A 4-day working week has become one of the most requested flexible arrangements in the UK, whether it’s a compressed hours deal, a genuine reduction in workload, or part of a trial following the growing number of 4-day week pilots across British companies. But agreeing to work 4 days instead of 5 raises an obvious question: what actually happens to your salary?
A Pro Rata Calculator UK tool answers this by scaling your full-time salary down to match your new working pattern. This guide walks through exactly how that calculation works for a 4-day week, how it differs from a genuine “4-day week, same pay” arrangement, and where the numbers commonly get misunderstood.
Two Very Different Types of “4-Day Week”
Before calculating anything, it’s important to know which version of a 4-day week applies to you, because the pay outcome is completely different.
- Pro rata 4-day week: You reduce your hours from 5 days to 4, and your salary reduces proportionally. This is the most common arrangement.
- Compressed hours 4-day week: You work the same total weekly hours as before, just squeezed into 4 longer days instead of 5. Your salary stays the same because your total hours haven’t changed.
- “100-80-100” 4-day week: Some companies (following the UK’s 4-day week trials) pay 100% of salary for 80% of the hours, as a genuine benefit rather than a pro rata reduction. This is still uncommon and depends entirely on your employer’s policy.
This article focuses on the standard pro rata 4-day week, since that’s what most employees actually encounter when they request reduced hours.
The Core Pro Rata Formula for a 4-Day Week
For a standard 5-day full-time role, moving to 4 days uses this formula:
Pro Rata Salary = Full-Time Salary × (4 ÷ 5)
Example: A role advertised at £35,000 FTE, worked over 4 days:
£35,000 × (4 ÷ 5) = £28,000
This 80% figure is the standard result most HR departments and payroll systems use when someone drops from full-time to a 4-day week.
When Full-Time Isn’t Based on 5 Days
Some roles are built around a different full-time baseline — a 37.5-hour week, a 6-day rota, or shift patterns with varying daily hours. In these cases, use hours instead of days for a more accurate figure:
Pro Rata Salary = Full-Time Salary × (Your Weekly Hours ÷ Full-Time Weekly Hours)
Example: Full-time is 37.5 hours/week at £30,000. You move to 4 days at 7.5 hours each (30 hours):
£30,000 × (30 ÷ 37.5) = £24,000
Notice this also comes out to 80%, because 30 out of 37.5 hours is the same proportion as 4 out of 5 days — but this only holds if your daily hours stay consistent with a full-timer’s. If your 4 days include longer shifts, the hours-based method will give a different (usually higher) result than the simple days-based formula.
Working Out Daily and Hourly Rates
Once you know your annual pro rata salary, breaking it into daily and hourly figures helps you sense-check your payslip and compare offers accurately.
- Daily rate: Pro Rata Salary ÷ Working Days Per Year
- Hourly rate: Daily Rate ÷ Hours Worked Per Day
Example continued: £28,000 ÷ 208 working days (4 days × 52 weeks) = £134.62/day
If your working day is 7.5 hours: £134.62 ÷ 7.5 = £17.95/hour
This is a useful check — your hourly rate on a pro rata 4-day week should be identical to your previous full-time hourly rate, assuming your daily hours haven’t changed. If it isn’t, something in the calculation (or your payslip) needs a closer look.
Table: 4-Day Week Salary at Different Full-Time Figures
| Full-Time Salary (FTE) | Pro Rata for 4 Days (80%) | Approx. Daily Rate |
|---|---|---|
| £26,000 | £20,800 | £100.00 |
| £30,000 | £24,000 | £115.38 |
| £34,000 | £27,200 | £130.77 |
| £38,000 | £30,400 | £146.15 |
| £42,000 | £33,600 | £161.54 |
These figures assume a standard 5-day full-time week and 52 working weeks a year, before tax and deductions.
Visit Now: MauricetteCalculetteHow Tax Works on a 4-Day Week Salary
Reducing your hours doesn’t change how you’re taxed — it just changes the amount you’re taxed on. The same Personal Allowance and tax bands apply regardless of your working pattern.
- Personal Allowance (2025/26): £12,570 tax-free, applied in full.
- Basic rate: 20% on income between £12,570 and £50,270.
- Higher rate: 40% above £50,270.
- National Insurance: Charged above the relevant threshold, same rates as full-time staff.
Why this matters for a 4-day week: dropping to 80% of your salary can sometimes move you out of the higher-rate tax band entirely, which softens the actual pay cut more than people expect.
Example: If your full-time salary was £52,000 (partly taxed at 40%), and your 4-day pro rata salary drops to £41,600, your entire income now falls within the 20% basic rate band — meaning your take-home pay doesn’t drop by a full 20%, since less of it is being taxed at the higher rate.
Read More: Best Rata Calculator UK-Pro Rata vs Full-Time Salary Explained 2026/27Holiday Entitlement for a 4-Day Week
Holiday entitlement is recalculated the same way pay is, based on the statutory minimum of 5.6 weeks per year.
Formula:
Holiday Entitlement = 5.6 × Days Worked Per Week
Example: For a 4-day week:
5.6 × 4 = 22.4 days of paid holiday per year
Some employers round this up to 23 days for simplicity, though 22.4 is the legally accurate statutory minimum. Anything beyond that is at your employer’s discretion, not a legal requirement.
If Your 4 Days Vary Week to Week
If your 4-day pattern isn’t fixed — for example, you work varying shifts that average out to roughly 4 days — your employer should apply the 12.07% accrual method instead of a flat weekly calculation:
Holiday Pay = Hours Worked × 12.07%
This method is recommended by ACAS and follows the Harpur Trust v Brazel Supreme Court ruling, giving an accurate result regardless of how irregular your hours are.
Step-by-Step: Calculating Your Own 4-Day Week Salary
- Confirm your full-time equivalent (FTE) salary — check your contract, offer letter, or ask HR if it isn’t clearly stated.
- Check whether full-time is genuinely a 5-day week for your role, or whether it’s based on hours or a different rota pattern.
- Apply the pro rata formula using days or hours, whichever matches your contract structure.
- Break the figure down into daily and hourly rates to confirm it matches your previous hourly pay.
- Recalculate your holiday entitlement separately, since it isn’t automatically included in the salary figure.
- Check your first payslip carefully, since pension contributions and tax codes sometimes need a cycle or two to fully adjust to the new pattern.
Overtime and Extra Shifts on a 4-Day Week
Many people who move to a 4-day week are occasionally asked to cover a 5th day — holiday cover, a busy period, or a one-off project deadline. This extra day should be calculated and paid separately from your base pro rata salary, not absorbed into it.
Read More: Pro Rata Calculator UK-Best Pro Rata Salary- Work out your daily rate first, using the method above (pro rata salary ÷ working days per year).
- Multiply by any premium your contract specifies — some employers pay standard rate for extra days, others pay time-and-a-half.
- Confirm whether the extra day counts toward your annual hours for pension or holiday accrual purposes, since occasional 5th days can sometimes push your average hours above your contracted 4-day pattern over the year.
Example: Your 4-day pro rata daily rate is £115.38. You’re asked to cover a 5th day at standard rate:
1 × £115.38 = £115.38 for that extra day, on top of your usual weekly pay.
If this becomes a regular pattern rather than an occasional request, it’s worth raising with your employer — regularly working a 5th day may mean your contract should be updated to reflect a genuinely different working pattern, rather than being treated as ad hoc overtime indefinitely.
Negotiating a 4-Day Week: What to Check Before You Agree
If you’re requesting a move to a 4-day week, or being offered one, a few checks upfront can prevent confusion later:
- Get the exact percentage in writing. Don’t assume 80% — confirm the precise figure your employer will use, especially if your hours aren’t a clean 4/5 split.
- Ask whether it’s a permanent contract change or a trial period. Some employers run 4-day week trials with a defined review date, which affects whether your terms are formally varied yet.
- Clarify how bonuses, commission, or performance pay are affected. These are sometimes calculated on a different basis to base salary and don’t always reduce in the same proportion.
- Check how it affects mortgage or loan applications. Lenders assess affordability based on your actual pro rata income, not your previous full-time salary, which can affect borrowing capacity if you’re applying around the same time.
- Confirm the start date for payroll purposes. A change partway through a pay period sometimes causes a one-off adjusted payslip before the new pattern fully applies.
None of these points are complicated, but they’re exactly the kind of detail that gets missed in an informal conversation with a manager and only becomes clear once the new contract is already signed.
How a 4-Day Week Affects Statutory Benefits
Dropping to 80% of your salary can occasionally affect eligibility for certain UK statutory payments, since several are based on earnings thresholds rather than job title or contract type.
- Statutory Sick Pay (SSP): You must earn above the Lower Earnings Limit on average to qualify. A reduced 4-day week salary rarely drops below this threshold, but it’s worth checking if your pro rata pay is already close to it.
- Statutory Maternity/Paternity Pay: These are calculated using your average weekly earnings over a set reference period, so a recent move to a 4-day week can affect the figure if the change happened shortly before the qualifying period began.
- Pension auto-enrolment: UK auto-enrolment uses specific qualifying earnings bands. A lower pro rata salary can sometimes fall closer to these thresholds, which is worth reviewing with your pension provider or payroll team.
- Universal Credit or other means-tested benefits: If you or your household receive these, a reduced salary should be reported promptly, since your entitlement is recalculated based on actual current earnings.
These aren’t reasons to avoid a 4-day week, but they’re worth checking before the change takes effect, particularly if you’re close to any of these thresholds already.
Common Mistakes When Calculating 4-Day Week Salary
- Confusing pro rata 4-day weeks with compressed hours arrangements. These have completely different pay outcomes, and mixing them up leads to wrong expectations.
- Assuming 4 days is always exactly 80% of salary. This only holds if your daily hours match a full-timer’s — longer or shorter days change the actual proportion.
- Forgetting to recalculate pension contributions. These should scale with your new pro rata salary, not stay based on your old full-time figure.
- Not checking whether reduced pay shifts your tax band. A drop to 80% of salary can sometimes remove you from a higher tax bracket, softening the real pay cut.
- Using a flat holiday formula for irregular 4-day patterns. If your days vary, the 12.07% accrual method gives a more accurate figure than 22.4 flat days.
Full-Time vs 4-Day Week: Quick Comparison
| Components | Full-Time (5 Days) | 4-Day Week (Pro Rata) |
|---|---|---|
| Salary basis | Full FTE salary | 80% of FTE salary (typical) |
| Personal Allowance | £12,570 | Same £12,570, full amount |
| Statutory holiday | 28 days (5.6 weeks) | 22.4 days (5.6 weeks × 4/5) |
| Pension contribution | % of full salary | % of pro rata salary |
| Hourly rate | Baseline | Same, if daily hours are unchanged |
1. How do I calculate salary for 4 days a week?
Multiply your full-time salary by 4, then divide by 5, assuming a standard 5-day full-time working week.
2. Is a 4-day week always 80% of full-time pay?
Only if your daily hours stay the same as a full-timer’s. If your 4 days include longer shifts, use an hours-based calculation instead.
3. What’s the difference between a pro rata 4-day week and compressed hours?
Pro rata means your total hours (and pay) reduce, since you’re genuinely working less overall. Compressed hours means you work the same total hours over 4 longer days instead of 5, so pay stays the same — only the shape of your week changes, not the total time worked.
4. Do I keep my full Personal Allowance on a 4-day week salary?
Yes. The £12,570 Personal Allowance (2025/26) applies in full, no matter how many days you work.
5. How much holiday do I get on a 4-day week?
22.4 days per year, based on the statutory 5.6 weeks entitlement multiplied by 4 days.
6. Can a 4-day week reduce my tax rate?
It can. If your pro rata salary drops below a tax band threshold, less of your income is taxed at the higher rate, softening the actual pay cut.
7. Does moving to 4 days affect my pension contributions?
Yes. Contributions are recalculated as a percentage of your new pro rata salary, not your previous full-time salary.
8. What if my 4 days aren’t the same every week?
Use the 12.07% holiday accrual method rather than a flat 22.4-day figure, since your hours vary from week to week.
9. Will my hourly rate change if I move to a 4-day week?
Not if your daily hours stay the same. Your hourly rate should stay identical — only your total weekly pay drops to reflect fewer working days.
10. Is a 4-day week with full pay realistic in the UK?
Some companies do offer full pay for reduced hours as part of 4-day week trials, but this is a specific employer policy, not a legal default or standard pro rata arrangement.
11. Does a 4-day week affect statutory maternity or sick pay?
It can, since both are based on average earnings over a set reference period. If you moved to 4 days shortly before a qualifying period, it’s worth checking how the calculation applies to your specific dates.
Final Thoughts
Working out salary for a 4-day week comes down to one core formula, but getting an accurate figure depends on knowing whether you’re looking at a genuine pro rata reduction or a compressed hours arrangement. A Pro Rata Calculator UK tool handles the maths instantly, but understanding the reasoning behind it means you can check your contract and payslip with confidence. Whether you’re negotiating reduced hours or reviewing an existing 4-day arrangement, these formulas put the numbers firmly in your control.
