The 40-hour week is the full-time standard in much of the UK private sector, and it is the baseline against which millions of part-time salaries are calculated. If your employer uses a 40-hour week, your pro rata pay follows a simple formula — but it gives a different answer from the same part-time hours on a 37.5-hour week, and that difference is worth understanding before you accept a job or query a payslip.
Quick answer: To use a pro rata salary calculator for 40 hour work week jobs, divide your weekly hours by 40 to get your fraction of full-time, then multiply the full-time annual salary by that fraction. For example, 20 hours a week is 0.5 of a 40-hour week, so on a £30,000 full-time salary you would earn £15,000 pro rata. Always confirm the 40-hour baseline in your contract, because the same hours on a 37.5-hour baseline would pay more.
The formula, step by step
Pro rata pay on a 40-hour week follows three steps. First, find your fraction: your weekly hours divided by 40. Second, multiply the full-time equivalent (FTE) annual salary by that fraction. Third, divide the annual pro rata salary by 12 for a monthly figure or by 52 for a weekly figure.
Worked example: a full-time salary of £32,000, working 24 hours a week. Step one: 24 ÷ 40 = 0.6. Step two: £32,000 × 0.6 = £19,200 a year. Step three: £19,200 ÷ 12 = £1,600 a month, and £19,200 ÷ 52 = £369.23 a week. Once you have done the calculation by hand, you can use a calculator for speed — but knowing the steps lets you check any figure an employer gives you.
Common part-time patterns on a 40-hour week
The 40-hour week produces particularly clean fractions, because 40 divides easily by 2, 4, 5, and 8. Working 32 hours is 0.8 FTE; 30 hours is 0.75 FTE; 24 hours is 0.6 FTE; 20 hours is 0.5 FTE; and 16 hours is 0.4 FTE. The table below shows what those patterns pay on three typical full-time salaries.
| Your hours per week | Fraction of full-time | On £25,000 FTE | On £30,000 FTE | On £35,000 FTE |
|---|---|---|---|---|
| 32 hours | 0.80 | £20,000 | £24,000 | £28,000 |
| 30 hours | 0.75 | £18,750 | £22,500 | £26,250 |
| 24 hours | 0.60 | £15,000 | £18,000 | £21,000 |
| 20 hours | 0.50 | £12,500 | £15,000 | £17,500 |
| 16 hours | 0.40 | £10,000 | £12,000 | £14,000 |
One pattern deserves a note: 30 hours a week is often described as “four days a week” in a 40-hour business, but the fraction is 0.75, not 0.8 — a difference that costs £1,500 a year on a £30,000 salary compared with the same hours on a 37.5-hour baseline. Job adverts that say “30 hours, pro rata” without stating the baseline leave this ambiguity open, which is why you should always ask.
Hourly rate on a 40-hour week
Your hourly rate is the full-time salary divided by full-time annual hours. On a 40-hour week, annual hours are 40 × 52 = 2,080. So a £30,000 salary gives an hourly rate of £30,000 ÷ 2,080 = £14.42 an hour. A £25,000 salary gives £12.02 an hour, and a £35,000 salary gives £16.83 an hour.
Compare that with a 37.5-hour week, where £30,000 gives £15.38 an hour. The same headline salary buys your employer more hours on a 40-hour week, which is exactly why the baseline matters in negotiations. If you are choosing between two jobs, compare hourly rates first — and if a job advertises a salary without stating the full-time week, treat the figure as incomplete until you have the baseline. You can test any combination on the free calculator on our homepage.
Where the 40-hour week is used
The 40-hour week is standard across much of retail, hospitality, manufacturing, logistics, and private-sector office work, and it is the default in many small businesses that have never formally adopted a shorter week. It usually means eight hours a day over five days, often 9am to 5.30pm with a half-hour lunch break, or 8am to 4.30pm on a similar pattern. Some employers advertise “40 hours” but actually work 37.5 paid hours plus unpaid breaks — check whether breaks are paid or unpaid, because an unpaid half-hour a day changes the effective hourly rate even when the headline week is the same.
There is no UK law that fixes the full-time week at 40 hours, or at any other figure. The Working Time Regulations set a maximum average week of 48 hours, but the standard full-time week is a matter of contract, not statute. That is why two employers in the same industry can use different baselines, and why your contract — not a general rule — is the authority for your pay.
Holiday, overtime, and other pro rata elements
As with salary, holiday entitlement usually scales by the same fraction: someone working 0.75 FTE on a 40-hour week typically receives 75% of the full-time holiday allowance, including bank holidays, often expressed in hours rather than days to keep the maths exact. Pension contributions are based on your actual pensionable pay. Overtime is where the 40-hour baseline can catch people out: some contracts only pay an overtime premium once you exceed the full-time week, so a part-time worker who covers an extra shift may be paid at their normal hourly rate up to 40 hours, with the premium only applying beyond that. Check your contract’s overtime clause before agreeing to extra hours.
Tax and National Insurance are calculated on actual earnings, so part-time earnings on a 40-hour baseline may fall below thresholds that a full-time salary would exceed. That can make the take-home proportion higher than the gross proportion — a welcome effect, but one that must be calculated on your own figures, not estimated from a full-time colleague’s payslip.
What if your hours vary from week to week?
Many 40-hour-week employers use rotating shift patterns, so part-time staff may work different hours each week. In that case pay is usually based on contracted average hours over the rota cycle: a contract averaging 24 hours a week pays the 0.6 FTE figure every month, with busier and quieter weeks balancing out. Keep your own record of hours worked, because a long-term pattern of working above your contracted average is evidence for a contract review — and if you are paid only for hours actually worked each month, with no averaging, your monthly pay will fluctuate, which makes budgeting harder and makes checking each payslip against the hourly rate even more important.
Casual and zero-hours staff in 40-hour businesses are paid by the hour rather than by a pro rata salary. The benchmark hourly rate still comes from the full-time salary: £14.42 an hour on a £30,000 salary over a 40-hour week. If a casual role pays materially less per hour than the contracted equivalent, the gap is worth querying, because the work is the same even if the contract type differs. Conversely, some casual roles pay a higher hourly rate to compensate for the lack of holiday pay and security, so compare the full package — hourly rate, holiday, pension, and predictability — not just one figure.
Worked example: a complete monthly picture
Putting it together helps. Take a warehouse operative on a £26,000 full-time salary, working 30 hours a week on a 40-hour baseline. The fraction is 30 ÷ 40 = 0.75, so the annual salary is £26,000 × 0.75 = £19,500. Monthly gross pay is £19,500 ÷ 12 = £1,625, and weekly gross pay is £19,500 ÷ 52 = £375. The hourly rate is £26,000 ÷ 2,080 = £12.50 an hour. Holiday entitlement, if the full-time allowance is 28 days including bank holidays, becomes 75% of 28 days, which is 21 days, usually expressed as 168 hours so that part-day shifts are exact. Pension contributions, sick pay, and any overtime premium all start from these same figures, which is why a single error in the baseline ripples through the whole employment package.
Checking your payslip
When your first payslip arrives, divide your annual pro rata salary by 12 and compare it with the gross monthly figure. Differences can come from a mid-month start, unpaid leave, or pension and student-loan deductions, but the basic pay line should match your own calculation. Also verify that the contracted hours shown match your contract — an employer recording fewer hours than you work will underpay you month after month.
Keep your offer letter, contract, and early payslips together. If a discrepancy appears, raise it with payroll in writing and promptly; payroll corrections are straightforward in the first month and progressively harder to untangle later. If payroll insists on a different baseline from the one in your contract, that disagreement is worth escalating, because the contract is what you agreed to.
Other working patterns: 20 hours a week · 24 hours a week · 30 hours a week · 37.5-hour week · 2 days a week · 3 days a week · 4 days a week · 37.5 vs 40 hours · part-time hours tables · pro rata hours · days vs hours · examples by days per week
Changing hours on a 40-hour baseline
Hours changes follow the same linear formula: new hours divided by 40, multiplied by the FTE salary. Moving from 20 hours to 30 hours on a £30,000 FTE salary takes you from £15,000 to £22,500, a £7,500 rise. Moving from 30 hours to 20 drops you by the same amount. Because the relationship is linear, you can model a hours change before requesting it, and go into the conversation knowing exactly what it is worth.
One final caution: if your employer moves the whole business from a 40-hour week to a shorter week, or vice versa, your pro rata salary should be recalculated from the new baseline — and if the change reduces your pay, that is a contractual change that normally requires your agreement. Knowing the formula means you can work out the effect of any proposed change in minutes, rather than discovering it on a payslip months later.