Flow rate and volume
A volume is an amount of water. A rate is an amount of water per unit of time, so it needs a duration attached before it means anything. A water meter shows a volume and a leak is described as a rate, and turning one into the other is the entire arithmetic of a meter test: divide the volume by the hours.
Last reviewed 14 August 2026
The short answer
One quantity has a time in it and the other does not, and that is the whole distinction.
| Volume | Rate | |
|---|---|---|
| What it is | An amount of water. | An amount of water per unit of time. |
| Units on this site | litres or cubic metres | L/hr or L/day |
| Where you meet it | A meter reading. A bill. | A leak description. A severity band. |
| How to get the other | Divide by the hours elapsed. | Multiply by the hours elapsed. |
A meter cannot show you a rate. It counts water, so it only ever shows a volume, and the rate is something you calculate from two of those readings and the time between them.
How this works in practice
The reason this is worth its own page is that the same meter movement produces completely different conclusions depending on a number that is not written on the meter.
| Test length | Rate | Which band |
|---|---|---|
| 1 hour | 4 L/hr | Small leak. |
| 2 hours | 2 L/hr | Small leak. |
| 8 hours | 0.5 L/hr | The bottom of the first band worth acting on. |
| 24 hours | 0.167 L/hr | Below what the meter reliably registers. |
One reading, four conclusions, and the only difference is a number the meter never told you. That is why writing down the time matters as much as writing down the digits, and why a test with an estimated duration carries an error straight into the rate.
It also explains why a longer test is a better one. The volume grows with the duration but the reading error does not, so dividing a larger volume by a larger number of hours produces a more reliable rate. take two readings over a measured interval is where the division happens.
What changes the answer
Three things, and they are all about the time rather than the water.
- An estimated duration. "About two hours" is not a measurement, and the rate is only as good as the worst of its two inputs.
- Water used during the test. It adds to the volume, so it inflates the rate, and one flush over a two hour test can move the answer more than the leak did.
- An intermittent loss. A rate is an average over the test, so a leak that runs for half the night and stops gives a rate that describes neither state accurately. Two shorter tests at different times tell you more than one long one.
None of those makes the arithmetic wrong. They make the inputs wrong, which is a different problem and a more common one.
Assumptions and sources
Hours in a day
Exact by definition
Definition of the day as 24 hours.
Test length below which a reading error matters
Editorial convention
A meter test is a difference between two readings, so a misread of one small division is a fixed error spread over the length of the test. The smallest division on a DN15 register is 0.02 litres, so a one division misread over two hours is 0.01 litres an hour, which is about 2 per cent of the 0.5 litres an hour band boundary. Over a twenty minute test the same misread is 0.06 litres an hour, which is more than a tenth of it. The two hours is still a chosen round number rather than a derived one, and it changes nothing in the arithmetic: it only decides when the tool adds a note.
Where the first severity band ends
Editorial convention
A practical threshold set just above the 0.4 litres per hour starting flow quoted for a DN15 meter with the TVM register. It is not a meter accuracy tolerance. Against the STANDARD it sits about 31 times below Q1, the 15.6 litres per hour at which OIML R 49-1 first defines any permitted error. Against THIS MANUFACTURER it sits about 6 times below 3 litres per hour, the lowest flow at which Itron volunteers a typical accuracy that the standard does not require. Neither figure reaches down to 0.5, so nothing quantifies a reading here on either footing. Meters with other registers start higher, so this threshold sits above the most sensitive case rather than above every meter.
Nothing on this page rests on a constant or a citation. That a rate is a volume divided by a time is a definition, and the table is that definition applied to one worked figure.
The severity bands the table refers to are this site's own practical classification rather than measured boundaries, and they are set out with that caveat wherever they are used.
What to do next
Two habits, and the first one costs nothing.
- Write the time down with every reading, to the minute. It is the input people estimate and it is exactly as important as the digits.
- Whenever you see a leak figure quoted anywhere, check whether it is a rate or a volume before comparing it to your own. A figure in litres and a figure in litres per hour are not comparable, and they are printed identically.
Common questions
- My meter moved by 4. Is that a rate?
- No, it is a volume, and on its own it is not yet an answer. 4 litres over two hours is 2 litres per hour. The same 4 litres over eight hours is 0.5. Those land in different severity bands, from the same reading, purely because of the time.
- Why does this matter more than the units?
- Because a units mistake has a fixed size and this one does not. Confusing litres with cubic metres is always a factor of a thousand, which is large enough to notice. Confusing a rate with a volume is a factor of however long your test ran, which could be anything, so the answer does not look obviously wrong.
- Which one does my supplier bill?
- A volume. Bills count cubic metres used over a billing period, with no rate anywhere on them. The rate is a diagnostic quantity, useful for judging severity and for comparing one test against another, and it never appears on a bill.
- Do I need both?
- Yes, for different things. The rate tells you how bad it is and whether it is worth acting on, because the severity bands are rates. The volume tells you what it costs, because charging is by volume. A complete answer has both, which is why every result on this site reports both.