When I time pickers in a small warehouse, the same thing shows up almost every time. More than half the picking time is walking and searching. The actual grabbing of product is a small slice.

Slotting is how you shrink the walking. It means deciding where each item lives based on how often it gets picked. And for a building with a few thousand SKUs or less, you can do it in a spreadsheet.

Start with pick lines, not units or dollars

The most common mistake I see is slotting by sales dollars or units sold. Neither one tells you how often a picker walks to that location.

What you want is pick frequency: how many order lines included that item. An item that ships 1,000 units on one order a month is visited once. An item that ships one unit on 300 orders a month is visited 300 times. The second one belongs up front.

Pull the export

From your ERP, export order line history for the last 6 to 12 months. You need, at minimum:

  • Item number
  • Order number (or order line ID)
  • Ship date
  • Quantity

If your business is seasonal, use 12 months, or slot separately for peak. If you have had big product changes recently, use the last 3 to 6 months instead. It depends on how stable your mix is.

Build the velocity ranking

In the spreadsheet, build a pivot table with item number as rows and a count of order lines as the value. Sort descending. That is your velocity ranking.

Now add a running percentage of total pick lines. In almost every warehouse I have done this for, something like 15 to 20 percent of SKUs account for 80 percent of the picks. That top group is what you are going to move.

Add cube

Pick frequency alone is not enough. A fast mover the size of a dishwasher cannot go in a shelf bin at waist height.

Add a column for the item's dimensions or at least a rough size class (small, medium, large, pallet). Then add one more column: average daily units picked times unit cube. That tells you how big a location the item needs so you are not replenishing it four times a shift.

Define the golden zone

The golden zone is the space between roughly knee height and shoulder height, closest to where picking starts and ends (usually pack stations or the shipping dock). Picking from there is faster and easier on backs.

Walk your building and actually mark it. For each aisle, note:

  1. Distance from the pack area
  2. Which levels are between knee and shoulder
  3. Which locations are on the main travel path

Give each location a simple score. I usually use something like 1 for golden zone near the front, 2 for golden zone farther back or above and below near the front, and 3 for everything else. Put it in a column next to the location list.

Match items to locations

Now you have two sorted lists. Items ranked by pick frequency and locations ranked by score. Start matching from the top, checking that the location size fits the item.

A few rules I apply while matching:

  • Put items that are often ordered together near each other. If you can, run a quick check on order history for common pairs.
  • Keep heavy items low, regardless of velocity. Nobody should be pulling a 40 pound case from above shoulder height 200 times a day.
  • Do not split the top movers across opposite ends of the building just because there was room.
  • Leave a few open golden zone slots for new fast movers. They will come.

Spread the very top movers a little

There is one exception to "put fast movers together." If your top five items are all in one bay, pickers will stack up waiting for each other at peak. Spread the very top handful across two or three bays at the front so people are not in each other's way.

Plan the move

A re-slot moves inventory, which means it can wreck accuracy if you are not careful. A few things that help:

  • Move during a slow period, a weekend, or a second shift with no picking
  • Move in a planned sequence so you do not need dozens of temporary spots
  • Do a location transfer in the system for every move, as you make it, not at the end of the day
  • Count each item at the new location after the move

That last step catches the mistakes while they are still easy to fix.

Measure before and after

Before you move anything, time a sample of picks. Maybe 30 order lines across a couple of pickers. Write down lines per hour.

After the re-slot settles in (give it two weeks), time the same kind of sample again. In small warehouses I typically see pick rates improve 15 to 30 percent from a first velocity slot. Buildings that were already reasonably organized see less.

If you see no improvement, the bottleneck was not travel. Go look at packing, replenishment, or how orders get released.

How often to re-slot

This is where people overdo it. A full re-slot is a lot of labor. You do not want to do it monthly.

What I recommend for most small operations:

  • A full velocity re-slot once a year, or before peak season
  • A quick quarterly review of the top 50 movers and any new items, moving only the ones that are badly out of place
  • Immediate slotting of new items into a reasonable spot on day one, instead of "wherever there is room"

The quarterly check takes an hour with the same spreadsheet. Rerun the pivot, compare the top 50 against where they currently live, and move the handful that drifted.

When a spreadsheet stops being enough

A spreadsheet works well up to a few thousand active SKUs and a single building. Past that, or if your mix changes weekly, slotting tools in a WMS start to pay for themselves. But I would not buy software for this until you have done it manually at least once. You will learn what your building needs, and you might find the spreadsheet is all you ever needed.