Guide
Shift pattern calculator: how the average week is worked out
Updated
Almost every argument about whether a shift pattern is legal is really an argument about which number to compare with 48. The calculation is one line, and getting it wrong in either direction is expensive: overestimate and you buy crews you do not need, underestimate and you run an unlawful rota.
The formula
Average weekly hours are days on, divided by the cycle length, multiplied by seven, multiplied by the shift length.
For 4 on 4 off on 12 hour shifts that is 4 over 8, times 7, times 12, which is 42 hours a week. The intuition that a 12 hour shift pattern must be near the 48 hour limit is wrong by six hours, because the pattern only works half the days in the cycle.
The same ratio gives the same answer whatever the cycle length. 3 on 3 off and 7 on 7 off both average 42 hours on 12 hour shifts, because both work half the days. Cycle length changes how the pattern lands on the calendar, not how many hours it produces.
Where the arithmetic is exact and where it is not
It is exact for a repeating cycle measured over whole cycles. It stops being exact the moment overtime, callouts, shift swaps or training days are added, and every one of those pushes the average up rather than down.
The limit is measured on working time, so unpaid breaks come out of the shift length before the calculation if your reference period excludes them. A 12 hour shift with an hour of unpaid break is 11 hours of working time and averages 38.5 hours a week rather than 42.
The reference period matters as much as the average
gov.uk states the limit as 48 hours a week on average "normally averaged over 17 weeks". A pattern is not unlawful because one week in it exceeds 48 hours. It is unlawful if the average over the reference period does.
That is why a seasonal operation can lawfully run heavy weeks in a peak, provided the shoulder weeks bring the average back. It is also why an employer relying on that has to actually measure the average rather than assume it.
The opt-out is individual, and it is not a formality
gov.uk describes the 48 hour limit as something a worker "can choose to work more" than by opting out. It is an individual choice made in writing, not a clause an employer can apply to a department.
For a pattern that averages over 48 hours, such as 6 on 4 off on 12 hour shifts at 50.4 hours, the opt-out is not a tidying-up exercise. It is the only thing standing between the rota and a breach, and it has to exist for every worker on the pattern.