Few payroll topics generate as many staff-room debates as overtime on a pro rata contract. If you work part-time and pick up extra hours, are you owed time-and-a-half from hour one — or only once you’ve passed full-time hours? The answer surprises most people, and getting it wrong costs real money in either direction. This guide sets out the rules, the maths, and how to check your own position.
The headline rule most part-timers don’t expect
Here’s the position under UK law as it stands: a part-time worker is generally only entitled to overtime premium rates once their total hours exceed the full-time equivalent — unless their contract says otherwise. This comes from the Part-time Workers (Prevention of Less Favourable Treatment) Regulations 2000 and the case law around them, notably the principle that comparing a part-timer working 25 hours with a full-timer working 37.5 isn’t comparing like with like until the hours match.
In plain terms: if full-time is 37.5 hours and you work 22.5, extra hours up to 37.5 are typically paid at your normal hourly rate (single time), not at an overtime premium. Only hours beyond 37.5 attract the premium — typically time-and-a-half or double time, depending on the employer.
Many employees find this unfair on first hearing. The legal reasoning is that the full-timer working 37.5 hours gets no premium for those hours either; the part-timer working extra hours at single time up to 37.5 is being treated the same as the full-timer, not worse. Whether you agree with the logic, it’s the framework most employers operate within.
The crucial caveat: your contract or employer’s overtime policy can be more generous, and many are. Some employers pay time-and-a-half for any hours above contracted hours; NHS Agenda for Change, for example, has its own detailed rules. Always start with your contract — statute is the floor, not the ceiling.
Worked example: extra shifts at single time vs premium
Let’s make this concrete. Nadia works 3 days a week — 22.5 hours — at £13.40 an hour (£15,681 pro rata). Full-time in her organisation is 37.5 hours. Her employer’s policy: single time up to 37.5 hours, time-and-a-half beyond, double time on Sundays. One month she works an extra 18 hours: 12 on weekdays, 6 on a Sunday.
Weekday extra hours (12): Her total for the week is 22.5 + 12 = 34.5 — still under 37.5. So all 12 are at single time: 12 × £13.40 = £160.80.
Sunday hours (6): Sunday premium applies regardless (it’s day-based, not hours-based in her policy): 6 × £13.40 × 2 = £160.80.
Total overtime pay: £321.60.
Now imagine she’d worked 20 extra weekday hours instead of 12: total 42.5 hours. The first 15 (up to 37.5) at single time = £201.00; the remaining 5 at time-and-a-half = 5 × £13.40 × 1.5 = £100.50. The premium only bites on the slice above full-time.
If Nadia’s contract had instead said “time-and-a-half for all hours above contracted hours”, those 12 weekday hours would have earned £241.20 instead of £160.80 — a difference of £80.40 in a single week. That’s why the contract wording matters so much. To check what your extra hours are worth under your own arrangement, our free pro rata salary calculator gives you the accurate hourly baseline to multiply from.
Table: how extra hours are typically paid
Assuming a 37.5-hour full-time week, £14/hour, and a standard policy (single time to 37.5, time-and-a-half beyond, double time Sundays/bank holidays):
| Your contracted hours | Extra weekday hours | Paid as | Overtime earned |
|---|---|---|---|
| 15 | 10 | 10 × single time | £140.00 |
| 22.5 | 10 | 10 × single time | £140.00 |
| 22.5 | 18 | 15 × single + 3 × 1.5× | £273.00 |
| 30 | 10 | 7.5 × single + 2.5 × 1.5× | £157.50 |
| 30 | 6 (Sunday) | 6 × double time | £168.00 |
| 37 (near full-time) | 5 | 0.5 × single + 4.5 × 1.5× | £101.50 |
The pattern is clear: the closer you are to full-time hours already, the sooner the premium kicks in — and Sunday/bank-holiday premiums are usually day-based, applying from the first hour.
Voluntary, compulsory, and guaranteed overtime
Not all overtime is equal, and the distinction affects your rights:
- Voluntary overtime (you choose whether to do it): the most common for part-timers picking up extra shifts. Paid per the policy above; no obligation either way.
- Compulsory overtime (your contract requires it when offered): rarer, but common in manufacturing and logistics. The rates are contractual — check the clause.
- Guaranteed overtime (the employer must offer it): usually for full-timers, but some part-time contracts include guaranteed extra hours. These count toward your regular pay for holiday-pay calculations.
Why does the distinction matter? Because regular overtime counts toward holiday pay. Following the Supreme Court’s line of case law, holiday pay must reflect “normal remuneration” — which includes overtime you work regularly. If you routinely do 10 extra hours a month, your holiday pay should reflect ~that, not just your contracted hours. Many employers’ payroll systems handle this automatically; some don’t. If your holiday pay looks like it’s based on contracted hours alone despite regular overtime, raise it.
There’s also the Working Time Regulations angle: you can’t be required to work more than 48 hours a week on average unless you’ve signed an opt-out. Voluntary extra shifts still count toward the 48-hour average — something to watch if you’re stacking shifts across two jobs.
Time off in lieu (TOIL): the alternative currency
Some employers offer TOIL instead of overtime pay — work an extra 8 hours, take 8 hours off later. For part-timers, check three things:
- The exchange rate. Is it hour-for-hour, or enhanced (e.g. 1.5 hours off per overtime hour)? Enhanced TOIL mirrors premium pay; hour-for-hour doesn’t.
- Expiry. Many policies require TOIL to be used within 3–6 months. Part-timers with fixed days can find it genuinely hard to schedule — negotiate this upfront if your working pattern is rigid.
- What happens to untaken TOIL. Does it convert to pay at year-end, or evaporate? Get it in writing.
TOIL suits people who value time over money; overtime pay suits the reverse. Neither is objectively better — but being given TOIL when you wanted pay (or vice versa) is a policy question worth asking about before you work the hours.
How to check you’re being paid correctly
Overtime errors are among the most common payroll mistakes, because the rules have thresholds and exceptions. A quick monthly audit:
- List your extra hours by category: weekday-under-FTE, weekday-over-FTE, weekend, bank holiday.
- Note the rate your policy assigns to each category.
- Multiply and total, then compare against the overtime line(s) on your payslip.
- Check the hourly baseline: divide a normal week’s basic pay by contracted hours — that’s the rate premiums should multiply. If it doesn’t match your assumed hourly rate, something’s off (often the 52 vs 52.14 annualisation — worth understanding in its own right).
Keep a simple log — dates, hours, rate applied. If a dispute arises months later, contemporaneous notes beat memory every time.
Frequently asked questions
Can my employer refuse to offer me overtime because I’m part-time?
They can’t exclude you from overtime opportunities because you’re part-time — that would be less favourable treatment under the Part-time Workers Regulations. But they don’t have to offer overtime to anyone; it depends on business need. If full-timers are routinely offered extra hours and you never are, ask why — and put the question in writing.
Do I get overtime premium for working a bank holiday?
Usually yes — bank-holiday premiums are typically day-based rather than hours-based, so they apply from your first hour that day. But the exact enhancement (time-and-a-half, double time, or a flat supplement) is contractual. Some employers offer a day off in lieu instead; check your policy before volunteering.
Does overtime affect my pro rata holiday entitlement?
Your entitlement (5.6 weeks pro rata) doesn’t change, but your holiday pay should reflect regular overtime. If you consistently work extra hours, holiday weeks should be paid at something close to your typical earnings, not just contracted basic pay. This is one of the most under-claimed entitlements among part-timers.
I’m on a zero-hours contract — is all my work “overtime”?
No — zero-hours workers have no contracted hours, so every hour is just… hours, paid at the agreed rate. Premium rates only apply where the contract or policy creates them (e.g. Sundays, bank holidays). The part-time overtime threshold concept doesn’t map onto zero-hours work because there’s no FTE baseline to measure against.
Know your rate, know your threshold
Overtime on a pro rata contract comes down to two numbers: your true hourly rate, and the full-time hours threshold where premiums begin. Get those right — from your contract, not from guesswork — and every extra shift’s pay becomes a simple multiplication. Start with an accurate baseline from our free pro rata salary calculator, check your employer’s overtime policy for the thresholds and premiums, and keep a log. That’s the whole system.