How pro-rata pay is calculated
Pro-rata means "in proportion". Your actual salary is your full-time contract salary multiplied by the fraction of full-time you work. If a full-time role pays £40,000 and you work 3 out of 5 days, your pro-rata salary is:
£40,000 × (3 ÷ 5) = £24,000
Income tax, National Insurance and (where applicable) student loan repayments are then calculated on that pro-rata figure, not the full-time one. Because the personal allowance and NI thresholds are fixed amounts, working part-time usually means a lower effective tax rate as well as lower pay.
What counts as full-time?
There is no single legal definition of full-time in the UK — it is whatever your employer defines in the contract. Use your employer's full-time hours or days to get an accurate proportion.