4 day week calculator

Moving to a 4 day week cuts your gross salary by 20%, but your take-home falls by less. The same arithmetic covers compressed hours and a nine day fortnight, where the hours move rather than the pay. 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.

Calculator

Figures update as you type. Press Calculate to jump to the result.

Current full-time salary

Days a week now

Days a week after

Where you live

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 28 August 2026 by Hadi against HMRC rates for 2026/27. For a change that is not about days, use the pro rata calculator.

How to use the 4 day week calculator

1
Enter your current full-time salary.
2
Choose whether the four-day week comes with a pay cut, and how much.
3
Read the new take-home, which falls by less than the gross does.
4
See the cost per day off, which is the figure the decision actually turns on.

When should you use the 4 day week calculator?

Use it when a shorter week is on offer or you are about to ask for one, because a day off costs less than a fifth of your take-home.

Your employer offers a four-day week

Giving up a fifth of gross pay costs less than a fifth of take-home.

You are asking to drop a day

Knowing the real cost helps you decide what pay cut you can accept.

You are comparing with reduced hours

Dropping a day and shortening every day are the same arithmetic with different logistics.

Your employer offers 100% pay for 80% hours

Then the only question is whether the workload actually shrinks.

Where to go next

All 51 calculators share one tested engine, so a figure from the 4 day week calculator agrees with every other page here. See the full calculator index or the pay and tax guides.