Irregular and zero hours holiday calculator

First, work out which set of rules applies to you

Before any calculation is worth doing, answer two questions: are you the kind of worker the 2024 reforms cover at all, and has your employer's leave year actually reached the date those reforms take effect for you? Both matter. Getting either wrong is the single most common reason this type of calculation comes out wrong.

Are you an "irregular hours worker"?

The Working Time Regulations define this precisely, at reg 15F(1)(a): your paid hours in each pay period are, under your contract, "wholly or mostly variable". In practice that's zero-hours staff, bank and on-call workers, and casual staff paid by the hour whose rota genuinely changes week to week. A contract that guarantees a fixed number of hours a week does not qualify here, even if those hours are low.

Are you a "part-year worker"?

This is a separate category, defined at reg 15F(1)(b): you're required under your contract to work only part of the year, with periods of at least a week that are unpaid and not worked. Crucially, this test says nothing about whether your hours vary — it's widely assumed that only "irregular hours" people are covered by the 2024 changes, and that assumption is wrong. A term-time classroom assistant on a permanent contract, working exactly the same 32.5 fixed hours every week she's in school, is a part-year worker in law. She has completely regular hours and is still inside reg 15F.

The old position: what Harpur Trust v Brazel decided

Until the 2024 reforms, every irregular-hours and part-year worker — including that term-time classroom assistant — sat under one set of rules, settled by the Supreme Court in Harpur Trust v Brazel [2022] UKSC 21, decided 20 July 2022. Mrs Brazel was a visiting music teacher on a permanent, term-time-only contract. Her employer paid her holiday pay as 12.07% of the hours she'd actually worked in each term — the same percentage this calculator can apply today. The Supreme Court held that was unlawful. The Working Time Regulations give every worker 5.6 weeks' leave, paid at a week's average pay calculated under the (now 52-week) reference period, with no statutory mechanism to pro-rate that figure down for someone who works fewer weeks than a full-year colleague. So her entitlement had to be calculated on the calendar-week method — 5.6 weeks at her average week's pay — regardless of how many weeks she'd actually worked to earn it, even though that produced a higher effective percentage of her annual earnings than 12.07% would have given her.

What changed on 1 April 2024

The Employment Rights (Amendment, Revocation and Transitional Provision) Regulations 2023 (SI 2023/1426) came into force on 1 January 2024 and inserted new regs 15B–15F and 16A into the Working Time Regulations. But the new regime only starts to apply, worker by worker, from the first leave year that begins on or after 1 April 2024 (reg 15B) — so an employer whose leave year runs 1 April to 31 March moved onto it immediately, while one running 1 July to 30 June didn't reach it until July 2024.

Accrual at 12.07%

For irregular hours and part-year workers whose leave year has reached that date, reg 15F now permits leave to accrue at 12.07% of the hours actually worked in each pay period — the exact method the Supreme Court had struck down two years earlier, now placed on a statutory footing for this defined group only.

Rolled-up holiday pay became lawful — for some people

New reg 16A goes further and permits rolled-up holiday pay: paying the 12.07% uplift with normal wages as it's earned, itemised as a separate line on the payslip, instead of paying it out when leave is actually taken. That, too, had previously been treated as unlawful under earlier case law. It is now lawful, but only for reg 15F workers, and only from their qualifying leave year onward.

Where the 12.07% figure actually comes from

12.07% isn't a number written into the regulations — it's arithmetic, and it's worth seeing done: 5.6 ÷ 46.4 = 0.1207, or 12.07%. The statutory entitlement is 5.6 weeks' leave (reg 13's 4 weeks plus reg 13A's 1.6 weeks). The denominator, 46.4, is what's left of a 52-week year once those 5.6 weeks of leave are taken out: 52 − 5.6 = 46.4. A worker who takes their full leave works, at most, 46.4 weeks a year — so expressing 5.6 weeks of paid leave as a percentage of those 46.4 working weeks, not of all 52, is what converts an annual entitlement into a per-hour-worked accrual rate that can be applied pay period by pay period. Divide by 52 instead and you'd under-accrue leave every single time.

A worked example under each regime

Take that term-time classroom assistant again — call her Priya — on a permanent contract: 32.5 hours a week at £12.00 an hour, worked across 39 term-time weeks a year, nothing paid or worked in the remaining 13. Her leave year runs from 1 September to 31 August. That gives weekly pay of £390.00 while she's working, and total annual earnings of £390.00 × 39 = £15,210.00.

Method	Basis	Holiday pay
Calendar-week (Harpur Trust)	5.6 × £390.00 average week's pay	£2,184.00
12.07% accrual (reg 15F)	12.07% × £15,210.00 annual pay	£1,835.85

Same worker, same hours, same pay — but £348.15 less under the accrual method, a 15.9% cut. That gap is not a rounding artefact: it's exactly the shortfall the Supreme Court identified in Mrs Brazel's own case, now reinstated by statute as lawful for anyone who falls inside reg 15F from the right leave year onward.

If you're outside the new rules

None of this makes 12.07% generally safe to use. It is lawful in exactly one combination of circumstances: you are an irregular hours worker or a part-year worker under reg 15F(1), and your current leave year began on or after 1 April 2024. Outside that combination — a fixed-hours, full-year employee; a part-year worker whose employer hasn't yet reached a qualifying leave year; or anyone assessing entitlement for a leave year that started before the cut-off — Parliament did not touch the underlying law at all. Harpur Trust v Brazel still binds those cases in full, and paying 12.07% of hours worked is exactly the practice the Supreme Court found unlawful in 2022. Check your leave year start date before choosing a method — it isn't a formality, it's the fact that decides which figure is correct.