If you’re moving to part-time work — or you’ve just been offered a job advertised at a full-time salary — knowing how to use pro rata salary calculator tools is the quickest way to find out what you’ll actually be paid. A good calculator takes the full-time salary, compares it with the hours you’ll work, and gives you your annual, monthly and hourly figures in seconds.
This guide walks you through the whole process: what each input means, how to enter your details correctly, how to read the results, and the mistakes that trip people up most often.
What a Pro Rata Salary Calculator Actually Does
“Pro rata” simply means “in proportion”. If a full-time role pays £32,000 a year for a 37.5-hour week, and you work half those hours, your pro rata salary is half of £32,000 — £16,000 a year. A pro rata salary calculator automates that proportion for you, and usually breaks the result down into annual, monthly, weekly and hourly figures so you can compare offers properly.
Most calculators also handle the reverse: if you know your hourly rate and your weekly hours, they’ll work out your annual pro rata salary. That’s handy when an advert quotes “£14.50 per hour, 22 hours a week” and you want to know what lands in your bank each month.
The Inputs You’ll Need (and Where to Find Them)
Before you open the calculator, gather four numbers. Getting these right is the whole battle — the maths itself is trivial.
1. The full-time equivalent (FTE) salary
This is the salary the job would pay if it were full-time. Job adverts in the UK usually quote this figure and add “pro rata” afterwards, e.g. “£28,000 pro rata”. If the advert only gives an hourly rate, multiply it by the full-time weekly hours and then by 52 to get the FTE annual figure.
2. Full-time weekly hours
In the UK this is typically 37 or 37.5 hours, but it varies — some employers use 35, others 40. Check the job advert, the contract, or ask HR. Using the wrong figure here is the most common source of error.
3. Your contracted weekly hours
The hours you’ll actually work. Be precise: 22.5 hours is not the same as 22, and the difference adds up over a year.
4. Days worked (optional but useful)
Some calculators ask how many days a week you work, mainly to estimate holiday entitlement or daily rates. It doesn’t change the salary figure, but it helps with the breakdown.
Step-by-Step: Using the Calculator
Here’s the exact process, using a worked example throughout. Say you’ve been offered a role advertised at £30,000 pro rata, full-time hours are 37.5 per week, and you’ll work 22.5 hours per week over three days.
Step 1: Enter the FTE annual salary
Type 30000 into the salary box. Don’t include commas or pound signs — most calculators want plain numbers.
Step 2: Enter the full-time hours
Type 37.5. This is the employer’s definition of full-time, not a national standard.
Step 3: Enter your hours
Type 22.5 — your contracted weekly hours.
Step 4: Read your annual pro rata salary
The calculator divides your hours by the full-time hours (22.5 ÷ 37.5 = 0.6) and multiplies by the FTE salary (£30,000 × 0.6). Your result: £18,000 a year.
Step 5: Check the monthly, weekly and hourly breakdown
A good calculator shows:
| Pay period | Amount |
|---|---|
| Annual | £18,000.00 |
| Monthly | £1,500.00 |
| Weekly | £346.15 |
| Hourly | £15.38 |
The hourly figure (£18,000 ÷ 52 ÷ 22.5) is worth noting — it’s identical to the full-time hourly rate, which is exactly how it should be. Pro rata pay never changes your hourly rate; it only changes how many hours you’re paid for.
Step 6: Sense-check the result
Roughly verify with mental maths: 22.5 hours is 60% of 37.5, and 60% of £30,000 is £18,000. If the calculator’s answer is wildly different, recheck your inputs.
Worked Example: Four Days a Week
Let’s run a second example — a very common pattern. A marketing role pays £36,000 FTE, full-time is 37 hours, and you’ll work 29.6 hours (four days of 7.4 hours).
- Proportion: 29.6 ÷ 37 = 0.8 (exactly 80%)
- Annual pro rata salary: £36,000 × 0.8 = £28,800
- Monthly: £28,800 ÷ 12 = £2,400
- Weekly: £28,800 ÷ 52 = £553.85
- Hourly: £28,800 ÷ 52 ÷ 29.6 = £18.71
Notice the hourly rate (£18.71) matches the full-time equivalent (£36,000 ÷ 52 ÷ 37 = £18.71). If it doesn’t match, something’s wrong with the inputs.
The Mistakes That Trip People Up
Using 40 hours as “full-time” by default. If the employer’s full-time week is 37.5 hours and you enter 40, your pro rata figure comes out too low — you’d be undervaluing the offer by about 6%.
Confusing monthly with annual FTE. Some adverts quote monthly salaries. A £2,500-a-month advert is £30,000 a year — make sure you’re entering the annual figure unless the calculator specifically asks for monthly.
Forgetting unpaid lunch breaks. If full-time staff work 9 to 5 with an unpaid hour for lunch, full-time hours are 35, not 40. Your contract should spell this out.
Rounding your hours. “About 20 hours” might actually be 19.5 or 21. Over a year, half an hour a week at £15 an hour is nearly £400 — worth getting right.
Ignoring the 52 vs 52.14 question. Most calculators use 52 weeks. Some payroll systems use 52.14 (365 ÷ 7). The difference is tiny — about 0.3% — but if your employer’s payslip differs slightly from the calculator, this is usually why. Our free pro rata salary calculator uses the standard 52-week basis that matches most UK payroll.
What the Calculator Won’t Tell You
A salary calculator gives you gross pay — what you earn before deductions. Your take-home pay depends on income tax, National Insurance, pension contributions and student loan repayments. A £18,000 pro rata salary in the 2026/27 tax year, for example, sits below the personal allowance for much of the year, so your take-home will be close to gross if you have no other income.
The calculator also won’t account for overtime, bonuses, shift allowances or salary-sacrifice schemes — those are calculated separately on top of (or as deductions from) your basic pro rata salary.
Using the Calculator in Reverse: Hourly-Rate Mode
Many calculators also work backwards, which is useful when a job advert quotes an hourly rate instead of an annual salary. The logic is the same, just rearranged.
Say an advert offers £16.20 per hour for 24 hours a week. The calculator multiplies hourly rate × weekly hours × 52:
- £16.20 × 24 × 52 = £20,217.60 a year
From there you can derive the FTE figure if you want it: divide by your proportion of full-time hours. If full-time is 37.5 hours, your proportion is 24 ÷ 37.5 = 0.64, so the FTE salary is £20,217.60 ÷ 0.64 = £31,590. That tells you the role is roughly a £31.6k full-time job — useful context when comparing it against salaried adverts.
This reverse mode is also handy at pay-review time. If your employer offers a 50p hourly increase and you work 30 hours a week, that’s 0.50 × 30 × 52 = £780 a year extra — a concrete number you can weigh against the offer.
Comparing Two Offers Side by Side
One of the most practical uses of a pro rata calculator is comparing competing offers with different hours. Run each through the calculator and line up the results:
| Offer A | Offer B | |
|---|---|---|
| FTE salary | £28,000 | £30,000 |
| Full-time hours | 37.5 | 37 |
| Your hours | 30 | 22.5 |
| Pro rata salary | £22,400 | £18,243 |
| Hourly rate | £14.36 | £15.58 |
Offer A pays more overall (£22,400 vs £18,243), but Offer B has the better hourly rate (£15.58 vs £14.36). Which matters more depends on your priorities — total income or pay per hour. The calculator gives you both numbers so the trade-off is explicit rather than a gut feeling. Remember to factor in things the calculator can’t see, like commute costs, pension contributions and holiday entitlement, before deciding.
FAQs
Do I need to know my FTE salary to use the calculator?
Yes — or at least the hourly rate and full-time hours, from which the calculator can derive it. If a job advert says “£24,000 pro rata” without giving the full-time figure, ask the employer; you can’t calculate your pay accurately without it.
Why does my result differ slightly from my employer’s figure?
Usually it’s the 52 vs 52.14 week divisor, or a small difference in the full-time hours figure used. A gap of a few pounds a month is normal and not worth disputing; a gap of hundreds suggests an input error.
Can I use a pro rata calculator for hourly-paid roles?
Absolutely. Enter your hourly rate and weekly hours, and the calculator annualises it for you: hourly rate × hours per week × 52. This is useful for comparing an hourly offer against a salaried one.
Does pro rata affect my hourly rate?
No. Your hourly rate stays exactly the same as the full-time equivalent. Pro rata only reduces the number of hours you’re paid for. If a calculator shows a different hourly rate for part-time, its inputs are wrong.
Try It Yourself
Now you know exactly how to use a pro rata salary calculator — gather your four numbers, enter them carefully, and sense-check the result against the proportion. It takes less than a minute and it means you’ll never accept a part-time offer without knowing precisely what it pays.
Run your own figures through our free pro rata salary calculator and see your annual, monthly, weekly and hourly pay in one go.