Little’s Law is the relationship stating that average work in progress (WIP) equals average throughput rate multiplied by average flow time within a defined process boundary.
The equation is WIP = throughput rate × flow time. Flow time includes all elapsed time between the agreed entry and exit points, including waiting. Units must match: throughput measured in units per hour requires flow time expressed in hours.
In a hypothetical cooling process handling 600 components per hour, a required residence time of five minutes implies an average cooling inventory of 50 components: 600 × (5 ÷ 60) = 50. A proposed target of 30 components would imply three minutes at the same throughput. That target needs evidence that the shorter cooling period preserves product quality.
Während der DMAIC measurement and analysis, Little’s Law helps teams test inventory targets and assess flow-time estimates. WIP should represent a time-weighted average or a defensible sampling estimate. Throughput must cover the same operating window. Growing queues and unfinished units at the window boundaries require investigation before the calculation supports a steady-operation forecast. The calculated average also does not establish the maximum storage capacity needed during variation.
SIPOC establishes the measurement boundary, while Value Stream Mapping distinguishes processing residence time from downstream queues. Operational definitions clarify whether local “lead time” terminology matches the flow time used in the equation. A Control Plan can track WIP alongside throughput and a quality balancing measure, with pilot testing used to validate proposed changes to residence time.
