Slack
Definition
Slack is the amount of time an order or activity can be delayed without missing its due date, calculated as time remaining until the due date minus the remaining work and setup time.
In Practice
Slack is the basic currency of scheduling priority. A job due in 12 working days with 7 days of processing left has 5 days of slack; zero or negative slack means the job is already late-trending and must jump the queue. The slack-time dispatching rule works whichever job has the least slack first, and slack per remaining operation refines it for jobs with many steps left.
In project scheduling the same concept appears as float on the critical path — activities with zero float determine the project end date. Planners protect slack deliberately, because it is the buffer that absorbs machine breakdowns, absenteeism, and quality holds.
Frequently Asked Questions
How is slack time calculated?
Subtract the remaining setup, run, and normal move time from the time left until the due date. A job due in 10 days with 6 days of remaining work has 4 days of slack. Negative slack means the job cannot meet its date at normal pace and needs expediting or rescheduling.
What is the minimum slack dispatching rule?
At each work center, run the queued job with the least slack first, since it has the smallest buffer against lateness. A common refinement divides slack by the number of remaining operations, recognizing that a job needing ten more steps has more chances to lose time than a job needing one.
Related Calculators
Related Terms
Critical ratio is a dispatching priority index calculated as time remaining until due date divided by work time remaining; a ratio below 1.0 means the job is behind schedule, and lower ratios get worked first.
Priority PlanningPriority planning is the function of determining what to make and when — setting and maintaining valid due dates and quantities for orders — as distinct from capacity planning, which determines whether the resources exist to do it.
Forward SchedulingForward scheduling is a planning method that starts a job at the earliest available date — usually today — and adds each operation's lead time to project the earliest possible completion date.
Lead TimeLead time is the elapsed time between initiating a process, such as placing an order, and its completion, such as receiving the goods. It is one of the most important inputs to inventory, planning, and customer promise decisions.