To contribute and also help move the discussion forward, let's first start with the definition of WIP:
WIP is the number of task items that a team is currently working on.
When rolling out a new workflow, you monitor the average number of task items in each iteration and determine the WIP limits. However, it is not possible to know if the WIP limit is low enough to promote smooth flow and avoid bottlenecks unless you experiment.
A lean management method exists that can make all the difference and this is Value Stream Mapping (VSM). The method was invented by Mike Rother, a US researcher who studied Toyota and particularly the practices of their supply chain development engineers.
Look at an indicative example of a map below:
Processes have been defined on the map (e.g., "Develop feature") and the Value Added time, Non-value added time and Efficiency have been calculated.
Value Added time (VA) is the average time spent actively working on a task.
Non-value added (NVA) time is the average time spent between each process step.
Efficiency is calculated as follows: Efficiency = VA / (VA+NVA)
Divide your team size by your efficiency to get the total number of tasks your team can perform simultaneously.
This total will be your total WIP for the entire board. :)
An excellent example is published by Evan Leybourn here:
I would like to mention a second approach. WIP can be also calculated by dividing the total Cycle time by Takt time. This is referenced as “Standard Work-in-Process” (SWIP).
SWIP = Total cycle time / Takt time
The Takt time is the rate at which you need to complete a product in order to meet customer demand and is calculated as:
Takt time = Available production time / Average customer demand
Note that available production time is the time that the team is actively working on creating value, excluding breaks and disruptions.
Cycle time is calculated as:
Cycle Time = End Date – Start Date + 1