Four-day week calculator
Dropping from five days to four cuts your gross salary by 20%, but your take-home falls by less. On £45,000 you would lose 18.0% of your take-home, not 20%, because your personal allowance does not shrink with your hours. The day off costs £124.62 a week while a working day earns you £138.15.
Going from 5 days to 4, your take-home falls by
£540.00 a month
You give up 20.0% of your gross salary but only 18.0% of your take-home pay, because your personal allowance does not shrink with your hours and the pay you drop comes off the top, at your highest rate first.
- New gross salary
- £36,000
- was £45,000
- New take-home, monthly
- £2,453.30
- was £2,993.30
- Real cost of a day off
- £124.62
- against £138.15 earned
Why the day costs less than it earns
Each working day currently brings in £138.15 of take-home pay, but giving one up only costs £124.62. The difference is the tax system. The pay you give up is the top slice of your income, so it is taxed at your marginal rate of 28.0%, while the pay you keep still gets the whole personal allowance and the whole basic rate band. The higher your salary, the wider that gap gets.
Two things this does not cover. Your pension contribution usually falls with your salary, so the pot grows more slowly, and it can affect statutory pay that is calculated from average earnings. It also assumes your employer prices the change pro rata, which is the usual arrangement but not the only one: some four-day-week trials hold pay flat.
For a pay drop that is not about days, use the pro rata calculator. To see the marginal rate behind this, try the take-home calculator.
Why a day off costs less than a day earns
Income tax is progressive, so the pay you give up is not average pay, it is the top slice of your income. Give up a fifth of £45,000 and the pound you lose first is the one that was taxed hardest, while the £12,570 personal allowance and the whole basic rate band stay exactly where they were. The higher your salary, the wider that gap gets, and it is widest of all between £100,000 and £125,140, where the allowance taper pushes the marginal rate above 60% and a day off can cost barely a third of what it earns. The £100k trap calculator shows that band on its own.
What the calculator does not price
- Your pension. A percentage contribution falls with your salary, so the pot grows more slowly and the employer match usually falls too. That is a real cost that does not show up in monthly take-home.
- Statutory pay. Maternity pay, sick pay and redundancy pay are calculated from average earnings, so a reduced salary reduces them for a period afterwards. Maternity pay and redundancy are worked out separately.
- Holiday. Your leave entitlement is pro rated too, so four days a week means four fifths of the days, not the same holiday in a shorter week.
- Pay held flat. Some four-day-week trials keep pay unchanged for reduced hours. This page assumes the usual pro rata arrangement, so it shows the worst case for your wallet.
Figures checked 2026-08-28 by Hadi against HMRC rates for 2026/27. For a change that is not about days, use the pro rata calculator.