Single Piece Flow


This is a concept which came out of Lean Manufacturing, but is incredibly powerful in other contexts. It’s something which has fundamentally changed the way I see certain problems.

Letter example

Single Piece Flow is demonstrated through the following conundrum:

You have 100 letters to send out. Each letter much be folded, put in an envelope, and have a label and stamp attached. How would you do this in the fastest way possible?

The usual solution

The solution which most people propose is as follows:

Get a batch of letters (perhaps 10) and fold them all, then place them in a pile. Then get 10 envelopes, attach a label and stamp to each one, then place them in a pile. And finally put the letters into the envelopes. Rinse and repeat this whole process 10 times, and voila! 100 finished letters.

There are two main problems with this approach:

  1. There’s a lot of work in progress. You will have 10 folded letters and 10 labelled and stamped envelopes at some points. If you had to leave the job for a while, or the job got cancelled half way through, then that’s a lot of waste.
  2. There’s a lot of handling:
    • Pick up a letter, fold it, then put it down (x10).
    • Pick up an envelope, label it, then put it down (x10).
    • Pick up an envelope, attach a stamp, then put it down (x10).
    • And finally, pick up the letter and envelope, combine them, then put it down (x10).

The right solution

With single piece flow we would do the following:

Pick up and the fold the letter, put it in an envelope, and add the label and stamp (x100).

We now have a batch size of one. This intuitively might sound like it will take even longer. But we’re no longer picking things up and putting them down as often. And we almost completely eliminate work in progress.


So why is this such a valuable insight? It’s one of the reasons Toyota became such a successful automobile manufacturer. And on a more prosaic level, it turns out we do a lot of tasks in our lives which are akin to sending out letters.


The letters example came from the Lean Thinking book - highly recommended for anyone interested in this area.