Quick answer: measure in centimetres, apply the shape formula, then divide cm³ by 1,000. A 40 × 30 × 25 cm box is 30,000 cm³ = 30 litres. From metres, multiply m³ by 1,000. And if it is water, 1 litre weighs 1 kilogram.
Why litres are the easy unit
Metric volume was designed so that the conversions disappear. A litre is defined as a cube 10 cm on each side, which produces three identities worth committing to memory:
and for water: 1 L = 1 kg · 1 m³ = 1 tonne
The practical consequence is that if you measure a container in centimetres, the cubic centimetres you calculate are already millilitres — there is no conversion step at all. Compare that with the imperial route, where a gallon is 231 cubic inches for historical reasons and a cubic foot is 1,728 cubic inches, and you have two unrelated numbers to remember before you start.
Litres conversion table
| Litres | cm³ / mL | m³ | US gallons | Imperial gallons | ft³ |
|---|---|---|---|---|---|
| 1 L | 1,000 | 0.001 | 0.264 | 0.220 | 0.0353 |
| 5 L | 5,000 | 0.005 | 1.32 | 1.10 | 0.177 |
| 10 L | 10,000 | 0.010 | 2.64 | 2.20 | 0.353 |
| 25 L | 25,000 | 0.025 | 6.60 | 5.50 | 0.883 |
| 50 L | 50,000 | 0.050 | 13.21 | 11.00 | 1.77 |
| 100 L | 100,000 | 0.100 | 26.42 | 22.00 | 3.53 |
| 200 L | 200,000 | 0.200 | 52.83 | 44.00 | 7.06 |
| 500 L | 500,000 | 0.500 | 132.09 | 109.98 | 17.66 |
| 1,000 L | 1,000,000 | 1.000 | 264.17 | 219.97 | 35.31 |
That last row is the one to anchor on: 1,000 litres = 1 cubic metre, and if it is water it weighs one tonne. IBC totes are built around exactly this — a "1,000 litre" tote is one cubic metre of usable capacity.
Everyday capacities in litres
| Container | Litres | Water weight |
|---|---|---|
| Kettle | 1.5–1.7 | 1.5–1.7 kg |
| Bucket | 10–12 | 10–12 kg |
| Watering can | 5–10 | 5–10 kg |
| Car fuel tank | 45–60 | — |
| Bathtub | 150–250 | 150–250 kg |
| Oil drum (standard) | 200–208 | 200–208 kg |
| Water heater | 150–300 | 150–300 kg |
| IBC tote | 1,000 | 1,000 kg (1 tonne) |
| Small garden pond | 1,000–5,000 | 1–5 tonnes |
| Above-ground pool (4.5 m round) | ≈ 17,000 | 17 tonnes |
The water-weight column is not decoration. A 250-litre bathtub puts a quarter of a tonne on the floor, and a modest 5,000-litre pond is five tonnes sitting on the ground — which is why pond liners and tank bases need proper support.
Worked examples
A rectangular water tank
A tank 120 cm long, 60 cm wide, filled to 80 cm: 120 × 60 × 80 = 576,000 cm³ ÷ 1,000 = 576 litres, weighing 576 kg when full.
A cylindrical drum
A drum 57 cm across and 85 cm tall: π × 28.5² × 85 = 216,900 cm³ ≈ 217 litres — which is why the standard oil drum is described as 200 litres, leaving headroom.
A round pond
A pond 3 m across with an average depth of 60 cm: π × 1.5² × 0.6 = 4.24 m³ = 4,240 litres, or 4.24 tonnes of water.
Common mistakes & pro tips
- Dividing cm³ by 100 instead of 1,000. A litre is a 10 cm cube, so the divisor is 10³.
- Mixing centimetres and metres. Convert to one unit first, or use the per-field unit selectors above.
- Measuring the container height rather than the fill depth. Nothing is filled to the brim.
- Using outside dimensions. Capacity is what fits inside; poly tank walls are thick enough to matter.
- Pro tip — measure in centimetres. The cubic centimetres are already millilitres. It removes an entire conversion step and the error that comes with it.
Frequently asked questions
How do I calculate volume in litres?
Measure in centimetres, apply the shape's formula, then divide the cubic centimetres by 1,000. A box 40 × 30 × 25 cm is 30,000 cm³, which is 30 litres. If you measure in metres instead, multiply cubic metres by 1,000.
How many cm³ are in a litre?
Exactly 1,000. A litre is defined as the volume of a cube 10 cm on each side, so 10 × 10 × 10 = 1,000 cm³. It also follows that one millilitre is exactly one cubic centimetre, which is why the two units are used interchangeably.
How do I calculate the volume of a cylinder in litres?
Use V = π × (diameter ÷ 2)² × height with all measurements in centimetres, then divide by 1,000. A cylinder 30 cm across and 50 cm tall holds π × 225 × 50 = 35,343 cm³, or about 35.3 litres.
How many litres are in a cubic metre?
Exactly 1,000. A cubic metre of water also weighs one tonne, which makes metric volume and weight calculations unusually simple — one litre of water is one kilogram.
How do I convert litres to gallons?
Divide by 3.785 for US gallons or by 4.546 for imperial gallons. A 200-litre drum is about 52.8 US gallons or 44.0 imperial gallons — a reminder that the two gallons differ by roughly 20 percent.