The pro rata salary formula looks simple on paper, but most people only really trust it once they’ve seen it working on real numbers. This guide does exactly that: it takes the standard formula and runs it through a series of worked examples covering the situations part-time workers actually face — different hour patterns, different full-time baselines, term-time contracts, and mid-year hour changes. Work through the ones that match your situation and you’ll know precisely what your pay should be.
The formula, briefly
For reference, the standard UK formula is:
Pro rata salary = (Full-time equivalent salary ÷ Full-time hours) × Your contracted hours
Every example below uses this formula. The only things that change are the three inputs. If you want the theory behind each part of the formula, that’s covered separately — here we’re all about the numbers.
Example 1: The straightforward case
Situation: Daniel works 4 days a week, 7.5 hours a day — 30 hours weekly. The full-time salary is £34,000 and the full-time week is 37.5 hours.
Step 1: £34,000 ÷ 37.5 = £906.67
Step 2: £906.67 × 30 = £27,200
Daniel’s pro rata salary: £27,200 per year (gross).
Monthly: £27,200 ÷ 12 = £2,266.67.
Check: 30 ÷ 37.5 = 0.8. £34,000 × 0.8 = £27,200. Confirmed.
This is the cleanest scenario: fixed hours, a standard full-time week, no complications. If your situation looks like Daniel’s, you can stop here — but most people’s don’t, so read on.
Example 2: A 40-hour full-time baseline
Situation: Aisha works 24 hours a week in a warehouse. The full-time salary is £27,000, but this employer’s full-time week is 40 hours, not 37.5.
Step 1: £27,000 ÷ 40 = £675
Step 2: £675 × 24 = £16,200
Aisha’s pro rata salary: £16,200 per year (gross).
Check: 24 ÷ 40 = 0.6. £27,000 × 0.6 = £16,200. Confirmed.
Notice how the baseline changes everything. If Aisha had assumed a 37.5-hour week, she’d have calculated 24 ÷ 37.5 = 0.64 and expected £17,280 — £1,080 more than she’s actually due. This is why the first question to ask any employer is always “what counts as full-time hours here?”
Example 3: Odd hours that don’t divide neatly
Situation: Tom works 17.5 hours a week in a café. Full-time salary £24,500, full-time week 37.5 hours.
Step 1: £24,500 ÷ 37.5 = £653.33
Step 2: £653.33 × 17.5 = £11,433.33
Tom’s pro rata salary: £11,433.33 per year (gross).
Monthly: £11,433.33 ÷ 12 = £952.78.
Check: 17.5 ÷ 37.5 = 0.4667. £24,500 × 0.4667 = £11,433.33. Confirmed.
Don’t round mid-calculation — keep the decimals until the final figure, then round to the nearest penny. Rounding early is how payslips end up a few pounds out.
Example 4: Term-time only
Situation: Grace works in a school office, 30 hours a week but term-time only — 39 weeks a year instead of 52. Full-time salary £25,000, full-time week 37 hours.
Term-time contracts need a two-stage calculation. First the hours fraction, then the weeks fraction:
Stage 1 — hours: 30 ÷ 37 = 0.8108
Stage 2 — weeks: 39 ÷ 52 = 0.75
Combined: 0.8108 × 0.75 = 0.6081
Pro rata salary: £25,000 × 0.6081 = £15,202.70
Grace’s pro rata salary: £15,202.70 per year (gross).
Her pay is typically spread evenly across 12 months (£1,266.89 a month) even though she doesn’t work in the holidays — that’s standard for term-time contracts and doesn’t mean she’s being paid for weeks she doesn’t work. The weeks fraction already accounts for it.
Example 5: Hours changing mid-year
Situation: Leo starts the tax year working 20 hours a week, then increases to 30 hours after 4 months. Full-time salary £31,000, full-time week 37.5 hours.
You can’t just average the hours — each period gets its own calculation:
Period 1 (4 months at 20 hours):
Annual equivalent: £31,000 × (20 ÷ 37.5) = £16,533.33
For 4 months: £16,533.33 × (4 ÷ 12) = £5,511.11
Period 2 (8 months at 30 hours):
Annual equivalent: £31,000 × (30 ÷ 37.5) = £24,800
For 8 months: £24,800 × (8 ÷ 12) = £16,533.33
Total for the year: £5,511.11 + £16,533.33 = £22,044.44
This is the example most payroll departments get asked to explain, because employees often expect a simple average. The split-period method is the correct one.
Example 6: Comparing two job offers
Situation: Nina has two offers and wants to compare them properly.
Offer A: £29,000 FTE, 25 hours a week, 37.5-hour full-time week.
£29,000 × (25 ÷ 37.5) = £29,000 × 0.6667 = £19,333.33
Offer B: £27,500 FTE, 22 hours a week, 35-hour full-time week.
£27,500 × (22 ÷ 35) = £27,500 × 0.6286 = £17,285.71
Offer A pays more in absolute terms — but look at the hourly rates:
– Offer A: £19,333.33 ÷ (25 × 52) = £14.87/hour
– Offer B: £17,285.71 ÷ (22 × 52) = £15.11/hour
Offer B actually pays a slightly better hourly rate. This is exactly the kind of comparison the formula makes possible — never compare FTE salaries directly when the hours differ.
Quick-reference table
What each common pattern pays on three different FTE salaries (37.5-hour week). A word of caution before you use it: tables like this are brilliant for ballpark figures, but they bake in one specific baseline — 37.5 hours. If your employer’s full-time week is 40 hours, every figure here will read high, and if it’s 35, every figure reads low. Always confirm the baseline first, then read the table. And remember these are gross annual figures: tax, National Insurance and pension still come off before you see a penny.
| Hours/week | Fraction | £26,000 FTE | £30,000 FTE | £35,000 FTE |
|---|---|---|---|---|
| 37.5 | 1.00 | £26,000 | £30,000 | £35,000 |
| 30 | 0.80 | £20,800 | £24,000 | £28,000 |
| 25 | 0.67 | £17,333 | £20,000 | £23,333 |
| 22.5 | 0.60 | £15,600 | £18,000 | £21,000 |
| 20 | 0.53 | £13,867 | £16,000 | £18,667 |
| 16 | 0.43 | £11,093 | £12,800 | £14,933 |
| 12 | 0.32 | £8,320 | £9,600 | £11,200 |
(Figures rounded to the nearest pound.)
Example 7: Compressed hours — full-time pay, fewer days
Compressed hours are the case that breaks people’s intuition. Situation: Jordan works full-time hours — 37.5 a week — but compressed into 4 long days instead of 5. The FTE salary is £31,000.
Run the formula: £31,000 × (37.5 ÷ 37.5) = £31,000 × 1.0 = £31,000.
Jordan gets the full salary despite working 4 days, because the formula cares about hours, not days. This is the mistake to avoid: seeing “4 days a week” and reaching for 4 ÷ 5 = 0.8. Days only matter when the hours differ.
Compare with someone genuinely part-time on the same rota pattern:
| Pattern | Hours/week | Fraction | Salary (£31,000 FTE) |
|---|---|---|---|
| Compressed full-time (4 long days) | 37.5 | 1.00 | £31,000 |
| Part-time (4 standard days) | 30 | 0.80 | £24,800 |
| Part-time (3 long days) | 28.5 | 0.76 | £23,560 |
Same number of days can mean very different pay. If you’re comparing a compressed-hours role against a standard part-time one, run both through a free pro rata salary calculator rather than eyeballing the day counts — the hours tell the true story.
Frequently asked questions
Why do two calculators give me slightly different answers?
Usually it’s the full-time baseline — one calculator assumes 37.5 hours and another assumes 40. A £500–£1,500 difference on the same inputs almost always traces back to that. Check which baseline each calculator uses before comparing results.
Should I round my hours up or down?
Neither — use your exact contracted hours, including fractions. If your contract says 22.5 hours, the calculation uses 22.5, not 22 or 23. Halves and quarters of hours are completely normal in these calculations.
Does overtime change the formula?
No. The formula uses your contracted hours only. Overtime is paid separately — usually at your normal hourly rate, or a premium rate if your contract says so — and doesn’t get folded into the pro rata calculation.
What if I’m paid monthly and my hours vary?
If your contracted hours are fixed, the formula works exactly as shown and your monthly pay is simply the annual figure divided by 12. If your hours genuinely vary week to week with no contracted minimum, you’re likely on a variable-hours arrangement and holiday pay — not the pro rata salary formula — is where the 12.07% accrual method comes in.
Run your own numbers
Pick the example closest to your situation, swap in your three numbers — FTE salary, full-time hours, your hours — and follow the steps. Then verify with the fraction method: your hours divided by full-time hours, multiplied by the FTE salary. If both routes agree, your figure is right. Keep a note of your working too — if a payslip ever disagrees with your calculation, having the steps written down makes the conversation with payroll far shorter. For a faster route, the free pro rata salary calculator handles all seven of these scenarios — including term-time, mid-year changes and compressed hours — without you touching a spreadsheet.