Pro rata means "in proportion", and the calculator for it is one multiplication: the full amount × the share of the period you are in it. That covers a part-time salary, a mid-month start, a holiday entitlement and a final invoice alike. £28,000 at three days a week is £16,800, and after 2026/27 tax and National Insurance that keeps £15,615.60: the second step people forget.
Build the fraction from the thing you are sharing
Hours over full-time hours for a part-time contract; days worked over working days for a partial month; months over twelve for a partial year. Keep the numerator and denominator in the same units and the fraction cannot lie. A 22.5-hour week against a 37.5-hour standard is 0.6 exactly; the 15th of a 30-day month is 0.5; joining on 1 October is 6/12 of the tax year: although PAYE handles the in-year spreading of your allowance for you.
Apply it, then tax the result: in that order
The bands only ever see your actual gross. £28,000 × 0.6 = £16,800; running that through the 2026/27 rules leaves £15,615.60 a year or £1,301.30 a month, an effective rate of 7.05%. Doing it backwards (taxing the £28,000 and then taking 0.6 of the net) overstates the deductions by hundreds of pounds, because the personal allowance would be scaled down when in reality it stays whole.
Where the proportion is a legal floor, not a convention
For employees the proportion is protected: part-time workers must not be treated less favourably than a comparable full-timer, so pay rate, holiday accrual and access to a pension scheme all follow the fraction rather than the employer's discretion. Where no statute is in play (pro-rating a bill or an invoice) the fraction is whatever the contract says, so write the denominator into the contract before you rely on it.
Questions people ask about pro rata calculator
What does pro rata mean on a bill or an invoice?
The charge covers only the share of the billing period you used, in proportion to time. Ten days of a £90 monthly service in a 30-day month is £30.
Is there a single pro rata formula?
Amount × (your units ÷ full units). Everything else (hours, days, months) is choosing the units.
Does pro rata change my tax code?
No. The standard code and allowance stay the same; a smaller gross simply uses less of the bands above the allowance.