Total inventory
Everything sitting between the processes, counted in pieces, and the biggest component of lead time on most maps.
Formula
In practice
Total inventory is the sum of the quantities on the map, but the figure that matters at each triangle is days of supply, and that is computed against the demand propagated to that node rather than against the customer's. The distinction is the bill of material: an assembly consuming four bushings loads the bushing feeder four times, so 1,200 bushings are one day of supply at 300 assemblies a day, not four.
Only stock on the longest path extends lead time. Everything else is still real, still counted, and still reported — the gap between declared WIP and what the critical path can account for is published where it exceeds 5%, because that gap is a quantity of inventory sitting outside the path that governs delivery, and it is usually the first thing a reader wants named.
How this is calculated
Common questions
How are days of supply worked out at a triangle?
Quantity divided by the demand at that point in the stream, on the production axis. Yield loss, rework loading, bill-of-material multipliers and branch shares are already in that demand, so a component feeder is measured against what it actually has to supply.
Does every triangle add to lead time?
No — only those on the critical path. A buffer on a parallel branch is stock the plant is paying for without it being what the customer waits through, and the two facts are reported separately rather than merged into one number.
Related terms
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Lead time
How long one piece takes to travel the whole stream, waiting included. Measured along the critical path, so parallel…
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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.
This definition is the one the product itself shows, read from its interface catalogue at build time.