The larger number after conversion does not mean the container holds more. It reflects the use of a smaller unit. This is similar to describing one metre as 100 centimetres.
Exact unit relationships do not make the original measurement exact. If a bottle is labeled approximately 1.5 L, writing 1,500.000 mL implies more precision than the label supports. Use digits that fit the measuring equipment and purpose.
For kitchen work, check whether a recipe uses millilitres or a named spoon or cup measure. Those household units can vary by convention. For laboratory work, use calibrated glassware or instruments appropriate to the required tolerance.
Volume and mass remain different. One millilitre of a substance does not universally weigh one gram; that familiar approximation is specific to water under limited conditions.
Reading a graduated container: if the smallest marked division is 10 mL, a result of 1,237 mL may be mathematically valid from the entered litres but cannot necessarily be dispensed to the nearest millilitre with that container. Round to a scale the equipment can deliver, or choose a more suitable measuring device.
Metric notation is case-sensitive. The accepted symbol for litre may be written L, and the prefix symbol for milli is lowercase m, giving mL. A capital M represents mega in SI notation and would describe a vastly different factor. In plain text, spelling out millilitres can prevent confusion.
For several portions, convert the total and then divide only if the portions are equal. For example, 1.5 L is 1,500 mL; six equal portions are 250 mL each. If measuring losses or residue matter, the theoretical division may exceed what can actually be poured.
When combining several amounts, put every value in one unit before adding. For example, 0.4 L plus 250 mL becomes 400 mL plus 250 mL, or 650 mL. Adding 0.4 and 250 directly would mix scales and produce a meaningless total.