Last Updated on 13 September 2023
A metric that is used often in Lean Six Sigma (and especially Lean) is cycle time, often written C/T. This is the time it takes for your process to produce one product. It one of the key metrics used when creating a value stream map, and vital for meeting customer demand and removing bottlenecks.
You can calculate cycle time for the entire product flow, or for each individual process. It is incredibly helpful to compare the cycle times for different processes in a line, as they need to be running at the same speed for the line to run smoothly. Differences between consecutive processes will lead to either bottlenecks (when the next process is slower) or waiting waste (when the next process is faster).
What is cycle time?
Cycle time is the length of time it takes to produce one good unit of output
You can apply it to either a process, which makes it the average time for a product to pass successfully through the process. It can also be applied to the whole value stream, in which case it is the time to make one good unit of output from raw materials. This is just the processing time and it ignores time waiting between processes, unlike lead time.
How do you calculate cycle time?
It’s vital that you go out into your work space (Gemba) to calculate cycle time. If you calculate it from your standard figures or from what people are telling you is happening, you’re likely to come to the wrong answers and wrong conclusions.
It’s worth averaging over a relatively large number outputs, preferably at least one batch so that you can include changeover time. A good example would be to average it over a morning shift. You need to calculate two things:
- Total time elapsed over the time that you were measuring. If they took a 15 minute break in the middle, deduct that off as it doesn’t count as net production time.
- Good items made during the same period. This is all items made less those which failed the inspection after the process.
From here, it’s simple to calculate; the formula you need is:
Cycle Time (C/T) = net production time / quantity of good items made
You should calculate it for each of the processes from start to finish, and then add them together for the total. If you are making a value stream map, these figures all go on the time line at the bottom of the map, below their respective processes.
Cycle time vs Takt time
These two metrics are often seen together, as comparing the two can tell you a lot about how your processes are working, and how efficiently you’re operating.
Takt time is the cycle time needed to meet customer demand.
It is therefore your optimal production speed to avoid either missing deadlines or having staff waiting around with nothing to do.
You need to compare the cycle time for each of your processes to the Takt time, to check that all of them are quick enough to meet demand. There will often be some where one figure is higher, and others where it is lower. As Takt time will vary over the year, just concentrate on what your system currently needs to be producing.
The easiest way to do this comparison is to create a value stream map, which will have all the processes in your value stream. You can then see your whole process chain and how cycle time compares to takt time for each process.
You can therefore balance out your flow by moving resources (usually this means people or shared tool time) from one process to another. You can sometimes rebalance by moving a part of a process from one process to another, rebalancing the size of the task.
A useful tool for this is an operator balance chart:
Using this, we can see which need improving and which have spare capacity. Spare capacity can either be used for savings, growing sales or moving to help processes that are struggling. In the example, Process 1 has spare capacity as it is below the takt time line, whereas Processes 2 and 3 need to speed up to meet demand.
Worked cycle time example
Say we have a nice simple 3 step process with three stages. Over a four hour shift with a 15 minute break, process 1 has made 342 items with 25 defective units, process 2 has made 138 items with 9 defective, and process 3 has made 200 items with 4 defective products. There is a takt time of 60 seconds.
Net production time
To calculate the cycle times, we need first to find net production time. We’re making a lot of items per shift, so I’ll calculate it in seconds:
There are four hours, which is 240 minutes, less a 15 minute break leaving 225 minutes net production time. Multiplying this by 60 gives a net production time of 13,500 seconds.
Good units produced
The other half of the formula is good units produced, which is total units produced less defective units, even if these defective units are fixed later:
- 1: 342 units produced – 25 defective = 317 good units
- 2: 138 units produced – 9 defective = 129 good units
- 3: 200 units produced – 4 defective = 196 good units
Calculate your cycle times
It’s now simple to calculate C/T, as they all have the same net production time:
- 1: 13,500 / 317 = 42.6
- 2: 13,500 / 129 = 104.7
- 3: 13,500 / 196 = 68.9
Comparing to a Takt time of 60 seconds, it’s clear that we won’t meet demand as two of the processes aren’t going fast enough! This will upset customers and reduce sales so we need to do something about it.
What can we do to improve it?
First of all, this variance is most easily viewed on an operator balance chart:
You’ll recognize that this example is the operator balance chart from above. The situation might not be as bad as we initially fear; process 2 is working slower than both process 1 and process 3. Process 3 may be slower than 1 and slower than the takt time, but this could just be that they are inefficient as they spend a lot of time waiting for Process 2.
Theory of constraints
Following the theory of constraints, we need to focus on improving process 2 as this one is the bottleneck. Process 1 has spare capacity, so we can afford to make it slower to aid process 2. Options are:
- Move operators from processes 1 and 3 to process 2 so it can be operated faster
- Change the processes so operations currently part of process 2 are moved to processes 1 or 3
- If the problem persists, you should investigate adding an extra station performing process 2, so that they can work in parallel.
When you’ve got the cycle time of process 2 below the takt time, process 3 might speed up. As the bottleneck at 2 is removed, process 3 will be able to work more efficiently and reduce waiting waste. If process 3 is now the problem, repeat the above but focusing on process 3.