To convert litres to millilitres, you can use the conversion factor that 1 litre is equal to 1000 millilitres.
Given that the container holds 2.5 litres of water, you can calculate its capacity in millilitres as follows:
\[ 2.5 , \text{litres} \times 1000 , \text{mL/litre} = 2500 , \text{mL} \]
So, the capacity of the container in millilitres is 2500 mL.