Realistic schedules: why most shops underestimate their capacity

Almost every make-to-order shop schedules its work the same way: someone experienced builds a schedule in a spreadsheet, spreading orders across machines and people based on what they remember about available capacity. It works reasonably well until the operation grows or the number of simultaneous orders exceeds what one person can keep in their head.

What is almost never in the spreadsheet

A manually built schedule rarely includes, consistently, the real changeover time between part numbers, each machine's scheduled maintenance, or efficiency differences between shifts. Not because nobody knows them, but because keeping that information up to date by hand, for dozens of active orders, quickly stops being feasible.

The result: optimistic capacity

When the schedule does not reflect these variables, it tends to assume more capacity than actually exists. Orders are committed with dates that look reasonable on paper, but that do not account for machine X being in maintenance on Thursday, or for a changeover on line Y taking two hours nobody planned for.

What changes with a schedule based on real data

A schedule fed by the real state of the plant — effective capacity, not nominal; real changeover times, not estimates; actual availability of each machine — lets you commit delivery dates the plant can actually meet. This does not remove the commercial pressure to promise shorter dates, but it does remove the surprise of finding out midweek that the schedule was never feasible.

The difference between an optimistic schedule and a realistic one is not noticeable on day one. It shows when, month after month, committed dates stop depending on luck and start depending on the operation's real capacity.