Recently when reading some material related to research grants, I came across the Latin phrase pro rata temporis in English text. It was easy enough to understand in the context. For example, a 600 000 € grant might be for three years, and if one applies for a shorter period of time, the maximum money is reduced pro rata temporis, so that for two years one can get at most 400 000 €.

But how do I parse the Latin phrase? I have difficulties seeing how it could mean what it does. Is the phrase an abbreviation of something longer? Does it have an origin that would help me understand how it works? Would the phrase make sense in classical Latin without context? I take it that rata is a perfect participle of reri, and L&S lists under II.B.1 pro rata parte/portione, which makes me think that perhaps an ablative like parte or portione is implied.

  • The phrase I'm familiar with for this practice is "pro-rating", which seems to come from the Latin expression?
    – Draconis
    Nov 11, 2017 at 1:14
  • @Draconis Apparently so, although it sounds more like "professional rating" to me at first.
    – Joonas Ilmavirta
    Nov 11, 2017 at 14:37

1 Answer 1


Yes, parte or portione is understood here; sometimes it's explicit.

OLD has a separate entry for the adjective ratus (in origin, the perfect participle of reor). Definition 3 is 'Determined, fixed, certain,' and definition 3b is as follows (bold added for emphasis):

~a pars, a fixed proportion; esp. pro ~a parte (portione), in proportion; also pro ~a alone.

One attestation is CIL 2.5181.27 (from Hispania), which actually uses the phrase pro rata temporis (without parte or portione):


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