Calculated fields follow most of the same rules as Digital Dice Rolling: Dice Notation but are defined with calc() instead of rollable().

calc([dice=]dicenotation,[dicelabel=label text,label=label text,compact,mods])

The differences between the two functions is that calc() fields do not have a dice rolling option, and if the mods keyword is used, it will not include a leading + symbol. Apart from that, the function the same.