pro ra ‧ ta /prəʊ ˈrɑːtə $ proʊ ˈreɪtə/ BrE AmE adjective [only before noun] especially British English
[ Date: 1500-1600 ; Language: Latin ; Origin: 'for the rate' ]
a payment or share that is pro rata is calculated according to how much of something is used, how much work is done etc:
Fees are calculated on a pro rata basis.
—pro rata adverb