37.5 vs 40-Hour Week: Pro Rata Compared

Here’s a question that catches out thousands of part-time workers every year — the pro rata 37.5 vs 40 hour week debate: if two jobs advertise the same full-time salary but one runs a 37.5-hour week and the other runs 40 hours, does it matter? The answer is yes — enormously. The full-time baseline is the hidden variable in every pro rata calculation, and the gap between a 37.5-hour week and a 40-hour week can be worth well over £1,000 a year for the same part-time hours. This guide compares the two directly so you can see exactly what’s at stake.

Why the baseline matters

The pro rata 37.5 vs 40 hour week question comes down to one fact: your salary is a fraction of the full-time figure, and the size of that fraction depends on what “full-time” means at your employer. Work 30 hours somewhere with a 37.5-hour week and you’re 0.8 of full-time. Work the same 30 hours somewhere with a 40-hour week and you’re 0.75. Same hours, same FTE salary, different pay.

Neither baseline is more “correct” — there’s no UK law fixing the full-time week. The NHS standardised on 37.5 hours. The civil service generally uses 37. Much of the private sector uses 40. What matters is which one applies to your contract, because every pro rata figure flows from it.

Head-to-head: the same hours, both baselines

Let’s take a £30,000 FTE salary and compare what identical part-time hours pay under each baseline:

The pattern is consistent: the 37.5-hour baseline pays about 6.7% more for any given part-time hours, because each of your hours represents a bigger slice of a shorter full-time week. On a £30,000 FTE salary at 30 hours a week, that’s £1,500 a year — £125 a month — for doing exactly the same work.

Worked comparison: two real offers

The situation: Maya is choosing between two admin roles, both advertised at £28,000 FTE, both 24 hours a week. Employer A runs a 37.5-hour week; Employer B runs 40 hours.

Employer A: 24 ÷ 37.5 = 0.64 → £28,000 × 0.64 = £17,920
Employer B: 24 ÷ 40 = 0.60 → £28,000 × 0.60 = £16,800

The gap: £1,120 a year, or £93.33 a month. The adverts looked identical — “£28,000 pro rata, 24 hours” — but the baselines make Employer A the better-paying job by a clear margin.

Now the hourly view, which tells the deeper story:
– Employer A: £17,920 ÷ (24 × 52) = £14.36/hour
– Employer B: £16,800 ÷ (24 × 52) = £13.46/hour

Employer A pays nearly a pound an hour more. When FTE salaries match, always compare the baseline before you compare anything else.

But wait — is the 40-hour job always worse?

Not necessarily, and this is where people oversimplify. A longer full-time week usually means one of two things, and you need to know which:

Scenario 1: Same FTE salary, longer week. Employer B pays £28,000 for 40 hours while Employer A pays £28,000 for 37.5. Here the 37.5-hour employer genuinely pays a better hourly rate (£14.36 vs £13.46 in Maya’s case). The 37.5-hour job wins outright.

Scenario 2: Higher FTE salary compensating for the longer week. Some 40-hour employers set higher FTE salaries precisely because the week is longer. If Employer B advertised £30,000 FTE for its 40-hour week: 24 ÷ 40 = 0.6 → £18,000. That beats Employer A’s £17,920. The baseline alone doesn’t decide it — you have to run both numbers.

The rule: never compare FTE salaries without the baseline, and never compare baselines without the FTE salary. You need both numbers, every time.

Which sectors use which?

There’s no rule, but there are strong conventions:

• 37.5 hours: NHS, most of the wider public sector, many charities, universities
• 37 hours: Civil service, local government (NJC)
• 40 hours: Retail, hospitality, warehousing, much of the private sector
• 35 hours: Some London offices, a handful of progressive employers

If you’re moving between sectors — say from an NHS role to a private clinic — expect the baseline to change, and re-run your pro rata figures from scratch rather than assuming your old fraction still applies.

The part-time trap: “full-time” isn’t always what you think

A subtle trap: some employers describe a role as “full-time” at hours below their own standard week, or advertise part-time roles against an unstated baseline. Red flags to watch for:

• An advert that says “pro rata” but never states the full-time hours
• “Full-time” described as 35 hours in one paragraph and 37.5 in another
• A baseline that changes between the advert, the interview, and the contract

Get the baseline in writing — ideally in the contract itself — before you accept. If the advert is vague, ask directly: “What are the full-time weekly hours this pro rata figure is based on?” It’s a completely normal question and any decent employer answers it immediately.

Comparison table: £32,000 FTE across baselines

A second reference table for a different salary level, showing monthly gross too:

The baseline also sets your overtime threshold

There’s a knock-on effect most people miss: the full-time baseline doesn’t just set your pro rata salary — it usually sets the point where overtime rates kick in. And the two baselines treat extra hours very differently.

In most contracts, overtime (paid at a premium, often time-and-a-half) only starts once you exceed full-time hours. Hours between your part-time contract and the full-time threshold are typically paid at your normal rate.

The situation: Two colleagues each contracted for 25 hours pick up an extra 15 hours in a busy month — 40 hours total that week. FTE salary £30,000 for both.

On the 37.5-hour baseline: 37.5 hours at normal rate, 2.5 hours at overtime rate. Normal hourly: £30,000 ÷ (37.5 × 52) = £15.38. That week’s extra pay: 12.5 × £15.38 + 2.5 × £23.08 = £192.25 + £57.70 = £249.95.

On the 40-hour baseline: All 40 hours at normal rate — the overtime threshold hasn’t been reached. Normal hourly: £30,000 ÷ (40 × 52) = £14.42. Extra pay: 15 × £14.42 = £216.30.

The gap: £33.65 for one week — and it repeats every busy week. The shorter baseline doesn’t just raise your base salary; it gets you into premium overtime rates sooner. When comparing offers, ask where the overtime threshold sits — it’s part of the package the headline salary doesn’t show. You can model both scenarios in the free pro rata salary calculator to see the full picture before deciding.

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 · part-time hours tables · pro rata hours · days vs hours · examples by days per week

Frequently asked questions

Can I negotiate the baseline?

Not usually — the full-time week is set at employer level, not per role. What you can negotiate is the FTE salary itself, or your contracted hours. If a 40-hour employer won’t move on salary, asking for an extra hour or two a week at the same pro rata fraction is sometimes an easier win.

Does the baseline affect holiday entitlement too?

Yes, in the same proportion. Holiday is usually quoted in days or hours per year for full-timers and scaled identically — 0.6 of the hours means 0.6 of the holiday. The baseline feeds both calculations, so getting it right matters twice.

What if my employer changes the full-time week?

If your employer moves from 37.5 to 40 hours (or vice versa), your pro rata salary must be recalculated — and if your hours stay the same while the baseline lengthens, your pay drops unless the FTE salary is adjusted to compensate. This counts as a contract change and should be agreed with you, not imposed silently.

Is a 37.5-hour week better for part-timers in every case?

For the same FTE salary, yes — unambiguously. But FTE salaries are often set with the local market and the working week in mind, so compare complete packages: baseline and FTE salary and benefits. The maths only answers the question you actually ask it.

Compare properly before you sign

Whenever you’re weighing part-time offers, run both numbers through the formula — FTE salary divided by that employer’s full-time hours, times your hours — and compare the results, not the adverts. The free pro rata salary calculator lets you plug in different baselines side by side, so a 37.5-hour offer and a 40-hour offer can be judged on what they’ll actually pay you, not what they appear to promise.

Related reading

• How to Check Pro Rata Pay on Your Payslip
• Starting Mid-Year: Pro Rata for Part-Year Workers