Methodology
Every capacity figure on this site comes from one formula: usable volume equals floor area × 8 ft ceiling × packing efficiency, less an aisle allowance where access is needed. Item volumes are packed cubic feet. Cost figures are national reference ranges, not quotes. This page documents each assumption and its known limits.
- Capacity model v1.0
- Reviewed 2026-08-02
- Cost data August 2026
The capacity formula
Three quantities, kept strictly separate throughout the site:
- Floor area = width × length, in square feet (sq ft). What facilities advertise.
- Gross volume = floor area × ceiling height, in cubic feet (cu ft). What the unit physically encloses.
- Usable volume = gross volume × packing efficiency × (1 − aisle allowance). What you can actually store.
Conflating the first with the third is the most common error in storage sizing, and the reason people rent units one size too small.
Assumptions, stated
| Assumption | Value | Why | When it is wrong |
|---|---|---|---|
| Ceiling height | 8 ft | The US standard across major operators | Newer facilities offer 10 ft ceilings, adding about 25% volume |
| Packing efficiency, default | 85% | Stacked to around 7 ft of the 8 ft height, furniture placed sensibly | Careless loading is nearer 70%; meticulous loading reaches 93% |
| Aisle allowance | 15% | Space lost to a walkway when you need access | Applied only when the user asks for access |
| Recommended fill ceiling | 90% | Above this, a unit stops tolerating the items you forgot | Short, well-planned loads can safely run higher |
| Item volume basis | Packed cubic feet | Matches a professional mover cube sheet | Unusual or oversized items should be measured directly |
Packing efficiency in detail
| Density | Efficiency | What it requires |
|---|---|---|
| Loose | 70% | Nothing stacked above head height, fragile items, room to move things around. |
| Moderate | 85% | Stacked to around 7 ft with furniture placed sensibly. The default. |
| Tight | 93% | Stacked to the ceiling, uniform boxes, disassembled furniture. Needs planning. |
These figures are the single largest source of variance in any result. We default to moderate rather than tight because tight packing is a commitment — disassembled furniture, uniform boxes, stacking to the ceiling — that most people do not make once the truck is unloaded.
Item volumes
The catalog holds 95 household items across 7 rooms, each with a packed cubic-foot value and a typical loaded weight. Packed volume is the space an item occupies as loaded, including what its shape wastes — not its bounding box. A recliner's packed volume exceeds its box dimensions; a dining table's is lower, because the legs come off.
The full catalog is published as our open dataset of packed volumes in CSV and JSON so any figure on this site can be checked independently.
Custom items entered by dimension are treated as bounding boxes with a 10% shape allowance added.
Knock-down furniture is valued disassembled. A professional mover's cube sheet prices a queen bed set at roughly 95 cu ft because it is carried assembled; the 60 cu ft recorded here assumes the frame comes apart, which is standard for storage and is what our packing guidance tells you to do. Published sources put that saving at 30 to 35 cu ft. If you plan to store a bed assembled, add it back — and the same applies to dining tables with the legs left on.
Home-size presets
Each home-size preset is a complete item list, not a rule of thumb. A "3 bedroom house" is a specific inventory of furniture, appliances, garage contents and boxes, and its cubic-foot total is computed from the catalog at build time. This means the number on a size page, the number in a table and the number the calculator returns cannot drift apart.
Presets describe a typical household. Real variance between two homes of the same size is routinely 200–300 cu ft, driven by time in the home, appliances and garage contents. That is why every preset is editable rather than fixed.
Cost data
Publisher-maintained national planning ranges for standard drive-up or interior units, month-to-month, before promotions, taxes and fees. They are not a reproducible market index or a facility quote.
Regional figures apply market-type multipliers to the national range:
| Market type | Low multiplier | High multiplier |
|---|---|---|
| Northeast metro (NYC, Boston) | ×1.35 | ×1.9 |
| West Coast metro (SF, LA, Seattle) | ×1.3 | ×1.85 |
| Southern metro (Atlanta, Dallas, Miami) | ×0.95 | ×1.25 |
| Midwest metro (Chicago, Minneapolis) | ×1 | ×1.3 |
| Suburban, most states | ×0.85 | ×1.1 |
| Rural and small town | ×0.6 | ×0.9 |
We deliberately do not publish ZIP-code prices. Facility rates move weekly with occupancy, and a precise figure that is stale misleads more than a range that is honest. Cost data is reviewed twice a year; the current review is August 2026, and every cost page carries that date.
One cost input is not a publisher estimate. The /data/storage-cost-index/ archives a federal price series in full, and it is the only figure in this section a reader can verify against an outside authority. It measures how storage costs moved, not what a unit costs, and the difference between those two things is stated on that page.
Error margins and known limits
Capacity: accurate to roughly one size step when the item list is complete. Error comes overwhelmingly from two sources — items omitted from the list, and optimism about packing — and both push the same way, which is why the recommended size targets 90% fill rather than 100%.
Cost: the ranges are wide because the market is. Treat them as a sanity check on a quote, not as a quote.
What the model cannot know: actual interior dimensions (advertised sizes are nominal and real units run a few inches under), door and corridor clearance, floor level, and how well you will pack. Measure before you rent.
When not to rely on this site
How this page is maintained
The capacity model is versioned. Changes to any assumption bump the version and are noted here with the date. The current version is v1.0, last reviewed 2026-08-02.
Content is maintained by the Storage Size Lab Editorial Team and passes the documented our internal review process before publication. This is internal data and consistency QA, not an external facility-operator endorsement. Where AI tools assist with drafting, every figure is verified against the dataset before it goes live, and no figure is generated by a language model. See the editorial policy.
Found an error? The how to report an error explains how to report it and what we do next.