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).The Free Dictionary.  Accessed May 29, 2008.
Example
For example, the cost of a pizza can be divided pro rata depending on the number of slices each person ate.
References

Category:Latin words and phrases
Category:Economics