Written and reviewed by James Whitfield · Updated for 2026/27 · Editorial standards · Methodology
How pro rata salary is calculated, what part-time take-home looks like after tax, and how to compare offers correctly.
Pro rata salary simply means 'in proportion'. If a full-time role working 37.5 hours per week pays £40,000, someone working 3 days per week (22.5 hours) receives 22.5 ÷ 37.5 × £40,000 = £24,000. Job adverts typically state the full-time equivalent salary with 'pro rata' or FTE in brackets, which is worth noting when comparing offers across different hours.
The calculation itself is straightforward. The complexity comes when you factor in tax. Part-time income might sit in a completely different tax band than the full-time equivalent, someone earning £18,000 pro rata pays no higher-rate tax and may pay little or no income tax at all if their earnings fall close to the personal allowance.
Student loan repayments are also calculated on the same gross income. A part-time salary of £18,000 falls below the Plan 2 threshold of £27,295, so no loan repayment applies. The same full-time role at £36,000 would cost £783 per year in loan repayments. This means part-time work near the threshold can produce better retention than the gross cut might imply.
Part-time employees receive the same personal allowance as full-time employees, £12,570 in 2026/27. This means someone working two days a week earning £15,000 per year pays income tax only on £15,000 − £12,570 = £2,430, at 20%, a tax bill of £486 per year and almost nothing per month. The full-time equivalent of the same role might pay £37,500 and incur income tax of around £5,000.
National Insurance starts at the same primary threshold (£12,570) and the same rules apply. A part-time worker earning £15,000 pays NI at 8% on £2,430 = £194 per year. Someone earning below £12,570 part-time pays no income tax and no NI at all, relevant for many workers doing minimal hours in a second job or alongside caring responsibilities.
Multiple part-time jobs complicate this. If you work for two employers simultaneously, your personal allowance can only be used by one of them, usually your main employer, who gets the full 1257L code. The secondary employer uses a BR code (20% on everything) or D0 (40% on everything if the income is higher rate). This can result in over-taxation across both jobs, with a potential refund at year end if cumulative earnings did not justify the combined deductions.
The biggest mistake is comparing two offers at the gross level without accounting for the different tax treatment. A full-time offer of £40,000 and a part-time pro-rata offer of £24,000 for three days look like the same deal per hour, but the after-tax comparison tells a different story. On £24,000, effective tax and NI is much lower in percentage terms because a larger share falls in the allowance zone. You keep a higher proportion of each pound earned, even though you earn fewer pounds.
Pension auto-enrolment thresholds are the other common surprise. Workers earning below £10,000 per year are not automatically enrolled (though they can opt in). Those above £10,000 are enrolled, with qualifying earnings calculated on income between £6,240 and £50,270. A part-time worker earning £18,000 would have qualifying earnings of £11,760 for pension purposes, with minimum employer contribution of 3% (£352.80/year) and employee contribution of 5% (£588/year).
When switching from full-time to part-time, your tax code should remain the same (1257L in most cases) and your payroll should adjust proportionately. If you are paid a monthly salary, your employer will simply pay a reduced amount. If anything looks wrong on your first reduced payslip, particularly if the tax deduction does not fall proportionately, contact payroll promptly.
Current tax-year thresholds used across this guide and calculator.
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Tax region, tax code, student loan plan, pension contribution and salary sacrifice are the key assumptions to check first.