Pro rata

Pro rata is a Latin term that is used in law and economics.
It is used to divide something "by calculation" or "by a ratio". 
Etymology.
Pro rata has a Latin etymology. It is from pro (according to, for, or by) and rata. Rata means "calculated" (rate or change).
Example.
For example, the cost of a pizza can be divided "pro rata" depending on the number of slices each person ate.