocked Pipes

Pipes can be blocked by limescale, rust, cooking oil, dirt or sometimes in the winter they can freeze. 

If this happens you will need a plumber to come and flush your pipes.

Clogged Appliances

Check your taps and showerheads for limescale, sometimes it builds up to the point where it can clog the outlet. 

You can buy descaler to take care of this problem or a mix of bicarb soda and vinegar will sort it out.

Obstructed Stop Cock

The Stopcock is the control tap for your mains water, it looks like a tap but without an outlet spout and it connects two lengths of pipe, this allows the stopcock to block the flow of water when it’s closed off.

If you have a leak or a burst tap the Stopcock is what you will need to find as soon as you can. Sometimes they can become obstructed by debris like rust or limescale and that can affect your water flow/pressure.

Photo from MyBuilder.com

How to Increase Flow Rate

  • Keep your pipes free of obstructions – flush your pipes semi-regularly

  • Check the water pressure – If your flow rate is low and it hasn’t been caused by low mains pressure then it’s probably time to …

  • Replace your pipes – if you’re living in an old house, they might be due to be replaced anyway

How to Increase Water Pressure

  • Replace your showerhead – even if the outside looks clean you might find there’s a lot of limescale built up inside. A new hose and unclogged showerhead can increase your shower pressure.
  • Fit a shower pump – a shower pump increases the amount of water flowing out of your showerhead, if you have narrow pipes and don’t want to replace them this is a good option.
  • Install a pump – this will increase your water pressure throughout your home and reduce flow problems

So now we’ve covered the difference between Flow Rate and Water Pressure, and I’ve become slightly more competent at my job, hopefully you’ve learned something new too. Go Team! 😊

I think I’ve earned a shameless plug….

If you have trouble with your Flow Rate or Water Pressure call SC Duncan today on 014389 64433

Quotes are free and our professional team of engineers will be happy to discuss your best options for work you’re looking to have done.

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