Rectangular diesel tank capacity
Rectangular storage tanks are the easiest to calculate. Multiply internal length, width and height. If the dimensions are centimetres, divide by 1,000 for litres.
Example: 200 × 100 × 50 cm fuel tank
200 × 100 × 50 = 1,000,000 cm³. Divide by 1,000 and the theoretical internal capacity is 1,000 litres. If you only fill to 45 cm depth, the liquid volume becomes 900 litres.
Cylindrical fuel tank capacity
For a full cylinder, use π × radius² × length. Keep radius and length in the same unit, then convert the cubic result to litres. For a horizontal tank that is only partly full, liquid depth does not increase linearly with litres because the cross-section is curved. Use a horizontal cylinder/tank calculator for that case.
Partial fuel level in a rectangular tank
For straight-sided rectangular tanks, replace full height with measured liquid depth. A 150 cm × 80 cm tank with 30 cm diesel depth contains about 360 litres: 150 × 80 × 30 ÷ 1,000 = 360.
Why external dimensions can be misleading
External dimensions include wall thickness and may include frames, insulation or supports. For a capacity estimate, internal dimensions are better. Manufacturer nominal capacity may also differ from a simple geometric calculation because of rounded corners, baffles and fittings.
Litres, US gallons and imperial gallons
| Unit | Approximate litres |
|---|---|
| 1 litre | 1 L |
| 1 US gallon | 3.785 L |
| 1 imperial gallon | 4.546 L |
Always identify the gallon system before converting a fuel-tank specification. A “70 gallon” tank has a different litre value depending on whether the rating is US or imperial.
Safe usable capacity is not always full geometric capacity
Fuel expands with temperature and storage systems may require freeboard. Suction pipes may also leave some fuel unusable. Do not treat a calculated brim-full geometric capacity as the recommended operating fill level. Follow the tank manufacturer, local fire code and fuel-storage rules.
Estimate remaining fuel
For a rectangular tank, remaining litres can be estimated from liquid depth. For a vertical cylindrical tank, litres are also proportional to liquid height. For a horizontal cylindrical tank, use a segment calculation because half the liquid depth is not the same as half the volume except at the exact midpoint.
Frequently asked questions
How do I calculate diesel tank capacity in litres?
Use the volume formula for the tank shape, then convert the cubic result to litres.
Can I use water to test the volume of a fuel tank?
Geometric volume is independent of the liquid, but testing or filling a fuel tank with another liquid should follow manufacturer and safety guidance.
How many litres are in a 200 × 100 × 50 cm rectangular tank?
The theoretical geometric capacity is 1,000 litres.
Why does a horizontal round tank need a special partial-fill formula?
Because the cross-sectional area changes nonlinearly as liquid depth rises.
Enter your own values, keep the assumptions visible and use this guide to understand the formula behind the result.