Pro rata

The phrase pro rata (pronounced IPA: /prəʊ 'rɑːtə/) means ‘in proportion’, and is used in the context of the distribution of benefits (or burdens) among a number of individuals. Benefits (or burdens) may be said to be distributed pro rata or on a pro rata basis – the phrase may be used either adjectivally or adverbially - when they are distributed proportionately to the claims (or liabilities) of each individual. The expression is typically used with benefits (or burdens) to which a precise numerical value, e.g., a cash value, can be assigned. For example, if an employer pays his full-time employees £2,000 a month for working a 32-hour week, and undertakes to pay his part-time employees pro rata or on a pro rata basis, he must pay them proportionately, according to the number of hours they work (so that, e.g., an employee working 16 hours a week receives £1,000, one working 24 hours a week £1,500, etc.)

Pro rata is an abbreviated form of the Medieval Latin phrase, now no longer used, pro rata parte (i.e., ‘in proportion to the calculated share’).. (Rata in this phrase is the feminine singular ablative form of the word ratus, the past participle of the deponent verb reor (infinitive rēri), ‘to think, suppose, reckon, calculate’).

See also Proration and Pari passu.