Flow Time

Flow time is the real-world elapsed time a unit experiences. It includes both active processing time and all the time spent waiting, in queues, between handoffs, waiting for approvals, or delayed by bottlenecks.

Flow time can be dramatically longer than system process time. A task that takes 10 minutes of actual work might take 25 minutes from start to finish because of queues and delays. The difference between the two is non-value-added time.

Note on “Lead Time” and “Cycle Time”: Lead time is frequently used as a synonym for flow time, especially in supply chain and operations management. In some contexts, lead time specifically means order-to-delivery time, which may include steps outside the process itself. “Cycle time” in Six Sigma and service operations often means flow time, which is the opposite of its Lean/manufacturing usage (where it means process time at a single step). This glossary uses “flow time” to avoid ambiguity.

Formula

Flow Time = System Process Time + Total Delay Time

Using Little’s Law: Flow Time = Inventory / Throughput

Example

A coffee shop order has a system process time of 10 minutes (four steps, 10 minutes of actual work). But during the morning rush, a customer waits 25 minutes from ordering to receiving their food. Flow time = 25 minutes. Of that, only 10 minutes is actual processing, the remaining 15 minutes is waiting.

Why It Matters

Flow time is what the customer experiences. They don’t care that your process time is 10 minutes, they care that it took 25 minutes. The gap between system process time and flow time represents waste (waiting, delays, queuing). Closing this gap improves customer satisfaction without necessarily adding capacity. Flow time is also the basis for Little’s Law calculations.