Part of a year, worked out from the dates
Somebody who starts or leaves part way through the leave year gets the proportion of the annual entitlement that matches the part of the year they were there. The arithmetic is simple. What goes wrong is everything around it, which year, counted how, and rounded which way.
Count days, not months
Months are not the same length, so counting them treats unequal periods as equal. Somebody in post through February and somebody in post through March have both done "one month" and one of them worked three days longer. Over a couple of months that is a rounding error. Over a part-year it is a day of holiday.
This counts the actual days of the leave year covered, inclusive of both the first and last day in post, and divides by the length of the leave year. That is 365 days, or 366 when the leave year happens to span a leap day.
Round up, never down
On termination, regulation 14 of the Working Time Regulations 1998 requires the figure to be rounded up to the nearest half day. There is no discretion in it. Rounding a leaver's holiday down takes money off them, and a payroll set to round to the nearest half day will get it wrong roughly half the time.
The leave year is not the tax year
Whose year matters is the employer's leave year, which they set. Plenty run 1 January to 31 December, plenty run 1 April to 31 March, and some run from each person's own start date. If a contract does not say, the default under regulation 13 is the anniversary of the start date.
That distinction is not academic. Which accrual rules apply to irregular hours staff depends on when the leave year started rather than on today's date, so an employer running a January leave year was on the old rules for months after the April 2024 change.
The first year works differently
During somebody's first leave year, regulation 15A lets an employer restrict leave to what has accrued: one twelfth of the annual entitlement on the first of each month. It accrues in whole twelfths in advance rather than day by day, so somebody one day into their third month already has three twelfths.
That is a limit on what can be taken early, not a cut to what is owed for the year. Somebody who works the whole of their first leave year still gets the full pro rata entitlement for the part of the year they were employed.
Changing a working pattern mid-year
If somebody drops from five days to three part way through, the accepted approach is to treat it as two periods and pro rate each: five days for the months worked on five, three for the rest. Run this twice and add the results. Leave already taken on the old pattern is not retrospectively revalued.
Nothing here is legal advice, and a contract can always be more generous than the statutory minimum. Where a page cites a regulation it names it so you can check it.
Names go in, and names are the part that matters
Dates, patterns and names stay on the page and are discarded when you close the tab. There is no account and nothing is uploaded.
Common questions
Why count days rather than months for pro rata holiday?
Because months are not the same length, so counting in months treats unequal periods as equal. Somebody in post through February and somebody in post through March have both technically done "one month", but one of them worked three days longer, and over a part year that becomes a day of holiday.
Should a leaver's holiday be rounded up or down?
Up, always, on termination. Regulation 14 of the Working Time Regulations 1998 requires the figure to be rounded up to the nearest half day with no discretion, so a payroll system set to round to the nearest half day rather than always up will get it wrong roughly half the time.
Is the leave year the same as the tax year?
No. The leave year is whatever the employer sets it to be, commonly 1 January to 31 December, 1 April to 31 March, or each person's own start date anniversary. If a contract does not say, regulation 13 defaults it to the anniversary of the start date.
Can an employer restrict how much holiday a new starter takes in their first year?
Yes. Regulation 15A lets an employer limit leave taken in the first leave year to what has accrued, one twelfth of the annual entitlement on the first of each month, building up in whole twelfths in advance. That only limits what can be taken early, not what is owed: someone who works the whole first leave year still gets the full pro rata entitlement for the time they were employed.
How do I work out holiday if someone changed their working pattern mid-year?
Treat it as two separate periods and pro rate each one against the pattern that applied at the time, for example five days for the months worked on five and three days for the months worked on three, then add the two results together. Leave already taken under the old pattern is not revalued afterwards.
Why does the leave year start date matter beyond just counting days?
Because which accrual rules apply to irregular hours staff depends on when the leave year started rather than on today's date. An employer running a leave year starting in January, for instance, remained on the old accrual rules for months after the April 2024 change took effect elsewhere.
Is this calculation legal advice?
No. It works out the statutory minimum from the dates given, and a contract can always be more generous than that minimum. Every regulation this cites is named so it can be checked directly.