Water flow rate:
time a bucket and turn it into L/min

Flow rate is volume over time

Flow rate answers how much water passes a point in a given time. It is the quantity that sizes a pump, tells you whether a tap will fill a tank overnight and decides whether a shower runs hot. All you need are two numbers that can be measured with a bucket and a watch.

A bigger sample reduces timing error. Collecting 120 L is a better test than filling a one-litre jug, because a one-second slip in the stopwatch matters less over four minutes.

Running a bucket test properly

The bucket test measures what a tap, hose or outlet delivers under its normal operating conditions. It is cheap and quite accurate if you take care with the details.

1. 1Choose a container of known size, such as a marked 10 or 20 litre bucket.
2. 2Open the outlet fully and let it run for a moment to clear air.
3. 3Start the stopwatch the instant the water enters the bucket and stop when it reaches the mark.
4. 4Repeat two or three times and average the results.
5. 5Divide litres by seconds, then multiply by 60 for litres per minute.

Both examples give the same 30 L/min, which shows that the answer depends on the rate and not on the size of the sample, as long as the flow is steady.

Using a water meter instead of a bucket

A household meter gives the same information without a bucket. Note the reading, run one outlet for a set time while everything else is off, and note the reading again. The difference is the volume collected, and dividing by the minutes gives the rate.

Meters show the volume in cubic metres, so remember to convert 0.120 m³ into litres before dividing. Make sure every other tap, cistern and appliance is off, or the reading includes their use as well.

Converting between flow units

Different trades use different units. Plumbers talk in litres per minute, pump makers in litres per second or cubic metres per hour, and some manuals in gallons per minute. Convert once and write down the unit with the number.

The m³/h figure comes from 30 L/min × 60 min = 1,800 L per hour = 1.8 m³ per hour.

What the flow means inside the pipe

The same flow moves faster in a narrow pipe than in a wide one. Velocity equals flow divided by the cross-section area of the bore. At 0.5 L/s, which is 0.0005 m³/s, and a 25 mm inside diameter whose area is about 0.000491 m², the water moves at about 1.02 m/s.

Domestic pipes are commonly designed to keep velocity below a few metres per second to limit noise and wear, though the acceptable limit depends on the material and the system, so refer to the design guidance for your installation.

Using a measured flow to size a tank or pump

Once the flow is known, planning questions become simple divisions. How long will a header tank take to fill from this supply? Does the pump deliver more than the outlet can take? Will a small borehole keep up with the garden's irrigation?

Flow also helps check demand. If three outlets each draw 12 L/min at once, the supply must provide 36 L/min, which is more than the 30 L/min measured, so pressure will drop or one outlet will run short. That is why peak demand, not average demand, is the number that matters.

• Add the flows of all outlets that can run together.
• Compare the total with the measured supply, and with the pump rating if there is one.
• If demand is higher than supply, a storage tank evens out the peaks.
• Leave a margin, since pumps and filters wear and flow drops over time.

When the measured flow misleads

A single test shows the flow at one moment. Supply pressure changes with the time of day, other users on the system and the state of a storage tank, so a morning test can differ from an evening one. Flow also falls with distance from the source and rises when more outlets are closed.

Seasonal change matters too: in many places the mains pressure is lowest in the evening peak or in the hot season when irrigation demand is high. A test taken at that time shows what the equipment will really receive, and a design based on it will rarely disappoint. Record the date, the time and which other outlets were open, so that a repeat test can be compared fairly.

If you are sizing equipment, test at the worst-case time and note the conditions. A capacity-planning tool for engineering systems can help organise a larger sum, but the flow figure it needs still comes from a measured or specified source like the one above.