# The FiFo Calculator – Determining the Size of your Buffers

In my previous post, I discussed how to Determine the Size of Your FiFo Lane – The FiFo Formula. My preferred method is still an expert estimate. However, if you are interested in the math, here is a small JavaScript calculator that estimates a FiFo size for two processes. For instructions on how to use it and some background, see below. The calculator requires JavaScript, and it may not work with some older web browsers. If you see no graphs below, then try a different browser.

## Information About the Two Processes

Please enter the information about the mean and standard deviation of the two processes in the fields below, using a time unit of your choice. You can hover over input fields for additional information.

 Mean Standard Deviation Process 1: Process 2: Minimum Time to cover for Breakdowns:

## Expected System Time

Shows the relation between the FiFo length and the expected overall time between parts for the system.

## Bottleneck Probabilities

Shows the likelihood of one of these two processes being the temporary bottleneck. The larger the FiFo, the more likely that the overall slower process will be the bottleneck.

## Normal Distributions

Shows the normal distributions of the two processes. The closer the distributions are and the more these distributions overlap, the more FiFo capacity is needed to achieve the same performance.

## How to Use the FiFo Calculator

The program below will calculate the effect of the capacity of a FiFo lane (or for that matter, also a random buffer) on these two processes. This program is most useful to calculate the FiFo length before and after the bottlenecks. For more on bottlenecks, see my posts related to bottlenecks, especially the Bottleneck Walk. For a general discussion on how FiFo works, please see Theory and Practice on FiFo Lanes. For some background into the calculations, see Determining the size of your FiFo lane – The FiFo Formula.

You would need information about the time between parts for each of the individual processes, both the mean and the standard deviation. This means collecting data directly from your processes (or making really good assumptions 😉 ). You have two options here:

• Collect data including every disturbance, breakdown, changeover, etc. during regular working hours. Your standard deviation may be quite large. In this case, such breakdowns, etc. are included in the assumptions about the effect of the FiFo. In this case, you can ignore the minimum time to cover for breakdowns by setting it to zero below if you wish.
• Collect data without major disturbances, breakdowns, changeovers, etc. during regular working hours. Your standard deviation will be smaller. In this case, such breakdowns, etc. are not included in the assumptions about the effect of the FiFo. In this case, I recommend to also fill out the minimum time to cover for breakdowns below. How long do your want your FiFo to last in case the slower process breaks down completely?

You will see that the larger the FiFo capacity is, the faster your system will be. However, a too-long FiFo will make your system less agile, hence you need a trade-off. The calculator will also suggest a trade-off. In any case, all the calculations below are only estimations. If you think the results to be strange, use your common sense or the common sense of someone familiar with the system.