How to Calculate Standard Work in Process (SWIP) Quantity

Standard Work is one of the more misunderstood concepts in Lean manufacturing. It is neither standardization nor work standards. You can learn more about Standard Work by looking at the following blog posts: Reflections on Standard Work, Standard Work is Kaizen Instruction for Managers, One Piece Flow & Standard WIP, Making Standard Work Stick, and The Toyota Job Description: Follow Standards & Find Better Ways if you are new to Standard Work. For step-by-step guidance, we have a Standard Work Course available in our learning library at Gembaacademy.com.

What are the Three Elements of Standard Work?

There are three elements to Standard Work:

1. Takt time is a fundamental concept of Lean manufacturing that is widely understood.
2. Work sequence is fairly intuitive.
3. Standard Work in Process or SWIP. This is a bit trickier.  Let’s dive in.

What is SWIP?

Standard work in process, or SWIP, is the minimum necessary in-process inventory (work in process or WIP) to maintain Standard Work. No more, no less. If our work-in-process stock is higher than SWIP, we have excess inventory. If we have less SWIP than our calculations specify, we risk running out and bringing work to a halt. Another term for SWIP is SIPS or Standard In-Process Stock.

How to Calculate SWIP?

How do we determine the quantity of Standard Work in Process (SWIP)? There are several important questions to consider. Technically, you can estimate it fairly accurately with the formula:

• SWIP = Sum of Cycle Times / Takt Time.

However, it’s essential to further analyze how much SWIP should be allocated to each area.

Determining SWIP

Here are the step-by-step rules for determining SWIP:

1) What is the crew size?

Standard Work represents the most efficient blend of manpower, material, and machinery, or in other words, the best-known method as of now. It is based on takt time, work sequence, and Standard Work in Process (SWIP). By definition, this implies the presence of manual work. If a process is fully automated or a ‘lights off’ operation, it doesn’t truly constitute Standard Work. Instead, it’s more likely you’re dealing with a Computer Numerical Control (CNC) program.

The size of the crew is determined by dividing the sum of manual cycle time by the takt time, which necessitates one piece of Standard Work in Process (SWIP) per person.

• SWIP (manual) = Crew size x (1 piece/person)

Rounding Rule: there is no need unless you somehow have less than a full person. In such cases, you should round up to the nearest whole number.

2) Which processes are single-piece automatic cycle machines?

Standard Work assumes that operations involve handling multiple processes or machines. It also assumes that wherever feasible, human tasks and machine tasks are distinct. The worker’s role in an automatic cycle is typically to load, unload, and then move on. Furthermore, it’s assumed that automatic cycle times are shorter than the takt time. This implies that for each automatic cycle process, there will be at least one piece of Standard Work in Process (SWIP) within the machine.

• SWIP (single piece auto) = Single-piece automatic cycle machines x (1 piece/machine)

Rounding rule: There is no rounding since you cannot have less than a full machine, if you do… round up to the nearest whole number.

Note that these are single-piece automatic cycles, meaning that you can unload and load one piece each takt time. The calculation for automatic cycles with batch processes or cycle times and lead times longer than takt is different (see below).

Another way to calculate this is:

• SWIP (single-piece auto) = Automatic Cycle Time / Takt Time

Rounding Rule: You should round up. However, bear in mind that doing this calculation could be considered wasteful, as the ratio must always be within takt time, and therefore always 1:1.

3) Which processes have a single-piece non-machine automatic cycle?

A Non-machine automatic cycle is an awkward phrase, but this includes things like the time for paint to dry, the time for epoxy to cure, the time for hot parts to cool, etc. There may be no machine involved, but it takes a certain amount of time for something to happen “automatically” with the parts left alone. The calculation is:

• SWIP (single-piece non-machine auto) = Automatic Cycle Time / Takt Time

Rounding rule: Always round up to the nearest whole number.

Often a turn table or a FIFO rack is used for parts needing to cure so that the first one in (been curing the longest) is the first one out (done curing). There should be one done curing every takt, and a new one with epoxy placed in the rack or turn table each takt.

4) Which processes have a batch automatic cycle?

These are processes in which the equipment design allows you to unload and load only a batch at a time, instead of one at a time. A good example would be heat-treating processes in which you need to pull a vacuum and cannot open the door for hours once the cycle starts. You take a batch of parts out, then load another batch. The cycle time per piece may be lower than takt, but the overall automatic cycle time is over takt.

In this case:

• SWIP(batch auto cycle) = (Automatic time / Takt time) x 2

Rounding rule: Round up to the nearest whole number, then multiply by 2.  Why times two? Any time you have a batch process that does not allow you to take one or add one each takt, you will need an extra quantity of complete parts.

Maintaining the Flow of Standard Work through Batch Processes with a Tsurbe System

Consider this scenario akin to a pulley and pail system used to draw water from a well, sometimes referred to as the “tsurube” system in Japanese. The “tsurube” system is comprised of a pulley, rope, and two pails at a well. Conceptually, you can imagine one pail at the end of the rope at the well’s bottom being filled with water, while the other pail at the top is being emptied.

Within the timeframe known as takt time, you progressively empty the top bucket and refill it, preparing to lower it back into the well. However, this analogy isn’t perfect, as it assumes the bucket is filled with water before descending into the well, and it undergoes a transformation in the well (indicating a batch process).

When to Include Manual Cycle Times in SWIP Calculation?

In formulas 2, 3, and 4 there is no manual cycle time included in the calculation. Why? This is because rule #1 takes care of that. Since every manual cycle time must be within takt by definition of Standard Work, and since the unload / load time will involve one piece, there is no need to add manual time back into the calculation (in most cases).

5) Which processes leave the cell and are processed at a vendor operation or at another department?

Rule #4 aligns closely with the concept of the pulley and pail. Although, there’s a slight variation in the formula. Rather than focusing on the automatic cycle time, it necessitates the calculation of lead time. This comprehensive measure involves various stages like sending the order to an external process, waiting in the queue, transporting it back, and so on. Despite the actual process only taking minutes, the lead time might stretch into days.

• SWIP (vendor op) = (Lead time / Takt time) x 2

Rounding rule: Round up to the nearest whole number, then multiply by 2.

This type of pulley & pail system will allow you to flow through a batch process, vendor operations, or shared resource and maintain one piece flow and Standard Work, with minimum WIP.

It’s really not that complicated, just a series of “if… then” statements. The key thing is to think in one piece flow and be able to see past so-called monuments, outside processing, and shared equipment which seem to prevent continuous flow.

The Final Ugly Equation

So when it comes time to answer “How to calculate the SWIP quantity?” the answer is slightly ugly:

• SWIP (total) = SWIP (manual) + SWIP (single-piece auto cycle)+SWIP (single-piece non-machine auto)+SWIP (batch auto cycle)+ SWIP(vendor op)

Someone who is a trained mathematician (volunteer?) could probably state this more elegantly as an algorithm.

Exceptions to the Rule

There are several special cases where there are additional considerations to the rules above. One example is when the direction of workflow (manual work sequence) is the reverse of the process flow (material flow). Some labor-optimized cells that have unbalanced automatic cycle times can improve labor productivity by having an extra piece ready to load at the previous process. This eliminates waiting for the machine cycle to end.

Another exception to rule #1 is if the operator is unloading and loading a series of batch processes (the type for rule #3) and the transfer quantity is not one piece but a batch. In that case, there will not be one in hand. But we are starting to approach “arcana“…

How to Limit WIP in Knowledge Work?

Knowledge Work work tends to be less repetitive in a cycle and more variable in content. Therefore, it’s not a good candidate for classic production-style standard work. However, even without achieving a precisely calculated SWIP, we can gain some of the benefits from limiting WIP. The Agile practice of visualizing knowledge work as tasks or user stories on an Agile kanban board works well. It is also easy to start from wherever you are.

Harnessing Lean Principles for Optimal Standard Work

Understanding Standard Work and SWIP needs attention and adaptability. It’s all about balancing people, machines, and materials to keep work flowing smoothly. These Lean principles apply even in unpredictable areas like knowledge work. Remember, whether you’re on a factory floor or in an office, Standard Work is all about continuous improvement – the essence of Kaizen.

---