The term flow in lean manufacturing primarily relates to the movement of product within the operation. There are various terms used in lean manufacturing training in the flow discussion. A few of these are "one piece flow", "continuous flow", "supermarket", "kanban", and "takt time".

One piece flow, as the name implies, is to have one product flow from the start of production to finish. Since the product is made one at a time, quality problems are easily identified as they occur. The opposite example would be a product made in large batches. Proponents of the batch method point to lower costs, but the lean manufacturing teaching would point out that the extra handling, damage, waste, storage, interest, and quality problems offset any productivity gains. In addition, machinery that produces one piece at a time usually costs less than high speed equipment.

Many processes are unable to economically produce one part at a time. For example, if a machine takes 80 minutes to set up and produces a part in 3 seconds, producing one piece at a time would be too expensive. Less than 6 different parts would be produced in a day.

The lean manufacturing discipline is not an "all or nothing" process. In other words, if producing one part at a time is too expensive and traditional batches were 20,000, the lean manufacturing way would be to reduce setup times and batch sizes accordingly over time. For example, reduce setup time to 20 minutes and cut the batch size to 5,000, then 10 minutes and 2000. Eventually setup time might reach single minutes or seconds and batches could be a few hundred. They key point is that lean cannot be implemented by the strictly as the definition of one piece flow would suggest. It takes common sense integrated with the tools of setup reduction, called SMED (single minute exchange of die), OEE (overall equipment effectiveness), takt time, TPM (total productive maintenance), and others.

The idea of continuous flow is parts are never (or rarely) idle waiting to be processed. If the batch size must be 1000, then the second process ideally begins to produce parts the moment the first process finish it’s first part. In this example, process #2 may notice a mistake and process #1 is only in it’s first few parts.

Not all processes can produce in such ideal conditions. For example process #2 might produce parts at 10 per minute while #1 produces at 80 per minute. In this case, a "supermarket" of parts might be created, which is a small buildup of inventory which is "pulled" and "filled" continuously.

Whatever production flow is chosen, it is important to eliminate or at least minimize handling, waste, and quality problems.

One piece and small batch flow enable drastically reduced cycle and lead times. It is critical to synchronize production rates, setup times, and supermarkets to reduce idle time and the potential for waste in process and handling.