You don’t need a spreadsheet, an app, or even a calculator to work out a pro rata salary. The maths is simple enough to do with pen and paper — and with a couple of shortcuts, simple enough to do in your head. This guide shows you exactly how to calculate pro rata manually, step by step, plus the mental arithmetic tricks that payroll professionals use when they need a quick figure without reaching for a device.
Why bother doing it by hand?
Two reasons. First, understanding the manual method means you actually understand what the numbers mean — which makes it much harder for an employer (or a dodgy online calculator) to hand you a wrong figure unchallenged. Second, there are times you just need a fast answer: reading a job advert on the train, comparing two offers over coffee, or sense-checking a payslip before a meeting. A two-minute manual calculation beats guessing every time.
What you need
Three numbers, all of which should be in the job advert or your contract:
- The full-time salary — the FTE figure, e.g. £28,000
- The full-time weekly hours — e.g. 37.5
- Your weekly hours — e.g. 22.5
And the method:
Pro rata salary = (Full-time salary ÷ Full-time hours) × Your hours
The pen-and-paper method, step by step
Let’s use a realistic example: FTE salary £27,500, full-time week 37.5 hours, your hours 25.
Step 1 — Work out the fraction first. Before touching the salary, divide your hours by the full-time hours:
25 ÷ 37.5 = 0.6667 (two-thirds, near enough)
Doing the fraction first is the key trick of the manual method. It turns the whole problem into one multiplication instead of a division followed by a multiplication, and fractions like this are usually friendlier numbers to work with.
Step 2 — Multiply the salary by the fraction:
£27,500 × 0.6667 = £18,333.33
That’s your pro rata salary: £18,333.33 a year.
Step 3 — Monthly, if you need it:
£18,333.33 ÷ 12 = £1,527.78 per month (gross).
The fraction-first approach works because multiplication is commutative — (A ÷ B) × C is the same as A × (C ÷ B). Same answer, easier arithmetic.
Mental maths shortcuts
Shortcut 1: Think in familiar fractions
Most part-time patterns reduce to fractions you already know:
| Pattern | Fraction | What to do with the salary |
|---|---|---|
| Half-time | 1/2 | Halve it |
| 4 days out of 5 | 4/5 | Multiply by 0.8 |
| 3 days out of 5 | 3/5 | Multiply by 0.6 |
| 2 days out of 5 | 2/5 | Multiply by 0.4 |
| 3 days out of 4 | 3/4 | Multiply by 0.75 |
So a £32,000 FTE role at 3 days a week: £32,000 × 0.6 = £19,200. Done in seconds.
Shortcut 2: The 10% method
For awkward percentages, build up from 10%. Say you work 22.5 hours of a 37.5-hour week — that’s 60%.
10% of £30,000 = £3,000. 60% = 6 × £3,000 = £18,000.
This works for any percentage: find 10%, then multiply. It’s the fastest reliable mental method for figures that don’t reduce to neat fractions.
Shortcut 3: Round, calculate, adjust
For a quick estimate, round the salary to something friendly, calculate, then adjust. FTE £28,750 at 60%? Round to £29,000: 60% = £17,400. The true figure is £17,250 — your estimate is within 1%, easily close enough for comparing job adverts.
Shortcut 4: Work out the hourly rate first
Some people find it easier to think hourly. £30,000 a year on a 37.5-hour week:
Annual hours: 37.5 × 52 = 1,950. Hourly: £30,000 ÷ 1,950 = £15.38.
Then your pay is just £15.38 × your weekly hours × 52. For 20 hours: £15.38 × 20 × 52 = £15,995.20 (the penny difference from £16,000 is rounding — the exact figure is £16,000).
A full pen-and-paper walkthrough
The situation: You’re reading an advert on your phone. FTE salary £26,000, full-time week 37 hours, the role is 28 hours a week. No calculator to hand.
Step 1 — the fraction: 28 ÷ 37. Long division: 37 goes into 28 zero times, so think of it as 280 ÷ 37 ≈ 7.57, meaning 0.757. Call it 0.76 for a manual estimate.
Step 2 — apply it: £26,000 × 0.76. 10% of £26,000 = £2,600; 70% = £18,200; 6% = £1,560. Total: £19,760.
The estimate: roughly £19,760 a year. The exact figure (using 28 ÷ 37 = 0.7568) is £19,675.68 — your two-minute paper calculation landed within £85. Good enough to decide whether the job is worth applying for.
Common manual mistakes to avoid
Dividing by 52 when you shouldn’t. The salary formula doesn’t need the number of weeks — it works directly on weekly hours. Only bring 52 into it if you’re converting to or from hourly rates.
Using monthly hours. Don’t multiply weekly hours by 4 to get “monthly hours” — months average 4.33 weeks. If you must work monthly, use weekly hours × 52 ÷ 12.
Forgetting the baseline. The most common error in manual calculations: assuming 40 hours when the employer uses 37.5. Before you start, confirm the full-time week — it changes every number downstream.
Rounding too early. Keep at least two decimal places through every intermediate step. Round once, at the very end, to the nearest penny.
Practice table: test yourself
Cover the answers, work each out by hand, then check. Try the fraction-first method on each one — it’s the fastest route when you’re without a calculator, and it keeps the arithmetic to a single multiplication.
| FTE salary | FT hours | Your hours | Answer |
|---|---|---|---|
| £24,000 | 37.5 | 18.75 | £12,000 |
| £32,000 | 40 | 32 | £25,600 |
| £28,500 | 37.5 | 15 | £11,400 |
| £35,000 | 37 | 29.6 | £28,000 |
| £22,000 | 35 | 21 | £13,200 |
(Working: 18.75 ÷ 37.5 = 0.5; 32 ÷ 40 = 0.8; 15 ÷ 37.5 = 0.4; 29.6 ÷ 37 = 0.8; 21 ÷ 35 = 0.6.)
Working backwards: from pro rata salary to FTE
Sometimes you need the formula in reverse — typically when an advert quotes only the actual part-time salary and you want to know the full-time equivalent, either to compare offers or to check the maths. The reverse is just as simple:
FTE salary = Pro rata salary ÷ Your fraction of full-time
The situation: An advert offers £19,250 for 28 hours a week. The employer’s full-time week is 35 hours. What’s the FTE salary?
Step 1 — the fraction: 28 ÷ 35 = 0.8
Step 2 — reverse it: £19,250 ÷ 0.8 = £24,062.50 FTE
Now you can compare that £24,062.50 against other FTE figures on equal terms. This reverse check is also how you audit an employer’s figure: take the salary they’ve quoted you, divide by your fraction, and see if the FTE it implies matches the FTE they advertised. If it doesn’t, one of the numbers is wrong — and it’s worth finding out which before you sign.
A handy mental shortcut for the reverse: dividing by 0.8 is the same as multiplying by 1.25; dividing by 0.6 is the same as multiplying by 1.667; dividing by 0.5 is just doubling. So £19,250 at 0.8 → £19,250 × 1.25 = £24,062.50. Same answer, no long division.
If you’d rather not do the reverse arithmetic at all, the free pro rata salary calculator lets you enter any two of the three numbers and solves for the third — FTE salary, your hours, or the pro rata figure.
Frequently asked questions
Is the manual method as accurate as a calculator?
Yes — it’s the same arithmetic. Calculators only win on speed and on avoiding transcription errors with long decimals. If your manual figure and a calculator disagree, check the full-time baseline first; that’s the culprit nine times out of ten.
What’s the quickest way to estimate in my head?
The 10% method. Work out 10% of the FTE salary (move the decimal point one place left), estimate your fraction of full-time as a percentage, and multiply. Two steps, no division of awkward numbers.
Do I need to account for tax when calculating by hand?
The pro rata formula gives gross pay — tax comes afterwards and is a separate calculation. For a rough net figure by hand, deduct roughly 20% for basic-rate tax and National Insurance combined on amounts above the personal allowance — but treat that as a ballpark only.
What if my hours aren’t a round number?
Use them exactly as contracted. 23.5 hours, 17.25 hours — the formula handles fractions fine. The fraction-first method is especially helpful here: 23.5 ÷ 37.5 = 0.6267, then multiply.
When to let a tool do it
Manual calculation is perfect for estimates, adverts, and sense-checks. But for anything contractual — signing an offer, disputing a payslip, splitting a year across two hour patterns — use a proper tool and keep the working. The free pro rata salary calculator runs the exact same formula shown here, handles the fiddly cases like mid-year changes automatically, and gives you a figure you can quote with confidence.