The formula
cycle = days on + days off, repeated from the first day
Where it comes from
A rotating shift repeats a fixed cycle without regard to the calendar, so the rest days keep landing on different days of the week. Working out how many fall inside a period is a matter of counting how many times the cycle fits and what is left over.
How to work it out by hand
- Add the days on and the days off to get the cycle length
- Count the days in the period you care about
- Divide by the cycle length to see how many complete cycles fit
- Multiply by the working days in each cycle and add those in the part cycle
What is worth knowing
The figure that surprises people most is the average working week, because a pattern that looks gentle can run past the legal maximum. A four-on two-off pattern of twelve-hour shifts works two days in every three: more than 55 hours a week on average, well above any ordinary working week. The other point is weekends: only cycles that are multiples of seven keep the same day off every time. On four-on two-off the rest days rotate and whole weekends off can be counted on one hand.