When employers calculate pro rata pay, most use one of two methods: by days or by hours. They usually give the same answer — but not always, and when they differ, the gap can be worth hundreds of pounds a year. If you’ve ever wondered about pro rata by days or hours and which method is fairer, this guide compares both head to head, shows you when they diverge, and tells you which one to insist on.
The two methods
By hours: Pro rata salary = (FTE salary ÷ Full-time weekly hours) × Your weekly hours
By days: Pro rata salary = (FTE salary ÷ Full-time weekly days) × Your weekly days
Both are lawful. Both are widely used. The hours method dominates in retail, hospitality, healthcare and anywhere shifts vary in length. The days method is common in schools, councils and offices where everyone works standard-length days.
When they agree
Take the simplest case: every working day is the same length. Full-time is 5 days × 7.5 hours = 37.5 hours. You work 3 days × 7.5 hours = 22.5 hours. FTE salary £30,000.
By hours: £30,000 × (22.5 ÷ 37.5) = £30,000 × 0.6 = £18,000
By days: £30,000 × (3 ÷ 5) = £30,000 × 0.6 = £18,000
Identical. When days are uniform, the two methods are mathematically the same calculation wearing different clothes — 3 days of 7.5 hours is 22.5 hours, so of course the fractions match. Most of the time, this is the situation, and the debate is academic.
When they diverge: unequal day lengths
The methods split apart the moment your days aren’t the same length as a full-timer’s days. And in part-time work, they very often aren’t.
The situation: Sam works 3 days a week, but long ones — 10 hours each, 30 hours total. The full-time pattern is 5 days × 7.5 hours = 37.5 hours. FTE salary £28,000.
By hours: £28,000 × (30 ÷ 37.5) = £28,000 × 0.8 = £22,400
By days: £28,000 × (3 ÷ 5) = £28,000 × 0.6 = £16,800
The gap: £5,600 a year. The days method values each of Sam’s days the same as a 7.5-hour full-timer’s day, silently discarding 2.5 hours per day — 7.5 unpaid hours every week. This is the single most expensive mistake in pro rata calculation, and it always works against the part-timer.
A subtler divergence: the short Friday
The situation: Elena works Monday to Thursday, 8 hours a day — 32 hours. Her employer’s full-time week is Monday to Friday, but Friday is a short day: 4 days × 8 hours + 4 hours = 36 hours over 5 days. FTE salary £33,000.
By hours: £33,000 × (32 ÷ 36) = £33,000 × 0.8889 = £29,333.33
By days: £33,000 × (4 ÷ 5) = £33,000 × 0.8 = £26,400
The gap: £2,933.33 a year. Elena works four full days while full-timers work four full days plus a short Friday — but the days method treats her four days as 80% of five equal days. The hours method captures reality; the days method doesn’t.
Side-by-side comparison table
Using a £30,000 FTE salary, 37.5-hour / 5-day full-time week:
| Your pattern | Hours method | Days method | Winner for you |
|---|---|---|---|
| 3 × 7.5h days (22.5h) | £18,000 | £18,000 | Tie |
| 3 × 10h days (30h) | £24,000 | £18,000 | Hours (+£6,000) |
| 4 × 7.5h days (30h) | £24,000 | £24,000 | Tie |
| 2 × 12h days (24h) | £19,200 | £12,000 | Hours (+£7,200) |
| 5 × 6h days (30h) | £24,000 | £30,000 | Days (+£6,000) |
| 4 × 6h days (24h) | £19,200 | £24,000 | Days (+£4,800) |
Look at the last two rows: when your days are shorter than full-time days, the days method overpays relative to hours worked. The pattern is simple — whichever method counts your actual working time more generously favours you, and the hours method is the one that always reflects reality.
Which method should you want?
The hours method, almost always. It’s precise, it handles every pattern — long days, short days, split shifts, compressed weeks — and it can’t silently undervalue your time. If your days are longer than standard, insist on it.
The days method is fine when every day in the building is the same length, which covers a lot of office and school roles. It’s simpler to administer and, in those cases, gives exactly the same answer.
Watch out when an employer defaults to the days method and your days run long. That’s the £5,600-a-year trap from Sam’s example. Ask how the calculation is done before you sign — “do you calculate pro rata by days or by hours?” — and if the answer is days while your shifts are longer than standard, query it.
What the law says
There’s no statute mandating one method. What the law requires — under the Part-time Workers Regulations — is that part-timers aren’t treated less favourably than comparable full-timers. A days-based calculation that systematically pays you less per hour than a full-timer, purely because your days are longer, is on shaky ground. You don’t need to quote legislation in a pay query; just ask the employer to show that their method values your hours equally. The hours method passes that test by construction.
How to check which method was used on you
Grab your payslip or offer letter and reverse-engineer it:
- Divide your quoted salary by the FTE salary — that’s the fraction they applied.
- Divide your weekly hours by full-time weekly hours — that’s the hours fraction.
- Divide your weekly days by full-time weekly days — that’s the days fraction.
Whichever of (2) or (3) matches the fraction from (1) is the method they used. If neither matches, ask for their working — something else is going on.
Quick example: You’re quoted £21,000 against a £30,000 FTE. Fraction applied: 0.7. You work 28 hours of a 40-hour week: 28 ÷ 40 = 0.7 — hours method. You work 3.5 days of a 5-day week: 3.5 ÷ 5 = 0.7 — also consistent. Either method gives 0.7 here, so no conflict.
Resolving a method dispute: a practical walkthrough
If you’ve run the numbers and suspect the days method has undervalued your long shifts, here’s how to raise it without turning it into a confrontation:
Step 1 — Gather your evidence. Your contract (stating your weekly hours and the FTE salary), plus the employer’s stated full-time week. Calculate both methods yourself so you can show the exact gap in pounds.
Step 2 — Ask for their working, in writing. Email payroll or HR: “Could you confirm whether my pro rata salary was calculated by days or by hours, and share the calculation?” This is a reasonable request — you’re not accusing anyone, you’re checking.
Step 3 — Present the comparison calmly. “By hours, my 30-hour week on a 37.5-hour baseline gives £22,400; the days method gives £16,800. My shifts are 10 hours against a 7.5-hour standard day, so the days method doesn’t reflect my actual working time.” Numbers persuade better than adjectives.
Step 4 — Know your fallback. If they won’t budge, you can raise a formal grievance, and ultimately the Part-time Workers Regulations are on your side where a method systematically underpays part-timers per hour. In practice, most disputes settle at step 2 or 3 — payroll errors are usually just that: errors.
Before you send anything, double-check your own maths with an independent tool — the free pro rata salary calculator works strictly from hours, so it gives you a clean second opinion to quote.
Other working patterns: 20 hours a week · 24 hours a week · 30 hours a week · 37.5-hour week · 40-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 · examples by days per week
Frequently asked questions
Can my employer switch methods after I’ve started?
Only with your agreement, as it changes your contractual pay. If a switch from hours to days would cut your salary, that’s a pay cut by another name — raise it formally and ask for the rationale in writing.
What about term-time contracts — days or hours?
Term-time calculations layer a weeks fraction on top (weeks worked ÷ 52), and the days/hours question applies to the first stage exactly as described here. Term-time staff working long days are the most exposed to the days-method trap, so check carefully.
Does the method affect my hourly rate for overtime?
It shouldn’t, but it can in practice. Overtime is usually paid at your normal hourly rate — which should be derived from the hours method (annual salary ÷ annual hours). If your base salary was calculated by days on long shifts, make sure overtime isn’t also undervalued. Derive the hourly rate from actual hours: salary ÷ (weekly hours × 52).
Which method do payroll systems use?
Most modern payroll software calculates from hours, because hours are what the system actually records. Days-based calculation is more often a manual HR convention, particularly in education. If your payslip figure looks like a days-based result, it’s worth a conversation — the system of record may disagree with HR’s spreadsheet.
Know your method, protect your pay
Two lawful methods, one right answer for your situation: the method that counts your actual hours. Work out both fractions for your own pattern — your hours over full-time hours, and your days over full-time days — and see whether they agree. If they don’t, you now know exactly how much the choice is worth. To run the hours-based calculation precisely, use the free pro rata salary calculator — it works from your real weekly hours, so unequal day lengths can never slip through the cracks.