Volume in, floor space out
A storage unit is sold by floor area and filled by volume, and the conversion between the two is where the size charts quietly make their assumptions. The chain is short: take the cubic feet of goods, divide by the height you will actually stack to, divide again by how densely the goods pack, then add whatever floor you intend to leave clear.
Work an example. Eight hundred cubic feet of a two-bedroom apartment, stacked six feet high, at 70 percent packing efficiency, needs 800 / 0.70 / 6, which is 190 square feet of stack. Add a 30 percent aisle and it is 248 square feet — a 10 by 25. Run the same 800 cubic feet the way a size chart does, stacked eight feet to the ceiling at 85 percent efficiency with no aisle, and it is 118 square feet, which fits a 10 by 15 with room to spare. Same goods, same arithmetic, two unit sizes apart. Every one of those differences is an assumption someone made on your behalf.
The three numbers the charts hide
| Assumption | What the chart assumes | What people actually do |
|---|---|---|
| Stack height | To the ceiling, 8 ft or more | 5 to 6 ft — what you can lift and reach safely |
| Packing efficiency | 85%+, as if it were all boxes | 60-75% once a sofa and a mattress are in it |
| Aisle | None. Solid to the door. | A walkway, if you ever want to reach the back |
None of the three is dishonest on its own. Together they are the difference between a unit you can use and a unit you can only fill. Packing efficiency is the one people underestimate: a stack of identical boxes packs beautifully, and a stack containing a bicycle, a floor lamp, a mattress and an armchair does not, because none of those things has a flat top to put the next thing on. Dismantling bed frames and tables and standing mattresses on edge against a wall is what moves that number up.
Stack height is a safety number, not a geometry number
The temptation, having paid for a unit with a nine foot ceiling, is to use all nine feet. Two reasons not to. Everything above shoulder height has to be lifted there and lowered back down, alone, standing on something, over a concrete floor — and the retrieval happens months later when you have forgotten which box is the heavy one. And the bottom of a column carries everything above it, so a nine foot stack crushes the boxes at the base, which are exactly the ones you cannot see are failing.
If you do stack high, the ordering rules are worth following properly: heaviest at the bottom, nothing you might want in the middle, fragile items only in the top two courses, and furniture never stacked on a finished surface. Label the sides of boxes rather than the tops, because in a full unit the top of a box is the one face you cannot see.
What the rate per square foot is really telling you
Comparing units by monthly price alone leads you to the smallest one that plausibly works, which is usually a mistake by one size. Divide the rate by the square footage instead and the picture inverts: small units are almost always more expensive per square foot than large ones, sometimes by half again, because the fixed costs of a door and a corridor are spread over less floor.
Set that against what the extra size buys. Not just room for the goods, but room to walk in, room to put the thing you need near the front, and room to not have to unload the entire unit onto the corridor floor to reach a box of documents. If you are storing for one move and one unload, buy the tight unit and pack it solid. If you are storing for a year while you look for somewhere to live, the aisle is worth more than the price difference, and you should decide that now rather than on the third visit.
Questions people ask
What size storage unit do I need for a 2-bedroom apartment?
The size charts say a 10 by 10, and that is true for the volume — around 600 to 1,000 cubic feet packed tight to an eight foot ceiling with no walkway. Stack to a realistic six feet, allow for the air around a sofa and a mattress, and leave floor to reach the back, and the same goods want 200 to 250 square feet, which is a 10 by 20 or 10 by 25. Which of those is right depends entirely on whether the unit is a sealed block you open once or a room you will visit. Decide that first, then pick the size.
Does a 10x10 storage unit really hold a two-bedroom apartment?
By volume, usually yes. A 10 by 10 with an eight foot ceiling is 800 cubic feet of space, and a two-bedroom apartment is commonly estimated at 600 to 1,000 cubic feet of goods. What the claim assumes is that you stack to the ceiling, that the goods pack efficiently, and that you leave no aisle — so the unit becomes a solid cube of possessions with a door on it. That works for a one-way store. It does not work if you need anything out of it before the end, and the packing efficiency assumption falls apart quickly once a mattress and an armchair are in the mix.
How much floor should I leave as an aisle?
Between 25 and 40 percent of the floor if you expect to go back in, and nothing at all if you do not. A 10 by 15 unit with a 30 percent aisle gives you a walkway roughly 3 feet wide down one side, which is enough to reach the back wall and to turn with a box in your hands. Less than that and you are climbing. The other half of access is ordering: things you might want near the door, labelled on the sides where you can read them, and a rough map of what went where taped inside the door.
Is a bigger unit cheaper per square foot?
Almost always, and the gap is large enough to change the decision. A small unit carries the same door, the same corridor frontage and the same administrative cost as a large one spread over a fraction of the floor, so the price per square foot falls as the size goes up. Work out the rate per square foot for each size you are quoted rather than comparing monthly totals — the step up from a 10 by 10 to a 10 by 15 is often a much smaller percentage of the rent than it is of the space, and the extra 50 square feet is the difference between a block of possessions and a room you can use.