The volume of a storage space is not the volume you can use, and the gap between the two is larger than people expect. Boxes are rectangular and rooms have doors, but more importantly boxes cannot be stacked floor to ceiling unless you never want the ones at the bottom again, and a space packed to a hundred percent is a space you cannot walk into. A realistic figure for a storage room is somewhere between fifty and seventy percent of its volume, and this shows both, with the box count that follows from each.
Room, packing and boxes
A room is never full of boxes. Somewhere between a half and three quarters is real.
70% if everything is boxed and stacked; 50% if you need to reach things.
A standard removal box is about 64 L; a banker’s box is about 40 L.
Result
The percentage is the whole calculation
A storage room's volume is a geometric fact and its usable volume is a behavioural one, and the gap is not small. Boxes do not tessellate perfectly around a door swing. Stacks need a gap between them so a box can be lifted out without dismantling the pile. Anything that is wanted more than once a year has to be reachable without moving six other things, which means aisles.
Seventy percent is the figure for a space where everything is boxed, uniform and stacked, and where you accept that reaching the bottom of a stack means emptying it. Fifty percent is the figure for a space you actually use: shelves on two walls, an aisle down the middle, and everything visible. Below about fifty, the space is under-used and you have paid for volume you are not filling.
Why boxes are the wrong unit for some things
Everything boxes well except furniture, and furniture is what usually dominates a storage space. A mattress, a sofa, a bicycle and a ladder have volume but no box, and they waste a great deal of the space around them because nothing stacks on top of them. The practical consequence is that the box count here is an upper bound: if a third of what you are storing is furniture-shaped, reduce it by a third.
Uniformity helps more than anything else. A room full of identical boxes packs to seventy percent; a room full of boxes of fifteen different sizes packs to fifty-five, because every odd-shaped gap is lost. Buying one box size in bulk is the cheapest way to increase what fits.
Things used more than once a year go at the front and at waist height. Everything else goes behind and above them.
Labelling matters more than tidiness. A box you cannot identify is a box you will open twice, and a stack you label on the side — not the top — is a stack you can read.
Shelving beats stacking
Floor-stacked boxes waste the headroom, because a stack you can safely lift is about 1.5 metres and a stack you can safely reach into is lower still. Shelving turns vertical volume into usable volume: at 40 cm spacing, a 2.2-metre room takes five levels, and each level is independently reachable without disturbing the others.
The trade-off is that shelving itself has volume and cost, and deep shelving wastes space at the back just as floor stacking does. The usual answer is shelves of 40 to 50 cm against the walls with a clear aisle, which suits a box of 40 to 64 litres and keeps everything visible from the aisle.
| Space | Volume m³ | Usable at 70% | Boxes of 64 L | Note |
|---|---|---|---|---|
| Under-stairs cupboard | 2.5 | 1.75 | 27 | Awkward shape; nearer 50% |
| Garden shed, 1.8 x 2.4 | 9.5 | 6.65 | 103 | Bikes and tools cut it down |
| Garage bay, single | 36 | 25.2 | 393 | Usually nearer 50% in practice |
| Storage room, 2.4 x 3.0 | 15.8 | 11.1 | 173 | The default case here |
| Loft, boarded | 20 | 10 | 156 | Low headroom; nearer 50% |
| Self-storage, 3 x 3 m | 27 | 18.9 | 295 | What a unit this size really holds |
A worked example
- Empty volume2.4 by 3.0 by 2.2 is 15.84 m³.
- Usable shareAt seventy percent that is 11.09 m³, or 11,088 litres.
- BoxesAt sixty-four litres a box that is 173 boxes.
- CostA hundred and seventy-three boxes at $3 is $519, which is usually more than people budget for boxes.
- Reality checkFive shelves fit at 40 cm spacing, and if a third of what is going in is furniture, think of it as about 115 boxes and some floor space.
Five hundred and nineteen dollars of boxes is the sort of number that makes buying fewer, better boxes — or reusing what is already in the house — worth considering. Cardboard boxes are the cheapest per litre and they collapse when empty, which is worth a lot in a space that changes.
What to do before filling it
Reduce first. The cheapest storage is the storage you do not need, and a space measured before a clear-out is always bigger than it turns out to be. The usual ratio is that a third of what goes into storage is never retrieved, which means a third of the volume is being paid for permanently for nothing.
Then measure the largest items rather than the average ones. Capacity is decided by the awkward things: the mattress defines whether a sofa fits beside it, and the bicycle defines the aisle. Boxes fill the gaps afterwards.
Where the space is a loft or a shed, weight matters as much as volume. A boarded loft is usually rated for a light distributed load, not for a hundred and seventy boxes of books, and the joists were not designed for it. The room volume figure gives the air volume of a habitable room, which is a different question from how much you can stack in it.
Do
- Reduce before you store — a third of stored items are never retrieved.
- Use one box size, which packs far better than a mix.
- Shelf the walls and keep a clear aisle.
- Label boxes on the side, not the top.
- Check the floor loading if it is a loft.
Avoid
- Assuming a room's volume is its usable volume.
- Stacking to the ceiling, where nothing is reachable.
- Mixing fifteen box sizes and losing every odd gap.
- Forgetting that furniture has volume and wastes space around it.
- Loading a loft with books without checking the joists.
Where the numbers stop being true
The usable percentage is a judgement, not a measurement, and it is the input that swings the answer most. Everything else on the page is arithmetic; that one figure is a guess about your own behaviour, and it is worth setting it honestly rather than optimistically.
Boxes are treated as perfect cuboids filling the volume exactly, which is why the count is an upper bound. Real boxes bulge, they do not stack perfectly, and the last ten percent of any space is lost to the gaps between rectangular things and rectangular rooms.
Nothing here accounts for shape. A long narrow space packs worse than a square one of the same volume, because the aisle has to run the length of it. A very low space packs worse still, because nothing can be stacked. And none of it accounts for what you will want to reach in February, which is the only thing that really matters.
How the figures are produced
Volume is width times depth times height. Usable volume is that times the percentage you entered, and the box count is usable litres divided by the volume of one box, rounded down because a partial box does not fit. Shelf count is the height divided by a 40-centimetre spacing, which suits a standard box.
Questions people ask
How much fits in a 3 x 3 m storage unit?
About nineteen cubic metres usable out of twenty-seven of space, which is roughly 295 boxes of 64 litres if everything boxes.
Why not a hundred percent?
Because you need aisles to reach things, gaps between stacks to lift them out, and headroom above them. A space packed solid is unusable.
What percentage should I use?
Seventy if everything is boxed and uniform, fifty if you need to reach things regularly, and forty if furniture is involved.
Is shelving worth it?
Almost always. It turns unusable headroom into reachable levels, and five levels at 40 cm fit in a 2.2 m room.
Are plastic boxes better than cardboard?
Cardboard is cheaper and collapses; plastic stacks better, resists damp and keeps its shape. For a damp shed, plastic. For a dry room and a finite job, cardboard.
Can I stack to the ceiling?
You can, and you will never see those boxes again. About 1.5 m is the limit for a stack you can safely lift into.