How Much Water?
Measure inside, in centimetres, then divide by 1000. 1 litre = 1000 cm³ exactly, and 1 m³ = 1000 litres = 1 tonne. A bath is about 80 L in use, an Olympic pool 2.5 million litres, and a person in the UK uses roughly 140 L a day.
Everyday water capacities
| Item | Litres | US gallons | Note |
|---|---|---|---|
| Kettle | 1.7 | 0.45 | Typical full capacity |
| Washing-up bowl | 10 | 2.6 | — |
| Bucket | 10–15 | 2.6–4.0 | Standard household |
| Toilet flush | 4–6 | 1.1–1.6 | Dual flush, modern |
| Toilet flush (pre-1990s) | 9–13 | 2.4–3.4 | Why replacing pays |
| Jerrycan | 20 | 5.3 | Weighs 20 kg full |
| Dishwasher cycle | 10–15 | 2.6–4.0 | Eco cycles use less |
| Washing machine cycle | 50 | 13.2 | Varies widely by model |
| Shower, 8 L/min × 8 min | 64 | 16.9 | Mixer shower |
| Bath, in use | 80 | 21.1 | Not brim full |
| Power shower, 12 L/min × 8 min | 96 | 25.4 | More than a bath |
| Bath, brim full | 150–200 | 40–53 | Nobody fills it this far |
| Hot water cylinder | 120–300 | 32–79 | 210 L is common |
The power shower row deserves attention: at 12 L/min, eight minutes uses more than a bath. The common assumption that showers always beat baths depends entirely on flow rate and duration.
| Item | Litres | Note |
|---|---|---|
| Watering can | 9 | — |
| Water butt | 100–250 | 210 L typical |
| Aquarium 120 × 50 × 50 cm | 300 | Brim full — see below |
| Hot tub | 1,000–1,800 | — |
| IBC tote | 1,000 | = 1 m³ = 1 tonne |
| Domestic rainwater tank | 1,500–5,000 | Below ground |
| Small garden pond 3 × 2 × 0.5 m | 3,000 | — |
| Domestic pool 8 × 4 × 1.5 m | 48,000 | — |
| Road tanker | ~30,000 | — |
| Olympic pool 50 × 25 × 2 m | 2,500,000 | 2,500 tonnes |
Working out anything not on the list
Three steps, and the first is the one people skip.
- Measure the inside. Capacity depends on internal dimensions — the walls hold no water. On a thick-walled tank the difference is substantial.
- Work in centimetres. Then the conversion is a decimal shift, because 1000 cm³ = 1 L exactly.
- Apply the shape formula, then ÷ 1000.
| Shape | Volume | Example → litres |
|---|---|---|
| Rectangular tank | l × w × d | 100 × 50 × 60 cm = 300 L |
| Cylindrical tank (upright) | πr²h | r 30, h 100 cm = 282.7 L |
| Cylindrical (on its side, part full) | circular segment | Not proportional to depth |
| Pond with sloping sides | average depth × area | Measure depth in several spots |
| Pool with a deep end | average depth × area | (shallow + deep) ÷ 2 |
Everything loses freeboard. Nothing is filled to the brim. An aquarium loses several centimetres at the top, plus the volume taken by substrate and decor — subtract 10–15% from the brim-full figure for a realistic dosing volume. A 300 L tank on paper is more like 255–270 L of actual water, which matters when you are adding treatments by the litre.
What it weighs
The metric system was built around this, so the numbers are unusually friendly:
| Volume | Weight |
|---|---|
| 1 litre | 1 kg |
| 10 litres | 10 kg |
| 1 m³ (1000 L) | 1 tonne |
| 1 US gallon | 3.79 kg (8.34 lb) |
| 1 UK gallon | 4.55 kg (10.02 lb) |
This is not quite exact — density peaks at 4 °C and falls as water warms — but for everyday purposes a litre is a kilogram.
It matters more than it sounds. A 300 L aquarium weighs over 300 kg once gravel and decor are added, concentrated on a small footprint. A full IBC tote is a tonne. A 210 L hot water cylinder puts 210 kg on the floor it stands on. Check the structure before installing any of them upstairs.
Daily use
| Measure | UK | US |
|---|---|---|
| Per person per day | ~140 L | ~300 L |
| Family of four, per day | ~560 L | ~1,200 L |
| Family of four, per year | ~205 m³ | ~438 m³ |
Water bills in many countries are charged per cubic metre — which is why the 1 m³ = 1000 L identity is the single most useful conversion on this page. A family of four in the UK gets through roughly 205 cubic metres a year, and each of those is a tonne of water.
Collecting rainwater
One beautifully simple rule: 1 mm of rain on 1 m² = 1 litre.
litres = roof footprint (m²) × rainfall (mm)
| Roof footprint | 10 mm rain | Per year at 800 mm |
|---|---|---|
| 20 m² (shed/garage) | 200 L | 16,000 L |
| 50 m² (small house) | 500 L | 40,000 L |
| 100 m² (typical house) | 1,000 L | 80,000 L |
Use the footprint — the plan area the roof covers — not the sloped surface area. Rain falls vertically, so the pitch does not increase the catchment.
Deduct 10–20% for splash, evaporation, gutter overshoot and first-flush diversion. Even so, a modest roof fills a 210 L water butt in a single decent shower, which is why one butt is almost never enough.
Common mistakes
| Mistake | Effect | Fix |
|---|---|---|
| Measuring outside dimensions | Overstates capacity | Capacity is internal |
| Filling to the brim in the calculation | Overstates by 10–15% | Subtract freeboard |
| Assuming a shower always beats a bath | Depends on flow and time | A power shower can exceed a bath |
| Using roof slope area for rainwater | Overstates yield | Use the footprint |
| Confusing US and UK gallons | 20% out | UK gallon is 4.546 L, US 3.785 L |
| Forgetting the weight | Floors overloaded | 1 L = 1 kg; 1 m³ = 1 tonne |
| Treating a side-on cylinder as proportional | Wrong at every level but half | It is a circular segment |
Frequently asked questions
How much water does a bath hold?
About 80 litres filled to a comfortable level, though the tub itself might hold 150–200 L brim full. Nobody fills to the rim, and a person displaces water on getting in. A shower at 8 L/min matches a bath at around ten minutes.
How much water is in an Olympic swimming pool?
50 m × 25 m × 2 m = 2,500 m³ = 2.5 million litres — the figure journalists use as a unit of comparison. At that size it weighs 2,500 tonnes, since a cubic metre of water is a tonne.
How do I calculate how much water a tank holds?
Measure the internal dimensions and apply the shape formula. Rectangular: l × w × d in cm, ÷ 1000 for litres. Cylindrical: πr²h. Always measure inside rather than outside — the walls hold no water.
How much does water weigh?
One litre is almost exactly 1 kg, and one cubic metre is 1 tonne. Not perfectly exact — density peaks at 4 °C — but close enough for everyday use. This matters for floors: a 300 L aquarium weighs over 300 kg once gravel and decor are added.
How much water does a person use per day?
In the UK, around 140 litres per person per day, so a family of four uses roughly 560 L daily or over 200 m³ a year. Showers, toilets and washing machines account for most of it. US figures are considerably higher, around 300 L per person per day.
How many litres in a cubic metre of water?
Exactly 1000, because a litre is defined as one cubic decimetre and there are 1000 of those in a cubic metre. A cubic metre is 1000 litres and weighs 1 tonne — which is why water bills in many countries are charged per cubic metre.
How much water does an aquarium hold?
Multiply internal length × width × height in cm and divide by 1000. A tank 120 × 50 × 50 cm is 300 L brim full — but the actual water volume is lower because of freeboard plus substrate and decor. Subtract 10–15% for a realistic dosing figure.
How much rainwater can I collect from my roof?
1 mm of rain on 1 m² = 1 litre. So a 50 m² roof in 10 mm of rain yields 500 litres before losses. Use the roof footprint, not the sloped area — rain falls vertically. Allow 10–20% lost to splash, evaporation and first flush.