Hundred SummersGoing part-time: what a four-day week really costs
The take-home you actually lose after tax and NI fall too, the pension contributions that fall with your pay, the child benefit that may come back, the State Pension earnings floor, and the price of each day off. Your figures never leave your browser.
Loading the calculator… (requires JavaScript; your figures never leave your browser)
Worked example
Computed when this page was built by the same engine that runs the calculator above, so the figures below and the live tool can never disagree. Change any input above to see your own.
| Inputs | |
|---|---|
| Full-time salary | £65,000 |
| Days a week | 5 → 4 |
| Pension contributions | 5% you + 3% employer |
| Children claimed for | 2 |
| Results | |
|---|---|
| New salary | £52,000 |
| Take-home before → after | £46,307 → £38,983 |
| Take-home lost a year | £7,324 (15.8% for 20% fewer hours) |
| Pension contributions lost a year | £1,040 |
| Child benefit regained a year | £205 |
| Cost of each day off | £137 |
The next question
Can we share an unused personal allowance? →Marriage allowance calculator
Or answer it with everything else: the full Hundred Summers plan connects your pay, pensions, savings, property, tax and spending year by year, and stress-tests them across 500 simulated market histories.
Read the guides
- Salary sacrifice, properly explained →How giving up pay for a pension contribution saves both income tax and National Insurance, the employer's saving and whether it is passed on, the 60% zone where a contribution costs 38p in the pound, the side effects, and the £2,000 cap announced for 2029.
- Maternity leave and money: what the year really costs, and what to protect →Statutory maternity pay week by week, why the take-home lost is less than the gross gap, the three things to protect while on leave (the child benefit claim, the pension, the State Pension record), and how part-time afterwards prices out.
Why it costs less than it looks
Pay is taxed from the top down, so the slice you give up was your most heavily taxed: a 20% cut in hours is usually a 13–17% cut in take-home, and for anyone in the £100,000–£125,140 zone or the £60,000–£80,000 child benefit band, the cut can pay for itself in part. Against that, the things that scale with pay quietly shrink: your pension contributions, your employer’s, the tax relief on both, and salary-linked benefits such as death-in-service and sick pay.
The long-run cost is the pension
A day a week over a decade is a tenth of a career’s pension contributions plus their growth, and it is the number people least often check before deciding. The pension projection calculator shows what a lower contribution does to the pot; the when can I retire calculator whether it moves the date; and the Hundred Summers app runs the whole plan with the new salary, so the trade is made with the retirement effect in view rather than the payslip alone.
Sources
- HMRC, Income Tax rates and National Insurance rates (the lower earnings limit).
- HMRC, High Income Child Benefit Charge.
- gov.uk, New State Pension: eligibility (qualifying years).