Prorating PTO for Part-Time and Four-Day Weeks

Your first part-timer signs a four-day-week contract, and the spreadsheet does the obvious thing twice. Her vacation allowance is prorated to 80%. Then she books a week off in October and gets charged 5 days, because a week is a week. She has now lost on both ends of the same deal, and she can prove it with ten minutes and a calculator.

Call it the double-cheat. It happens because leave has two separate calculations, what you grant and what you charge, and most setups remember to adjust only one. Fair proration means getting both right, in the same unit, from the same source of truth.

The fairness test: a week off costs the same fraction of the year

Run a 26-day vacation policy through both arrangements:

• A full-timer gets 26 days and spends 5 for each week away: 26 / 5 = 5.2 weeks of leave.
• A Monday-to-Thursday employee at 0.8 FTE gets 26 × 0.8 = 20.8 days and spends 4 for each week away: 20.8 / 4 = 5.2 weeks.

Identical. Prorate the grant but keep charging 5 days a week and she gets 4.16 weeks: cheated. Charge her 4 days but forget to prorate the grant and she gets 6.5 weeks: now part-time quietly carries a leave bonus, and sooner or later the full-timers do this arithmetic too. Both mistakes come from treating one side of the ledger as an afterthought.

FTE counts days, not hours

The grant side runs on FTE, and the definition is where people slip. FTE should be a fraction of standard working days, not hours. Three full days a week is 0.6. Five short days is still 1.0, because that person needs a day off for every weekday; halving their hours didn't change how many days they must book to take a fortnight away.

The days rule also settles the four-day week. Whether it's reduced hours or the same hours compressed into four longer days, the person has four working days, a week away costs them 4, and 0.8 of the grant buys the same 5.2 weeks as everyone else. Leave is spent in days, so it has to be granted in days. Hours still matter, but for a different job, which we'll get to.

Schedules are history, not a setting

A schedule change can't be a simple edit to a profile field, because leave calculations refer to dates in the past and the future. Model it as a slice: which weekdays someone works, their hours (a weekly total, or a per-day map when days are uneven), and their FTE, all effective from a date. Changing an arrangement appends a new slice; the old one keeps governing the dates it always governed.

This matters the moment a request crosses a change. Each date in a request resolves whichever slice was in force on that date, so a booking that straddles 1 September counts its August days against the old five-day week and its September days against the new four-day one. And a date only charges the balance at all if it falls on a working weekday for its slice, and isn't a public holiday the policy excludes.

The mid-year switch, with real numbers

Meet Aleksandra. She joined on 1 March under a 26-day annual policy, so her prorated grant for the year was 21.5 days (the accrual post walks through that rounding). From 1 September she moves to Monday-Thursday at 0.8 FTE. Two things have to happen, and both are checkable.

The charge changes immediately. A week-long request after 1 September charges 4 days, not 5. Friday stopped being a working day for her, so it's excluded from the count the same way a weekend is.

The grant gets retargeted. Her 21.5 days assumed full FTE for the rest of the year, so the year is reweighted month by month against the schedule actually in force: full FTE from March through August, 0.8 from September through December. That lands on a target of 20.0 days. She holds 21.5, so a correction of 1.5 days posts against her balance as a visible ledger entry, not a silent rewrite.

Two rules keep corrections humane. They never claw back days already taken or reserved by a pending request; someone who already spent more than the new target keeps what they took. And they touch only the current year; if an earlier year's grant was also wrong, fix it with a deliberate manual adjustment, not a retroactive surprise.

Expected hours ride along

The same slice should feed timesheets. A day's expected hours come from the schedule in force that day: the per-day map if one is set, otherwise the weekly total split evenly across the working days. From September, Aleksandra's Friday expects 0 hours, so no report shows her mysteriously 8 hours short every week. This is the job hours do: they price the working day for timesheets, while FTE prices the year for leave.

Honest caveats

Local law wins. Some jurisdictions define statutory minimums in weeks or hours, and part-time protections vary by country. The days model maps cleanly onto most EU-style arrangements, but verify your floor before promising anyone a number. This post is arithmetic, not legal advice.

Corrections cut both ways. A move up to full-time mid-year adds days by the same reweighting, and a move down late in the year can push the target below what's already booked. The system's job is to make the number visible; deciding how generous to be about an overage is still a human call.

Sync the paperwork. The slice's effective date should match the contract amendment. If the contract says 1 September and the system says 15 September, every calculation between the two dates inherits the gap.

Where this lives in SquadBear

Everything above is shipped behavior. Each profile has a Schedule tab showing the current slice and its full history; people can see their own (managers, a managed employee's), and only an admin can change it, from the same tab. Requests resolve every date against the slice in force on that date, and saving a new slice recomputes the current year's grants on the spot, posting the correction exactly as in Aleksandra's numbers.

With an AI assistant connected, the whole switch is one instruction, correction included:

"Set Aleksandra's work schedule to Monday-Thursday, FTE 0.8, effective 1 September, and explain the correction it posts to her Vacation balance."

The employee-side sanity check is just as short:

"Show me my work schedule history and how many days a request for the last week of September would charge."

Start free and set up your first part-time schedule in minutes, or ask the demo to run Aleksandra's correction on sample data.

Related: PTO accrual, explained, for how grants, rounding and carryover work before schedules enter the picture.