Warehouse Storage Myth Debunked – Equal Space Strategy versus Equal Time Strategy

Written By: Lauren Noyes

Warehouse Storage Dilemma

Ask any warehouse manager how to stock a forward pick area and you may get one of these two methods:

  1. Allocate the same amount of space to each SKU
  2. Store an equal time supply of each SKU in the area

The first answer, known as the Equal Space Strategy (EQS), tends to ignore the differences in SKU popularity and size when stocking. Moreover, the second method is called the Equal Time Strategy (EQT)–appropriate amounts of each SKU used are stocked for use during the time before the next replenishment. Logically, it seems to be a better method because more popular SKUs are allocated more space. However, assuming is risky business, so let’s take a closer look at these situations.

I can show you with simple algebra that there is no difference in the number of restocks with the EQS versus EQT strategy. Thus, each method will theoretically have the same restock labor costs.

I’ll use the following variables so you can see how I come up with the numbers in the table.

  • i ,j, k = subscripts to distinguish SKU A, SKU B, SKU C
  • n = the number of SKUs you have in the forward pick location
  • Fi = rate of flow of the SKUs per year
Allocation of Space 1/n fi /(I fi)
Restocks for SKU n*fi I fi
Total restocks over all SKUs n*I fi n*I fi

For the sake of argument, I have one unit of storage shared by 3 SKUs with flows of 26, 3, and 1 unit per year, respectively.

Flow of units per year (Fi) 26 3 1
EQS Allocation of Space in 1 unit storage 1/3 1/3 1/3 1
EQS Number of Restocks per Year 78 9 3 90
EQT Allocation of Space in 1 unit storage 26/30 3/30 1/30 1
EQT Number of Restocks per Year 30 30 30 90

For the EQS case, note that SKU B stays on the shelf longer than in the EQT case because it is not restocked from inventory as often. If expiration dates are an issue, EQS may not be the best choice.

Both EQS and EQT strategy offer uniformity that can simplify warehouse management. If space management is an issue, EQS may be better, especially when old SKUs are being phased out and new SKUs introduced. Because all storage slots are the same size, a newly-arrived SKU always fits into a space in the fast-pick area. One needs to keep in mind that EQS demands erratic restocking frequencies, which may be difficult to manage.

When you look at designing your restock method, don’t dismiss either strategy based on restocking concerns. There may be no difference in labor costs!

If you need assistance with SKU layout and re-stocking best practices, our supply chain consultants can help. Contact us at info@bastiansolutions.com or review our consulting services.

Reference: Warehouse & Distribution Science by J. Bartholdi and S. Hackman, August 19, 2014

Lauren Noyes is a Logistics Consultant for Bastian Solutions in Los Angeles, CA. She has a degree in Industrial Engineering from California Polytechnic State University and experience working in both automotive and hydraulics manufacturing industries before joining Bastian Solutions. While at the company, Lauren has worked on multiple consulting projects for 3PL’s, warehouses, and ecommerce distribution centers, making her well versed in data analysis, picking design, and planning.

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