An eight-day cycle against a seven-day workweek. Model the 56-day superblock and see why two consecutive paychecks never match.
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Patterns are generated automatically — click any calendar day to override it.
Standard FLSA: overtime applies after 40h in any Sun–Sat workweek within this period.
Gross Pay — This Period
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Four on, four off, twelve-hour shifts. It is the cleanest rotation to remember and the hardest one to predict a pay cheque from, and the reason is a single arithmetic fact: the cycle is eight days long and the legal workweek is seven days long.
Every other element of your pay is stable. Your wage does not move, your differentials do not move, your shift length does not move. But because eight and seven share no common factor below fifty-six, the number of shifts that falls inside any given Sunday-to-Saturday workweek keeps sliding. Some workweeks catch three shifts. Some catch four. The order in which they arrive is not random, but it does not repeat until day fifty-six.
Fifty-six is the lowest common multiple of seven and eight. That makes it the true period of a 4-on/4-off roster — eight calendar weeks, seven full rotations, and four bi-weekly pay periods before the pattern of shifts-per-workweek returns to where it started.
Inside that superblock, the overtime is unevenly distributed. A three-shift workweek is 36 scheduled hours, or 34.5 paid after a thirty-minute unpaid break, and produces nothing. A four-shift workweek is 48 scheduled, 46 paid, and produces six overtime hours. Because the Fair Labor Standards Act evaluates each workweek in isolation and forbids averaging across a pay period, those six-hour weeks are the entire overtime story, and they arrive on an eight-week rhythm rather than a fortnightly one.
The practical effect is that two consecutive bi-weekly cheques will differ even though nothing about your roster changed. Workers new to the pattern routinely assume a payroll error on the lighter period. It is not an error — it is the drift. Use the period arrows to step across four consecutive pay periods and you will see the full oscillation before it resets.
The same eight-versus-seven mismatch moves your weekend differential around. On a fixed weekly roster you either work weekends or you do not. On 4-on/4-off, your on-block walks steadily backwards through the calendar week, so across the fifty-six-day superblock you will pick up Saturdays and Sundays in an uneven distribution — clusters of weekend work followed by stretches with almost none.
If your weekend premium is a meaningful percentage, this is the largest single source of variance in your pay after overtime. It also means any estimate of weekend earnings based on one month of roster is unreliable. Model the full superblock instead.
A calendar year contains 365 days, which is 45.625 eight-day cycles — so a 4-on/4-off worker is scheduled for roughly 182 or 183 twelve-hour shifts a year, around 2,190 scheduled hours before break deductions. That is a genuinely high-hours roster, well above a 2,080-hour salaried equivalent, and the gap is almost entirely made up of premium-rate hours.
Because the cycle does not divide into the year, the number of shifts you work in any given calendar year varies slightly depending on where the rotation sits on 1 January, and leap years shift it again. Annual projections built from a single fortnight will therefore be wrong in both directions across a career. The annualised metric in this calculator extrapolates from your modelled period, so run it against a period that contains a representative mix of three-shift and four-shift weeks rather than one that happens to be all-heavy or all-light.
There is also a daily-overtime exposure specific to twelve-hour patterns that does not exist on eight-hour rosters. Colorado triggers overtime at twelve hours in a workday, or twelve consecutive hours worked including unpaid breaks — which a twelve-hour shift with a thirty-minute meal break reaches by definition, since the span from clock-in to clock-out is twelve and a half hours. California applies a daily eight-hour trigger with double time past twelve. In those jurisdictions a 4-on/4-off roster generates premium hours on every shift regardless of the weekly total, and the eight-day drift only determines how much sits on top.
One last thing worth checking on your own roster. Some employers run 4-on/4-off with a fixed day crew and a fixed night crew; others rotate the same crew from days to nights at the end of each on-block. If you rotate, every second on-block carries night differential and the drift compounds with the rotation, producing a sixteen-day effective cycle for differential purposes even though the shift pattern itself is still eight. Use the calendar day coding to set the day and night blocks explicitly rather than relying on a single assumption for the whole period.
