Somebody is standing in an empty warehouse with an agent, and the number in their head is the floor area on the brochure. They divide it by the footprint of a pallet, get a figure that sounds generous, and sign.

What they will actually store is a fraction of that, and the gap is not small. We are called in after the lease is signed far more often than before it, and by then the constraints are fixed. Everything that decides your real capacity is a property of the building, and you have just committed to it for three years.

Here is what actually determines the number, in the order it bites.

Height is the first number, and it is not the height on the brochure

Pallet positions are won vertically. Floor area sets your footprint; height sets how many times you use it.

But the height that matters is not the roof apex, and it is not the figure in the listing. It is the clear height under the lowest obstruction across the area you intend to rack. Walk the space looking up, and the things that will cap you are:

• Sprinkler heads, and the clearance that has to be maintained below them
• Lighting, and whether it can be relocated or is cast into the structure
• Roof beams, purlins and bracing, which are lower than the ridge and often lowest at the eaves
• Services running at high level — ducting, cable tray, pipework
• Any overhead crane or gantry, which sterilises the space beneath its travel

A unit advertised on its apex height can have substantially less usable height at the eaves, and if your racking runs are laid out along the eaves you get the smaller number, not the bigger one.

You do not get to use all of that height either

Beam levels are not spaced at the height of your pallet. Each level needs the height of the loaded pallet plus clearance for the forklift to lift it clear of the beam and withdraw it without catching. That clearance is real and it is per level, so it compounds — the taller your load, the fewer levels you fit, and the more you lose to the clearances between them.

The top is where it hurts most. You will frequently find you have enough clear height for most of another level but not quite enough with the clearance included, and that near-miss costs you a full level across the entire warehouse. It is the single most common reason a real layout comes back below what an operator expected.

This is also why the answer changes if your pallets are not all the same height. A racking layout designed around your tallest load wastes space on every level holding your shortest one, which is a separate problem covered in mixing pallet sizes on selective racking.

Your forklift decides how much floor you lose to aisles

Aisles are the largest single deduction from a warehouse, and you do not get to choose the width — the truck does.

A counterbalance truck needs the most room to turn and place. A reach truck needs less. Narrow-aisle and very-narrow-aisle equipment needs least of all, but brings its own requirements around floor flatness and guidance.

The consequence people miss is the direction of the trade. Narrowing the aisle buys you more racking, and it also commits you to a truck type — and to replacing that truck with the same type forever, because the aisles will not accommodate anything wider. A fleet change made by someone in a different department later can strand a perfectly good racking installation. We wrote about how little clearance separates workable from unworkable in forklift turning radius and aisle width.

The other consequence is that density and access are traded against each other. Selective racking gives you every pallet reachable at any time and the lowest density. Drive-in and double-deep give you more pallets in the same footprint and less freedom to reach any given one. Which is right depends entirely on how many SKUs you carry and how fast they move — a question about your business, not your building.

The floor may cap you before the ceiling does

Racking concentrates load. The weight of a fully loaded bay arrives at the slab through a handful of base plates, which is a very different thing from a distributed floor load.

On a ground-floor slab in a purpose-built warehouse this is usually straightforward. It stops being straightforward when you are on an upper floor of a ramp-up building, on a mezzanine, or in a converted unit. There, the permitted floor loading can limit how high you may load regardless of how much clear height is above you — see racking a ramp-up warehouse.

Ask for the permitted floor loading in writing, from the building's approved records, before signing. Not what the agent believes.

Columns, doors and everything that is not storage

The remaining deductions are unglamorous and they add up fast:

• The column grid. Columns landing inside a rack run break it, and a run that has to stop and restart loses bays at each break. A regular grid that suits your bay length is worth a great deal; an awkward one quietly costs you several percent of the building.
• The dock and staging area. Goods have to land somewhere before they are put away and be marshalled somewhere before they leave. Racking to the roller shutter is not a plan.
• Escape routes and access. Gangways and clearances are not negotiable and cannot be borrowed back later.
• Charging, battery changing and equipment parking. Real floor area that has to come from somewhere.
• Anything you already do that is not storage — repacking, QC, an office, a workshop bench.

What to measure at the viewing

Take a laser measure and fifteen minutes, and you will know more than the brochure tells you:

• Clear height to the lowest obstruction, taken in at least three places including at the eaves
• The column grid spacing, both directions
• The permitted floor loading, requested in writing
• Whether the sprinkler system is in place and where the heads sit relative to your intended top beam level
• Door and shutter heights, and whether a loaded truck can pass through them
• The slab condition and whether it is level enough for the truck type you intend to use
• Where power and lighting are, and whether either has to move

Bring your own pallet dimensions and your tallest load height. Those two numbers, with the clear height and the column grid, are enough for a realistic estimate rather than an optimistic one.

Plan for the operation you will have, not the one you have

The most expensive version of this mistake is not underestimating capacity. It is fitting the building exactly, then growing.

A layout designed to the last bay leaves nowhere to put the extra volume, and the options at that point are all bad: rent a second unit, convert to a denser system mid-operation, or start stacking on the floor in the aisles, which is where damage and near-misses begin. Leaving deliberate slack, and choosing a configuration that can be densified later without a full reinstall, costs very little at the outset. We covered that in racking patterns that absorb growth.

Get the number before you sign, not after

We will lay out a unit you are considering and tell you what it genuinely holds — including when the honest answer is that the building next door will hold more for the same rent. That conversation is far more useful before the lease is signed than after, and it takes very little time.

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