Capacity and shift planning under variable demand

Why it matters

Capacity planning is broader than dividing the number of available workers by the quantity demanded. The same unit demand can create a different load when the product mix changes. A product that is long to assemble by hand and short to run automatically does not use the same resources to the same extent as one that is the opposite. For this reason, you first write down what will be completed, by when, and under which acceptance condition. The quantity of acceptable product to be delivered to the customer is not confused with the quantity released to production. When the limits of the plan are explicit, you can check which product and which scope of work the time data belongs to; the total unit count alone stops being an indicator of sufficiency.

Labor minutes describe the load of work done by people; machine minutes describe the occupancy of a particular piece of equipment. Even if both are written with the same unit name, one does not stand in for the other. If a worker can prepare another product while the machine runs automatically, the person and the machine carry load at the same time. Adding these times together to produce a single capacity gap hides the real resource. Conversely, finding slack in the total does not prove that simultaneous jobs are feasible. The order of setup, loading, inspection and transport must be examined on a common time axis. Resource totals are the first screen; the detailed task sequence is the second check.

With variable demand, the choice of period determines the result. The week's total work may fit within the usable time, yet orders with early delivery dates may create a resource gap on a specific day. The slack of a later day cannot retroactively cover today's delivery. Early production is an option only if material, shelf life, change risk and the delivery condition allow it. When early production is not allowed, the gap requires extra time, a different resource or a delivery discussion. A schedule prepared on average demand hides these decisions. Daily demand, completed quantity and backlog must be kept in the same sequence.

The consistency of the time base is also part of the plan. Subtracting an allowance already contained in a standard time from the usable time again, for the same reason, shrinks capacity needlessly. On the other hand, leaving the shared break out of the standard time and counting the whole shift as open for production inflates capacity. The standard labor times used here include the allowance share for the defined scope of work; the shared break kept outside the shift is not added to them again. This boundary is written next to the calculation. That way different plans are compared with the same time definition, and the fact that the numbers come out close does not make different assumptions invisible.

Formula