The formulas
Rectangular: Volume = Length × Width × Height
Cylinder: Volume = π × (Diameter ÷ 2)² × Length
1,000 litres = 1 m³ = 264.2 US gallons = 220 UK gallons
Worked example
A rectangular underslung tank 80 × 40 × 30 cm holds 96 litres (25 US gallons). At 15 L a day that's about 6 days of water.
Planning tips
- Water weighs 1 kg per litre (8.3 lb per US gallon). A full 100 L tank adds 100 kg to your payload.
- Usable volume is often a little less than the calculated volume because of fittings and the outlet height.
- Inboard tanks stay above freezing more easily than underslung tanks in winter.
Frequently asked questions
How much water does a van lifer use per day?
Typically 10–20 L (3–5 US gal) per person for drinking, cooking and washing up; 30–40 L or more with daily showers.
How do I calculate litres in a tank?
Measure inside dimensions in cm, multiply them together, and divide by 1,000.