Part-time hours,
full-time clarity

How a pro-rata salary is worked out

There is only one sum. Take the full-time salary, multiply by the days or hours you work, and divide by the days or hours a full-time person works. Nothing else in the advert changes: the pro-rata figure is your actual contractual salary, and it is the number your payslip, your mortgage application and HMRC all use.

Use whichever unit your contract is written in. Days work when a role is five days a week and you do three. Hours work when the full-time week is 37.5 hours and yours is 22.5, or when your days are not all the same length. Mixing the two, comparing days against hours, is the one way to get the wrong answer.

Your tax is not pro-rated, and that works in your favour

The Personal Allowance is £12,570 a year for everyone who qualifies, full time or not. National Insurance starts at £12,570 of pay. Neither figure shrinks because you work fewer days, so a part-time salary keeps the whole of the tax-free band while the taxable part gets smaller.

The practical result is that going from five days to three does not cost you 40 per cent of your take-home. It costs less, because a larger share of what you still earn falls inside the allowance. The calculator above shows the real figure rather than the proportion, which is why the take-home line and the gross line move at different speeds.

The same logic runs the other way at the top. Dropping a day can take you back under the £50,270 higher-rate threshold, or out of the Personal Allowance taper that starts at £100,000, and the saving on those pounds is larger than the pay you gave up. Switch the tax year or the region above to see how that lands on your own numbers.

Holiday: the part most pro-rata adverts get wrong

Paid holiday is pro-rated too, and it is pro-rated in the same proportion as your hours. The statutory minimum is 5.6 weeks of paid leave a year, so a three-day week earns 5.6 weeks of three-day weeks, not 5.6 weeks of five-day weeks. Counting leave in days rather than weeks is where part-time staff most often lose entitlement without noticing.

Bank holidays follow the same rule. If your employer gives full-time staff the eight English and Welsh bank holidays on top of the statutory minimum, a part-time colleague is entitled to a pro-rata share of them, whether or not those particular days happen to fall on days they work. Employers usually settle this by converting the whole allowance into hours.

None of this changes your salary, and so none of it changes the figures above. Holiday pay in a salaried job is already inside the annual salary: you are paid the same amount every month whether or not you took leave that month. It matters for what you are owed, not for what you take home.

Where a pro-rata figure can still mislead

• Pension contributions. Auto enrolment is assessed on your actual earnings, so a low pro-rata salary can fall under the enrolment trigger even though the full-time rate is well above it. Ask before you assume you are in the scheme.
• Benefits quoted as a full-time value. Life cover at a multiple of salary, a car allowance and a bonus target are usually pro-rated. A private medical policy usually is not.
• Salaries advertised as a full-time equivalent. An advert reading "£35,000 FTE, part time considered" is quoting the full-time number. The pro-rata figure for the hours on offer is the one to compare against your current pay.
• Term-time only roles. These are pro-rated by weeks worked as well as hours, which is a second multiplication the advert rarely spells out.