How do I measure the water flow rate at my taps?
The jug-and-stopwatch test that gives you the single number every combi boiler, shower and unvented cylinder specification is written around.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 3 min
Short answer
Fully open the kitchen cold tap with nothing else running, fill a container of known volume and time it. Litres divided by seconds, times sixty, gives litres per minute. Do it at the kitchen cold tap because that is fed directly from the main, and repeat at a busy time of day to see your realistic worst case.
Do this first
This is the most useful number in the house and almost nobody knows theirs. It decides which combi boiler will actually work for you, whether an unvented cylinder is viable, whether a mixer shower will be any good, and whether the answer to a weak shower is a new fitting or a new supply pipe. It takes five minutes and needs nothing you do not already own.
What you'll need
- A measuring jug or a bucket of known volume
- A stopwatch or phone timer
Step by step
- Pick the right tap.The kitchen cold tap, because on every system type it is fed directly from the incoming main. Any other cold tap on a vented system comes from the loft tank and measures the tank, not the supply.
- Turn everything else off.No washing machine, no dishwasher, no toilet refilling, nobody in the shower. You are measuring what the supply can deliver to one outlet.
- Open the tap fully.All the way, and check the isolating valve under the sink is fully open too — a part-closed service valve is a very common reason a measured flow comes out low.
- Fill a known volume and time it.A ten-litre bucket is ideal. Start the timer as the water starts and stop it when the bucket is full. If you use a two-litre jug, fill it several times and average, because the start and stop error matters more on a short run.
- Do the arithmetic.Litres collected divided by seconds taken, multiplied by sixty, gives litres per minute. Ten litres in forty seconds is fifteen litres per minute.
- Repeat at the worst time of day.Early evening in a residential street, when everyone is home. Mains flow varies with neighbourhood demand and the figure that matters for sizing a boiler is your realistic worst case, not your best.
- Test a shower outlet as well if that is the complaint.Same method, holding the head in the bucket, with the shower on full. Comparing that with the kitchen figure tells you whether the shortfall is in the supply or in the fitting and the pipework to it.
- Write it down and keep it.Every quotation for a boiler, an unvented cylinder or a shower will ask. Having a measured number rather than 'it seems alright' changes the conversation, and stops you being sold something the supply cannot feed.
Tips
- Take the figure to the boiler quotation, not the other way round. A combi rated well above what your main can feed will never deliver its headline performance.
- If you are considering an unvented cylinder, the flow rate is the gating question — below a certain figure the strong-shower promise does not materialise.
- Compare hot and cold flow at the same tap. On a combi the hot will be lower by design, because the boiler restricts it to get the temperature rise.
- Measure again after any supply pipe work — it is the only proof the money achieved anything.
If it doesn't work
Flow is much lower than expected at every tap
Cause: A part-closed stopcock, a scaled or undersized supply pipe, or a genuinely poor main — Fix: Check the stopcock is fully open first — it costs nothing and it is frequently the answer. If that is fine, the supply pipe or the main is the constraint and the water company can tell you the pressure at the boundary.
Kitchen tap is good, everywhere else poor
Cause: Internal pipework restriction, scale, or on a vented system those taps are tank-fed and measuring something different — Fix: Establish the system type first. On mains-fed pipework, look at pipe size and at isolating valves under each fitting.
Flow drops sharply after a few seconds
Cause: A partly blocked filter or aerator, or an isolating valve with a failing washer being pushed shut by the flow — Fix: Unscrew and clean the tap aerator first. If it recurs across several outlets, look at the main strainer or a service valve.
Good in the morning, poor in the evening
Cause: Neighbourhood demand on a marginal main — Fix: Nothing wrong with your pipework. Size any new appliance around the evening figure, and consider an accumulator if the shortfall is significant.