Capacity planning that respects reality: three honesty rules
Every factory has a plan. The question is whether the plan is a statement about reality or a wish with dates on it. The difference usually comes down to three honesty rules that finite capacity planning enforces and spreadsheet planning quietly breaks.
Rule one: capacity is finite, so the plan must choose
A line has so many operators, so many hours, and one calendar. Loading it beyond that is not planning; it is deferring the decision to the floor, which will make it for you at the worst possible moment. Finite scheduling forces the trade-off up front: when an order will not fit, the planner sees the conflict at booking time and decides, move the order, split it, add capacity, or renegotiate the date, while every option is still cheap.
The alternative, infinite loading, produces the familiar theater: a plan that everyone knows is overbooked, daily firefighting to decide what actually runs, and delivery promises whose basis nobody can state.
Rule two: efficiency is never silently 100 percent
Planned output is capacity times efficiency, and the efficiency you assume is a claim about the future. The discipline is to make the claim explicit and sourced: this line, on this product family, has run at 62 percent over the last measured window, so the plan books 62, not the 100 a spreadsheet cell defaults to, and not the 80 that optimism suggests.
A plan that states its efficiency can be argued with, which is exactly what makes it useful. A plan with an implicit 100 percent is unfalsifiable until the week it fails.
Rule three: a start date needs its materials
The most common planning fiction is the start date that ignores the warehouse. Sewing cannot start before cutting, cutting cannot start before fabric, and fabric has its own upstream reality of dye batches and knitting programs. An honest plan floors every start date on material readiness: not the purchase order's promise, but the pegged supply, what is actually allocated to this order, and when it credibly lands.
When the floor is upstream production rather than a delivery truck, the floor moves with upstream reality. A dye batch that slips two days should move the sewing start before anyone has cut a single ply against fabric that does not exist.
What changes for the planner
Finite planning does not make the planner's job easier; it makes it earlier. The conflicts that used to arrive as morning emergencies arrive as booking-time decisions instead. The 07:30 meeting changes character: instead of discovering what really ran yesterday, it compares actuals against a plan that was achievable, so a variance means something happened rather than the plan was fiction all along.
And promises to buyers change basis. A ship date backed by finite capacity, stated efficiency, and material floors is a commitment. Anything else is a hope with a customer attached.
Where to begin
Pick one product line. State its measured efficiency, book it finitely for two weeks, and floor its starts on real material. Run the fortnight and count the surprises. The comparison with the rest of the factory makes the argument better than any article can.
Further reading: Backward Scheduling from the Ship Date and Seeing the Capacity Crunch Six Weeks Out.