How water is measured in the UK
UK water is billed by the cubic metre, and one cubic metre is exactly 1,000 litres. That single conversion sits behind every figure on this site, because a leak is felt in litres per hour and charged in cubic metres per year.
Last reviewed 14 August 2026
What this section covers
The units every figure on this site is expressed in, how to convert between them, and the one distinction that causes most misreadings of a water meter.
- The cubic metre, which UK water is billed in
- The litre, which losses are felt in and meters show as fractions of a cubic metre
- The imperial gallon, which older meters and almost every published pool volume use
- The bar, which sealed heating system pressure gauges are marked in
- The difference between a rate and a volume, which is not a unit at all but decides whether a reading has been understood
None of this is difficult and all of it is definitional, which is exactly why it is worth writing down once and linking to rather than repeating on every page.
How it works in the UK
A UK household meets three of these units in three different places, and nothing on a bill or a meter explains how they relate.
| Unit | Where you meet it | What it is |
|---|---|---|
| Cubic metre | Your bill, and the black digits on your meter. | The SI unit of volume. A cube one metre on each side. |
| Litre | The red digits on your meter, and every leak figure anywhere. | Exactly one thousandth of a cubic metre. Listed in the SI Brochure's table of non-SI units. |
| Imperial gallon | Older meters, and nearly every published swimming pool volume. | Exactly 4.54609 litres, by UK weights and measures law. |
| Bar | The pressure gauge on a sealed central heating system. | A pressure, equal to 100,000 pascals. Not a volume, and not convertible into one. |
That last row is the one worth being careful about. A bar measures pressure, not quantity, so no arithmetic turns a pressure drop into a number of litres without knowing the system volume, the expansion vessel charge and the water temperature at both readings. That is why the heating calculator on this site refuses to give a volume from a pressure reading.
The figures that matter
Four conversions, and three of them are exact by definition rather than measured.
| From | To | Multiply by | Status |
|---|---|---|---|
| 1 cubic metre | litres | 1,000 | Exact. The litre is defined as one cubic decimetre. |
| 1 imperial gallon | litres | 4.54609 | Exact. Defined in UK weights and measures law. |
| 1 cubic metre | imperial gallons | 219.969 | Follows from the two above. Not a separate definition. |
| 1 litre | millilitres | 1,000 | Exact. Milli is a factor of one thousandth. |
Nothing in that table is an estimate, an average or a rule of thumb. Every row is either a definition or arithmetic on two definitions, which is why none of them carries a caveat and none of them will ever change.
The one thing that is not exact is the mass. A cubic metre of water weighs about a tonne, and "about" is doing real work there: water is densest near 4 degrees Celsius and slightly less dense warmer, so the figure moves by a fraction of a per cent across ordinary temperatures. It is a useful mental image rather than a conversion to calculate with.
How this changes your loss figure
Units do not change how much water you are losing. They change whether you can see it, and one of them changes whether you have understood it.
- Reading only the black digits on a meter makes a household leak invisible. A loss of 0.5 litres per hour moves them by 0.012 in a day, which is nothing you would notice.
- Mixing imperial and US gallons on a pool overstates or understates the volume by about a fifth, and every loss figure derived from it by the same proportion.
- Treating a meter reading as a rate rather than a volume is the error that changes an answer by a factor of the test duration, which can be any number at all.
That last one is the only genuinely conceptual mistake on this list, and it is common enough to have its own page. the difference between a rate and a volume sets out how to tell which one you are holding.
Assumptions and sources
Litres in a cubic metre
Exact by definition
SI definition. The litre is defined as one cubic decimetre, so one cubic metre is exactly 1,000 litres.
Litres in an imperial gallon
Exact by definition
UK legal definition. The imperial gallon is defined as exactly 4.54609 litres in United Kingdom weights and measures law.
Millilitres in a litre
Exact by definition
SI prefix definition. Milli is a factor of one thousandth.
Pascals in a bar
Exact by definition
Definition of the bar as exactly 100,000 pascals, that is 100 kilopascals.
The unit definitions come from the SI Brochure, the current edition of which was updated in June 2026, and from UK weights and measures law for the imperial gallon. The cubic metre is a coherent derived SI unit. The litre and the bar are listed in the Brochure's Table 8, which the June 2026 edition titles simply "Non-SI units".
That title is worth stating precisely, because nearly every secondary source still gives the older wording, "non-SI units accepted for use with the SI". The current edition dropped it. On a page whose entire job is defining units, quoting a superseded formulation would be exactly the wrong place to be approximate.
The only figure here that is not exact is the mass of a cubic metre of water, which varies slightly with temperature and is given as an approximation on purpose.
What to do next
Two things, and the first one takes ten seconds.
- Look at your own meter and identify which digits are cubic metres and which are fractions of one. Everything else on this site depends on reading both.
- If you are working with a pool volume in gallons, check whether it is imperial or US before converting. A figure quoted in the UK is almost always imperial, and almost always unlabelled.
With the units settled, the measurement itself is two readings and the time between them. take two readings over a measured interval is where that happens.
Common questions
- Why are bills in cubic metres when leaks are in litres?
- Because the two are describing different scales of the same thing. A cubic metre is a sensible billing unit for a household using hundreds a year, and a hopeless unit for a leak: a loss of 0.5 litres per hour moves the cubic metre digits on a meter by 0.012 in a day. The litre is what you can actually see change.
- How many litres in an imperial gallon?
- Exactly 4.54609, by UK law. That is the imperial gallon, not the US gallon of about 3.785 litres, and the two differ by about 20 per cent, which is enough to matter on a pool.
- Is the litre an SI unit?
- Not strictly. The cubic metre is the SI unit of volume, and it is a coherent derived unit. The litre appears in the SI Brochure's table of non-SI units, defined as exactly one cubic decimetre. In practice both are used freely and the distinction only matters when you are quoting a definition.
- What is a bar?
- A unit of pressure equal to 100,000 pascals, listed in the same table of non-SI units. It is what UK sealed heating system gauges are marked in, and on this site it is only ever read, never converted into a volume.
- What is the difference between a rate and a 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 to mean anything. Confusing the two is the most common way a meter reading is misinterpreted, because a meter shows a volume and a leak is described as a rate.