Guide
Carrying over annual leave: the 8 day rule and its exceptions
Updated
Carry-over is where a leave tracker earns its keep, because the balance that matters is not the one on 31 December: it is the one at the end of your leave year, and how much of it survives the boundary.
The general rule
gov.uk states that "workers can normally carry over a maximum of 8 days into the next leave year, with the agreement of their employer".
Two conditions sit in that sentence. It is a maximum, not an entitlement, and it needs agreement. An employer who declines is not doing anything unlawful in the ordinary case, which is why a leave year ending with a large untaken balance is a management problem rather than a legal one.
Family leave is different
Where a worker cannot take their statutory holiday because they are on maternity or other family related leave, gov.uk states that "an employer must allow" them to carry over all their holiday entitlement to the following leave year.
That is an obligation rather than a discretion, and it covers the whole entitlement rather than eight days of it. It is the single carry-over rule most often got wrong, and getting it wrong takes leave away from the group least able to argue about it at the time.
Sickness
Long-term sickness has its own treatment and it is not the eight day rule. Where a worker has been unable to take leave because of sickness absence, carry-over is permitted on different terms and over a longer horizon.
If you are handling a specific case, read the position in full rather than applying the general maximum: this is an area where the general rule and the correct answer point in different directions.
Practical consequence for a tracker
A tracker that only reports the current year's balance will tell somebody they have nine days left in March and nothing about the fact that eight of them are the maximum that can move.
The useful figure late in a leave year is the balance capped at the carry-over maximum, which is what the tool above reports alongside the raw balance. The difference between the two is the leave that will be lost if nobody books it.