How many liters in 375 ml?

It is dependent of the density of material which you are trying to determine. If for example you had water then 375 milligrams woul be equal to 375 millilitres.

density = mass/volume

so:-

density x volume = mass

thus:-

volume = mass/density

be sure to have all your number in standard units though i.e volume in cm3 density in gcm-3 and mass in g