The Aisle Width Decision

By Dave Piasecki

Inventory reduction efforts over the past couple of decades have increasingly changed the focus from warehousing and storage density to material handling and speed. Industries that would previously manufacturer a six-month supply of an item, store it in high-density storage such as bulk floor storage or drive-in rack, and ship out in full-truck-load quantities are now manufacturing a two-week supply and shipping one pallet, one case, or one piece at a time to their customers. While this doesn’t negate the need for a warehouse or the importance of space utilization, it does change the storage characteristics from long-term high-density storage to temporary selective storage.

Determining optimal aisle width is a critical part of an overall storage/material-handling strategy. Aisle width decisions must attempt to achieve the best combination of productivity, space utilization, flexibility, safety, and equipment costs for the specific application. Assuming you’ll continue to use the aisle widths “you’ve always used” or “other warehouses use” could be a costly mistake.

The primary constraints to aisle width are the type of lift trucks used and the characteristics of the loads being handled. Since you probably can’t significantly change the characteristics of the loads being handled, you’ll find that the aisle width decision is actually a material handling equipment decision.

Aisle Types

Lift trucks used for handling unit loads in racked storage are categorized by the aisle widths they are designed to be operated in.

Wide Aisle (WA) trucks are the standard counterbalanced lift trucks that have become synonymous with the term “forklift”. Wide aisle trucks generally operate in aisles greater than 11’ with 12’ being the norm for handling 48” deep loads.
Narrow Aisle (NA) trucks operate in aisles of 8’ to 10’. This is primarily the domain of the stand-up reach and double-deep reach trucks.
Very Narrow Aisle (VNA) trucks generally operate in aisles of less than 6’ and often use guidance systems (wire, rail, optical) to travel within the aisles. Standardized VNA vehicles consist of man-up order selectors used to manually handle less-than-pallet-load quantities and man-up turret trucks used to handle unit loads (man-up turret trucks can usually also perform as order selectors)(Manufacturers of turret trucks include Raymond Corporation, Crown Equipment Corp, Yale Materials Handling Corporation, Hyster) . There are also several non-standardized VNA designs including a counterbalanced sit-down lift truck where the mast swings to 90 degrees (Drexel SwingMast)and another where the mast has been mounted to the steer axle located on the front of the truck (Landoll Bendi).

There are many other designs available that function in narrower aisles that bridge the gaps between wide and narrow aisle or narrow aisle and very narrow aisle. These include stand-up counterbalanced, stand-up straddle, side loaders, walkie counterbalanced stacker, walkie straddle stacker, and walkie reach stackers. The very affordable walkie designs can provide narrow aisle functionality to even the smallest warehouses and storage areas.

The most significant difference between VNA trucks and all others is that VNA trucks turn only the load while wide aisle and narrow aisle trucks must turn the entire vehicle in the aisle. The man-up VNA trucks also allow efficient operation at heights greater than their man-down counterparts (up to 40 feet).

Warehouse Aisle Widths

Space Savings

There are many factors to consider when calculating potential space savings. These would include vehicle specs, load size, load weight, rack height, and facility factors such as warehouse dimensions/shape and structural support locations. To give some general comparisons, assuming a standard 40”x48” load weighing approximately 1500 lbs and using wide aisles as the baseline, you can expect to be able to store 20%-25% more product by going to a narrow aisle system and 40%-50% more by going to a VNA system. A more complicated calculation is required when considering the option of going to narrow aisles and double-deep storage. Whenever you use higher density storage solutions where one unit load blocks access to another unit load, you will generally limit each location to unit loads of the same SKU. In doing this you will have unusable storage locations whenever the number of unit loads for a specific SKU is not an exact multiple of the capacity of the locations (often referred to as honeycombing). In the case of double-deep storage you can assume one unusable location for every two multiple unit load SKUs and every one single unit load SKU unless you have a system in place that ensures product is always received, manufactured, consumed, and shipped in multiples of two unit loads. In comparison to wide aisles, the double deep option can result in anything from a loss of 20% product storage to a gain of 60%. The extremes compare all SKUs being single unit loads to all SKUs being multiples of two.

Productivity, Flexibility, Safety and Cost

With the significant space saving available with NA and VNA options, certainly everyone should be converting to this type of storage, right? Not necessarily. As previously mentioned, warehouses are becoming more movement and productivity focused and, in rapidly changing markets, flexibility and cost are getting a lot of attention. And please, let’s not forget safety.

The fact is, the standard wide aisle counterbalanced lift truck design that’s been used for more than 50 years still remains a very viable option. The flexibility to pick a load out of pallet rack and load it immediately onto a trailer combined with fast travel speeds, a short learning curve, low cost, higher weight capacities, and a great variety of options and attachments will keep the standard forklift and wide aisles around for while. Wide aisles also provide more flexibility with diverse load sizes and weights.

Moving to narrow aisles and a reach or double deep reach provides greater storage density with only marginal additional equipment investment. Reach trucks are also capable of greater storage heights in excess of 30’ compared to the 24’ or less of wide aisle trucks. Disadvantages include slower travel speeds, slower putaway/extraction rates, longer learning curves and the inability to load trailers. Also, even though reach trucks operate at heights above 30’, it is much more difficult to place and extract loads at these heights with a man-down vehicle. Options including lift height selectors and tilt control are highly recommended for these high-lift applications to help increase productivity and reduce damage and driver fatigue. Even with these options, 30 plus feet is still a long way to look straight up when positioning a load. Neck strain, eyestrain, potential product damage and operator safety need to be carefully considered when storing at these heights.

VNA storage with order selectors and turret trucks has been gaining in popularity. For less-than-pallet-load storage, order selectors provide quick access to product in rack up to 40’ in height (20’-30’ most common) and aisle widths under 6’. Order selectors are very affordable although optional guidance systems can drive up costs. Turret trucks provide unit load storage up to 40’ and also provide the flexibility of order selector functionality for case picking operations. The man-up design of most turret trucks combined with the functionality of the turret can provide higher putaway/extraction rates than their wide and narrow aisle counterparts. The required guidance systems can also provide high travel speeds in aisles. Cost is the primary disadvantage of turret truck systems as both the vehicles and the guidance systems will cost significantly more than other systems. These systems also require much tighter tolerances for level floors and racking. This makes them more likely applications for new construction than for retrofitting existing facilities. Although the initial costs of turret trucks and the guidance systems are substantial, the savings associated with both space and productivity can easily offset these costs in moderate to large operations. Safety is obviously a concern with man-up vehicles and strict enforcement of safety belts or harness use is critical. While some of the non-standard VNA vehicles available are capable of trailer loading, man-up vehicles (turrets and order selectors) are not designed to load or unload trailers, so an additional staging step will be required with these vehicles.

Vehicle Type	Space Utilization	Cost
Standard Wide Aisle Forklift	Baseline	$
Narrow Aisle Reach	+20% to +25%	$ $
Narrow Aisle Double-Deep Reach	-20% to +60%	$ $
VNA Turret	+40% to +50%	$ $ $ $
VNA Swing Mast or Bendi	+35% to +45%	$ $ $ $

When evaluating specific equipment and designing racking configurations you will also need to make decisions on the actual aisle width you use. For turret trucks and order selectors this is pretty straightforward and should be done by the equipment supplier. For other vehicles you have more flexibility and need to balance the space savings associated with going with the minimum aisle width for a specific vehicle/load combination, and the productivity gains by going with a wider aisle. You may even choose to divide your warehouse and use wider aisles for your faster moving product and narrower aisles for the slow items.

The aisle width decision may not be an easy one, but it is one well worth the time it takes to do it right. Working closely with your material handling equipment supplier and getting additional outside help as needed can ensure the best methods are being applied to meet your specific business needs.

For more on Aisle Widths and related topics, check out my book

Saving Warehouse Space: Equipment, Methods, Strategies, and a little Math. .

Most businesses that carry inventory will experience space issues at some point in time. It’s important to understand how to make the most of the space you have. While storage equipment and material handling equipment can be an essential part of making the most of the space you have, there is more to it than just equipment.

Saving Warehouse Space explains storage capacity. Not just theoretical capacity (how much you can potentially store), but more importantly, the book explains utilization and working capacity. Without an understanding of utilization and working capacity, you may be making the wrong equipment and layout decisions. In addition, Saving Warehouse Space explains how choices related to pick faces and slotting impact utilization and working capacity.

And, of course, Saving Warehouse Space covers equipment choices and options for aisle widths. Going to narrow or very narrow aisles can provide significant space savings, but you need to make sure you are making the right choices for your specific facility. Saving Warehouse Space even covers lower-cost options to go very narrow aisle.

Other topics include options for offsite storage, and how cost models such as Economic Order Quantity and Equipment Return-On-Investment calculations may need to be tweaked to account for space limitations.

Visit my Lift Truck Pics Page and read my article on Lift Truck Basics for more info.

Go to Articles Page for more articles by Dave Piasecki.

Dave Piasecki, is owner/operator of Inventory Operations Consulting LLC, a consulting firm providing services related to inventory management, material handling, and warehouse operations. He has over 25 years experience in operations management and can be reached through his website (https://www.inventoryops.com), where he maintains additional relevant information.