Definition
The actual quantity of material and semi-finished goods released to production but not yet completed; it is the measured value on the floor, while standard WIP (SWIP) is the value set by takt and work sequence. The two are not interchangeable.
In detail
Work in Process is a type of inventory; it ties up cash and lengthens lead time. On a value stream map, the inventory triangles between processes are Work in Process (the raw material and finished goods inventory triangles at the beginning and end of the map do not count); dividing by daily demand gives how many days this segment covers. The textbook treats it as a Kaizen target: use Continuous Flow and pull to squeeze out excess Work in Process, rather than managing it with warehouses and racks. When judging, distinguish two numbers—how much is actually on the floor (Work in Process) and how much is minimally needed per Standard Work (Standard Work in Process). Being above standard for a long time indicates Flow is broken or batches remain; being below standard will cause material shortage and line stoppage.
Example in practice
A cell's standard WIP is 6 pieces, yet a count finds 40 on the line. Instead of adding racks, the team finds an upstream step still batching; after the batch is split, WIP returns to 8 and the lead time falls from three days to half a day.