The mL = gram rule of thumb is only accurate for water, besause its density is very close to 1g/mL.
Generally speaking:
density = mass/volume
Hence volume = mass/density, which basically means that for liquids thicker than water(d>1), 30 grams of sauce for instance is less than 30 ml.
Anyway, the point is that if the calories are listed for mls, and you go by their weight in grams, you are actually overestimating calories, so don't bother, just go by mls.
Sorry guys, my nerd mode was activated.