Lead time

How long one piece takes to travel the whole stream, waiting included. Measured along the critical path, so parallel branches do not add up.

Formula

In practice

Lead time is the longest path through the map, not the sum of everything drawn on it. A sub-assembly line running alongside the main line does not add its queue to what the customer waits through, and a tool that adds it produces a number nobody in the plant recognises.

Each node contributes what a unit actually spends there: an inventory triangle contributes its quantity divided by the demand at that point in the stream, a process contributes unit residence time plus the wait for its batch to release plus the wait behind any standard WIP, an external operation contributes its turnaround, and a material link contributes transport. Accumulation happens entirely in production seconds, and the conversion to working and calendar days happens once, at the end — a five-day week makes one working day 1.4 calendar days.

How this is calculated

Lead time is the longest path, not the sum of everything on the map.

Parallel branches run concurrently: a sub-assembly line alongside the main line does not add its queue to the customer's wait. Longest-path relaxation in topological order, which is why the primary flow must stay acyclic (ADR-002).

Dwell time per node:

unit residence time = cycle time × mix factor      (machines do NOT divide it)
PCE                 = value-added time / total lead time

Residence, not interval (ADR-026). Four desks working a claim each deliver one every 300 s and hold each claim for 1 200 s. The first figure sizes capacity; the second is what the customer waits through, and it is the one the timeline charges. ProcessMetrics.effectiveCycleTime and unitResidenceTime are both published for exactly this reason, and they are equal wherever a process has one machine — which is every process on the Acme benchmark.

Value-added time is mix-weighted on the same basis, so it is a part of process time rather than a figure computed against a different denominator.

Accumulation happens entirely in production seconds; conversion to working and calendar days happens once, at the end, through the calendar.

For a process or transfer batch of Q pieces:

average batch release wait = ((Q − 1) / 2) × effective cycle time

This is the post-processing wait for a completed piece to leave with its transfer group. Upstream batch formation belongs in an explicit inventory node, so it is not counted a second time. Batch wait is reported separately from touch time and included in queue time and total lead time.

For a process holding standard WIP:

standard WIP wait = standard WIP × effective cycle time

Little's Law inside the box: a piece arriving at a station holding two others waits for both to clear. Charged here and nowhere else — standard WIP is stock inside the process, so a map that also drew a triangle for it would bill the same pieces twice. Reported separately and included in queue time, like batch wait, because the two are removed by different work.

Inventory uses the throughput propagated to its own node, including quantityPer and branch shares. Four bushings per finished assembly therefore turn 1,200 bushings into one day of supply at 300 assemblies/day, not four.

Methodology — Lead time

Common questions

Why is the lead time lower than my inventory adds up to?

Because stock on a branch that is not the critical path does not govern delivery. The engine reports the gap against Little's Law where it exceeds 5%, and that gap is informative rather than an error: it quantifies inventory parked outside the path that decides when the customer is served.

Is the figure in working days or calendar days?

Both are available, and they are not interchangeable. Production seconds are what the arithmetic accumulates in; working days are the days the stream operates; calendar days are what the customer experiences. The conversion is applied once, through the calendar, rather than at each node.

Related terms

• Process cycle efficiency

  Value-added time as a share of lead time. Single-digit percentages are normal, which is the point of drawing the map.

• Total inventory

  Everything sitting between the processes, counted in pieces, and the biggest component of lead time on most maps.

This definition is the one the product itself shows, read from its interface catalogue at build time.

Open the editor