Where the other 37 percent of the slab goes
A 180 by 120 building is 21,600 square feet of foundation, roof and property tax. Divided the way the form does it — 10 ft units either side of a 6 ft corridor, 10 ft wide, 4 in partitions, 6 in exterior wall — 13,600 of those square feet are inside a unit and can be rented. That is 63 percent, and the remaining 8,000 square feet are not waste in any careless sense. They are 4,296 square feet of corridor, 2,685 square feet of a strip left over across the width, 720 square feet of partition and the tail end of each row, and 299 square feet of exterior wall. Every one of those is a real thing you pay to build and cannot rent, and the page lists them separately rather than folding them into an efficiency factor, because they behave completely differently when you change the plan.
Corridor width is a staircase, not a slope
The obvious lever is the corridor, and it is the one that does the least. At the default depths, a 4 ft corridor, a 6 ft corridor and a 9 ft corridor all give the same four corridor runs, the same 136 units and the same 13,600 rentable square feet. Nothing at all is bought by narrowing it. Then at 10 ft the module reaches 30 ft, four of them no longer fit in 119 ft of interior width, and the building drops to three corridors, 102 units and 10,200 square feet — 47.2 percent. One foot of corridor costs 34 units.
That is the shape of the whole problem. Rentable area is a step function of every dimension in the layout, and the value of a change depends entirely on which side of a step you are standing on. A ratio quoted on its own tells you nothing about that, which is why the table on the page prints the neighbouring widths rather than a slope.
The strip nobody draws
The largest single loss at the defaults is not the corridor. It is the 15 ft strip left across the width because 119 does not divide by 26 — 2,685 square feet of floor, inside the building, with a roof over it, that the geometry threw away. It usually appears on a drawing as a wider corridor at one end, or as nothing at all.
It is also the easiest to recover, because it does not need a step to be crossed. Deepen every unit from 10 ft to 11 ft 10 in and the module becomes 29.75 ft, four of which is exactly the 119 ft available. The corridors stay at four, the unit count stays at 136, the strip goes to zero and rentable area goes from 13,600 to 16,150 square feet — 74.8 percent instead of 63. At the same 1.10 dollars a square foot that is 17,765 dollars a month against 14,960, from the same slab and the same roof. Deeper units are harder to rent and harder to reach the back of, which is a genuine trade and not a free win, but it is the trade worth making consciously.
Why the ratio divides by gross
Rentable to gross means rentable divided by the outside footprint, not by the interior clear. That is deliberate and it is the harsher of the two. You buy land, pour slab and pay tax on the outside dimension, so that is the denominator that matches the cost. Dividing by the interior instead makes the exterior wall disappear from the arithmetic and flatters every building by a point or two. Whichever you use, the corridor is part of the difference and not part of the numerator, and getting that the wrong way round inverts the answer completely.
Questions people ask
What is the rentable to gross ratio for a self-storage building?
It is rentable unit floor divided by the gross outside footprint. For the layout in the form — 180 by 120 ft, 10 ft units either side of a 6 ft corridor — it comes out at 63 percent, with the corridor taking 19.9 percent, a leftover strip across the width taking 12.4 percent, partitions and row tails 3.3 percent and exterior wall 1.4 percent. This page has no view on whether any particular ratio is good; it takes your own dimensions and shows where the difference goes.
How many storage units fit in a 180 by 120 foot building?
At 10 by 10 units on both sides of 6 ft corridors, 136 of them: four corridor runs across the 119 ft of interior width, two rows each, and 17 units along each 179 ft row once a 4 in partition is taken at every joint. Change the unit width to 5 ft and the count roughly doubles while the rentable area barely moves, because the partitions eat more of the run.
Does a narrower corridor give me more rentable space?
Only if it crosses a step. At the default depths, anything from 4 ft to 9 ft of corridor gives four corridor runs and exactly 13,600 rentable square feet, so narrowing it buys nothing whatsoever. At 10 ft the fourth run no longer fits and the building loses 3,400 square feet at once. The table on the page prints the neighbouring widths so you can see which side of the step you are on before deciding.
What is the leftover strip and can I get rid of it?
It is the width left over when the interior does not divide evenly by one corridor module, and at the defaults it is 15 ft running the whole 179 ft length — 2,685 square feet. Deepening every unit from 10 ft to 11 ft 10 in makes four modules fill the 119 ft exactly, which takes rentable area from 13,600 to 16,150 square feet and the ratio from 63 to 74.8 percent with no change to the unit count. Deeper units are a real trade, but the strip is worth converting into something.
Can this handle a mix of unit sizes?
Not in one pass. It lays one unit width and one pair of depths into the whole footprint, which is what makes the area reconcile to the square foot. A real building has lockers along one wall, a few oversize units at the end of a run and a corridor that turns a corner. Run the page once for each band, add the results, and treat the ratio it gives as the ceiling a clean rectangle allows rather than what the finished drawing will show.