Lean Manufacturing

How to Calculate Standard Work in Process (SWIP) Quantity

By Jon Miller Updated on September 3rd, 2020

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 at blog posts here, here, here, here and here if you are new to Standard Work. For step-by-step guidance, we have a standard work course in our learning library.

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. Then there is 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 will run out and bring work to a halt. Another term for SWIP is SIPS, or Standard In-Process Stock.

How to Calculate SWIP?

So how do we calculate Standard Work in Process quantity? There are a number of questions you need to ask. Technically you can get fairly close if you say SWIP = Sum of Cycle Times / Takt Time but you still need to go back and figure out how much SWIP goes where.

So here are the step by step rules to determining SWIP:

1) What is the crew size?

Since Standard Work is the most effective combination of manpower, material and machine i.e. the best known method today, based on takt, work sequence, and SWIP, by definition there should be manual work. If it’s a fully-automated “lights off” process, you really don’t have Standard Work. At that point you probably have an CNC program.

In any case, because crew size is determined as the sum of manual cycle time / takt time, you will need one piece of SWIP per person.

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

Rounding rule: There is no rounding, unless you have found a way to have less than a full person, in which case you round up to the nearest whole number.

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

Standard Work presupposes that there is multi-process or multi-machine handling. It also presupposes that human and machine work are separated, wherever possible, and that when there is an automatic cycle, the worker will unload, load and walk away. It also presupposes that automatic cycle times are less than takt time. This means that for every automatic cycle process, there will be at least one piece of SWIP in 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 you can state this is that:

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

And then round up, but it is a waste to do this calculation since it must always be within takt, and therefore 1:1.

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

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.

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

Think of it as a pulley and pail used to get water from a well. Sometimes called tsurbe system in Japanese. A tsurube is the contraption of pulley, rope and pails at a well. The idea is to have one bucket at the end of the rope in the bottom of the well, being filled with water, and one bucket at the top being emptied of water. During takt you empty out the bucket one by one, and fill it back up one by one, getting ready to lower the bucket into the well. The analogy is not perfect since the water is in the bucket before it goes down the well, and there is a transformation that happens to the bucket in the well (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?

The concept of the pulley & pail is the same as rule #4. The formula is slightly different, since instead of the automatic cycle time you need to determine the lead time which includes time to get the order to the outside process, time in queue, time to transport it back, etc. and can be days even for processes that take only minutes.

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 (volunteers?) 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 work flow (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 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.

1. Erick Mortera

February 10, 2007 - 6:46 am

i just want to add for item# 2 – single piece automatic machine; as you have mentioned “There are several special cases where there are additional considerations to the rules above. One example is when the direction of work flow (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, in order to eliminate waiting for the machine cycle to end”, therefore this will result to item 2 having:
SWIP (single piece auto) =
if forward human movement = 1 in each auto machine
if reverse human movement = 2 in each machine; 1 in-process and 1 in-front of the machine (stand-by).
…hope this helps. thank you.

2. robert thompson

February 10, 2007 - 8:55 am

Training Within Industries had a direct impact on the development and use of kaizen and Standard Work at Toyota. In fact, kaizen is a direct descendant of Job Methods, and most likely Job Relations had an impact on the development and function of the Team and Group Leader structure in Toyota. Of course, the inability to hold standard work is one of the major reasons why lean initiatives stagnate instead of progressing on toward autonomous, daily improvement.
Rob
http://www.qualityhero.co.uk (six sigma)
http://www.63buckets.co.uk (lean)
http://www.rob-thompson.net

3. Ron

February 10, 2007 - 11:19 am

Thanks Jon. You rock.

4. Chris

February 11, 2007 - 6:32 pm

Thanks for clarifying the formulas for Standard Work calculations. I bookmarked this site and will be referring to again.

5. Keerthi Abeywickrama

February 17, 2007 - 8:53 pm

This is immensely helpful post to all learners of manufacturing systems engineering. If SWIP is practiced, it will reduce the “Idle Time” in many garment factories in an around Asia, and alleviate many ‘ills’ associated with garment manufacturing, and improve the living standards of the sewers. Great work Jon! Thank you.

6. Jon Miller

February 18, 2007 - 1:11 pm

Hello Keerthi. I could ask for nothing more than to have you use this information to improve the living standards of people.

7. kk

September 5, 2009 - 5:32 pm

hi, my question is if i have different models with many customers, how do i calcualte the takt time? does it mean with different customers i have different takt time of the same model?

8. Jon Miller

September 5, 2009 - 11:22 pm

Hi KK
The takt time is calculated by dividing net available time to produce by the customer demand over that period. The answer to your question depends on your mode of operation – dedicated lines, mixed model lines, or lot production where various models share a line but do not run as a mix.
The short answer to your question is that you do NOT have different takt times for different customers. The takt time is the average speed of customer demand for all customers for that product.
So let’s say you have product models A, B, C etc. For A you might have customers X, Y, Z. You would total the demand for X, Y and Z for product A to get the customer demand.
Please feel free to e-mail me more details to jon dot miller at gemba dot com or post it here if this does not answer your question.

9. CY

September 16, 2009 - 9:15 pm

Hi, add on to KK question, the takt time must be calculate by product? Can I calculate the takt time base on several products demand (sum of all products)?

10. Jon Miller

September 17, 2009 - 10:19 pm

Hi CY
You can calculate takt time by several products, not just one. These products should belong to the same “product family” or group with similar process flows. For example if you have several models that are made on the same production line the takt time for that production line would be calculated based on the sum of those products. When you have mixed production with different processes, work content and time it is often better to calculate separate takt times for different products or product families, even if they are produced on the same production line.

11. Jorge Wong

May 18, 2010 - 6:21 am

Hi Jon,
Just a question and a comment:
Question-
Equations of paragraphs 1 and 2 above are:
1) SWIP(manual) = Crew size / (1 piece/person)… units are (1 / piece)
2) SWIP (single piece auto) = Single-piece automatic cycle machines / (1 piece/machine)…units are (time/piece)
Shouldn’t these equations be
1′) SWIP(manual) = Crew size x (1 piece/person)..units are pieces
2′)SWIP (single piece auto) = Number of Single-piece automatic cycle machines x (1 piece/machine) …units are pieces
(replace “/” with “X”? so the resulting WIP units are pieces.
In general any WIP can be estimated using Little’s law
WIP = throughput (units/min) x LT = 1/takt x LT (units are in pieces)
since desired throughput to match customer demand = 1/Takt
JW

12. Jon Miller

May 18, 2010 - 6:43 am

Hi Jorge
Thanks for the careful proofreading. You are right, the / should be x. I made the changes.
In general you can use Little’s Law, but not always.

13. Steve

May 27, 2010 - 10:17 am

Stumbled across this as I was working on developing standard work and looking for operator balance. Here is my scenario. I have a machining cell that has a TAKT of 78 seconds. Works Station – 1 OP10 has 2 machines 1 provides a CT of 6 parts / 300 seconds and the other 6 parts / 660 seconds. Manual unload / load = 70 seconds / machine total 140 seconds. Work station – 2 OP20 that has 2 machines each produce 4 parts / 210 seconds and have a manual unload / load time 70 seconds per machine for a total of 140 seconds. Work station 3 OP 30 Wash 4 parts / 36 Seconds manual, OP40 testing CT = 4/60 manual 4/parts / 12 seconds OP50 Visual inspect and pack 4 parts manual 25 seconds. Currently I have 3 people in the cell and am trying to get it to 2. Having problems with calculations here. Any help would be appreciated.

14. Tony

July 11, 2010 - 7:32 pm

Dear Sir,
Can I ask you a question: Standard WIP = One piece flow?

15. Jon Miller

July 13, 2010 - 4:06 am

Hi Tony
No, standard WIP does not equal one piece flow. One piece flow requires SWIP. The minimum necessary amount of stock to keep flowing one piece at a time is known as SWIP.

16. Mike

September 27, 2010 - 11:20 am

What is a NC program as mentioned under step 1?

17. Jon Miller

September 27, 2010 - 12:17 pm

Hi Mike
NC = numerical control. It is using computer program to operate machine tools. More modern term is CNC or computer numerical controlled.

18. Russell

July 6, 2011 - 8:15 am

So, my process has a “batch auto cycle”, like #4, but my auto cycle is five separate batches due to cycle time limitations. How do I use your calculation for SWIP? Do I divide by five?

19. Laurie Smidutz

July 26, 2011 - 3:41 pm

My question is:
How can I use lead time and takt time in hospital warehousing of medical supplies? I currently have a team of 9.5 FTE’s.
The average scan time (order materials)Per person is 31.4 minutes
The average time to replenish the patient units are 45 minutes, 277 Par locations.
All other tasks total 284 minutes.

20. Mostofa

October 7, 2020 - 6:49 am

In garments Industry ( Apparel manufacturing) how can we calculate the SWIP in sewing line? In a sewing line there are about 30 operators who are doing 30 manual process (individual time requires). Is there any way to calculate SWIP in that sewing line?

• Jon Miller

October 7, 2020 - 10:03 am

Hello Mostofa. It sounds like a fully manual line. If so, the SWIP calculation is one piece per person. SWIP would be 30 pieces. This assumes everyone is working to takt time.

• Farhan

October 21, 2021 - 5:53 am

SWIP is 30. So, in every process, there should be 30pcs WIP? Or in total 30 pcs Or 1 in all 30 processes?

• Jon Miller

October 21, 2021 - 10:25 am

SWIP is the total WIP for the whole process in a specified process chain. In this case 30 is not fo 1 process but for the mentioned series.