Pro rata is Latin for "in proportion", and on a job advert it means the salary shown is the full-time figure, to be scaled by the hours you actually work. "£30,000 pro rata" for a three-day week pays £18,000 a year gross; after 2026/27 income tax and National Insurance that is £16,479.60, or £1,373.30 a month. The advert's number is the yardstick, never the payslip.
Why adverts quote the full-time figure at all
Because it is the comparable one. Two part-time roles at different hours cannot be ranked by their actual pay, but their full-time equivalents line up directly, and pay scales, union agreements and job-evaluation bands are all written in FTE terms. The convention only misleads when the reader stops at the headline: the fraction printed next to the hours is half the offer, and dividing your actual pay by the fraction is how you check the employer's arithmetic.
What the proportion legally covers beyond salary
Under the part-time worker regulations the proportion runs through the whole package: paid holiday accrues at the same fraction, the pay RATE per hour must match the full-timer's, and access to the pension scheme and its employer contribution cannot be withheld for working fewer hours. What does not shrink is your personal allowance: the first £12,570 stays tax-free however few hours you work, which is why part-time pay keeps a noticeably larger share of itself.
The arithmetic, worked once
Take the FTE salary, build the fraction (your weekly hours ÷ the role's full-time hours), multiply, then tax the result. £30,000 at 30 of 37.5 hours: fraction 0.8, gross £24,000, take-home £20,799.60 in 2026/27 at an effective 13.34%. The same salary at half hours: gross £15,000, take-home £14,319.60 at 4.54%, most of it inside the allowance. The calculator this page links runs any variant of that in one step.
Questions people ask about what is pro rata
Does pro rata ever apply to a full-time job?
Yes: for a part YEAR. Join or leave mid-year and that year's pay is pro-rated by time served, even on full-time hours.
Is a pro rata salary worse than the advertised one?
It is smaller in gross by exactly the hours fraction, but it is taxed more lightly per pound, so the net falls by less than the fraction suggests.
What should the contract state?
Both figures: the full-time equivalent and your actual gross, plus the hours used to derive the fraction. That makes every later calculation checkable.