Convert volume units between units, using exact SI factors.
A cubic metre holds exactly 1,000 litres. The litre is by definition one cubic decimetre, and the BIPM SI Brochure accepts it for use with SI even though the SI unit of volume is the cubic metre. Water and heating bills use cubic metres; everyday life uses litres.
No, and the gap is substantial. A US gallon is 3.785 litres; an imperial gallon is 4.546 litres, about 20% more. Neither is an SI unit. This difference is a routine source of serious errors when comparing fuel consumption figures.
For a rectangular tank the volume is length × width × height; for a cylinder it is πr² × height. If the dimensions are in centimetres the result is cubic centimetres, which you divide by 1,000 for litres. Measuring in metres gives the answer directly in SI cubic metres.
Because volume units scale by the cube. A metre is 100 centimetres, and 100 cubed is 1,000,000. It is also why a small error in a length measurement triples in a volume calculation — the most common surprise in SI conversion.