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.
Talk to a specialist Three numbers in, and you have the cycle, the average week, the crews and the Working Time position.
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How this calculator works
- The 48 hour average weekly limit is applied as gov.uk states it: "you cannot work more than 48 hours a week on average, normally averaged over 17 weeks". A worker may choose to opt out of the 48 hour week in writing, and several categories of work sit outside the limit entirely, including jobs requiring 24 hour staffing, the armed forces and emergency services, and roles where working time is not measured and the worker controls their own decisions (gov.uk).
- Average weekly hours are computed from the SHAPE of the cycle rather than from a calendar: days on divided by cycle length, times seven, times the shift length. That is exact for a repeating cycle and is why a 4 on 4 off 12 hour pattern produces 42 hours a week rather than the 48 people assume from the shift length.
- Daily rest is 11 hours between working days and the weekly rest is either an uninterrupted 24 hours each week or an uninterrupted 48 hours each fortnight. Workers also have the right to one uninterrupted 20 minute rest break during the working day where they work more than 6 hours (gov.uk).
- The daily rest figure assumes shifts on consecutive days start at the same clock time, so the gap is 24 hours minus the shift. A pattern that rotates forward from nights to days inside a cycle can produce a shorter gap than this figure shows, and that transition is the one to check by hand.
- Night workers are staff who regularly work at least 3 hours during the night period, which is 11pm to 6am unless the worker and employer agree a different period. They "must not work more than an average of 8 hours in a 24-hour period", usually calculated over 17 weeks, or up to 52 weeks by agreement. Where the work involves special hazards or heavy physical or mental strain the limit is absolute: they "cannot work longer than 8 hours in any 24-hour period" (gov.uk).
- The crew count is arithmetic, not a rota: shifts needed a day multiplied by the cycle length over the days on. It tells you how many crews the pattern implies for the cover you asked for. It does not account for holiday, sickness, training or the relief factor, all of which push the real headcount above it.
- Under 18s are not modelled. gov.uk states that under 18s "cannot work more than 8 hours a day or 40 hours a week", that 16 and 17 year olds "cannot work between midnight and 4am", and that they usually cannot work between 10pm and 6am, with narrow industry exceptions. Any pattern involving young workers needs checking against those rules directly.
- No figure here is a market rate, a typical pay rate or a benchmark. Everything the tool returns is derived from the pattern you enter and the published statutory limits, and nothing is estimated.
The formula any rota calculator should use
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.