Perimeter, not area
The usual way to choose a shed is to add up floor area and pick something bigger. It gives the wrong answer often enough to be worth abandoning. Almost everything in a shed lives against a wall — tools hang on it, the bench sits against it, the bags lean on it, the bikes park along it — and the middle of the floor is the part you have to keep clear in order to reach any of it. What the shed has to supply is wall run, and wall run scales with perimeter.
That is why a 10 by 12 and an 8 by 15 both give 120 square feet and are not the same shed. The 8 by 15 has 46 feet of perimeter against the 10 by 12's 44, but only 8 feet of width, so once you put a two foot deep storage band down both long walls there are four feet left to walk in and no room at all for a mower to stand in the middle. Long and thin buys wall and spends floor. Square does the opposite.
The three placement letters
| Letter | Means | Costs you | Typical items |
|---|---|---|---|
| w | Against a wall | Wall run equal to its width, and sets the band depth | Bench, cabinets, bags, long-handled tools, wheelbarrow on end |
| f | Free on the floor | Its footprint, plus room to get around it | Mower, bikes, anything wheeled that you move in and out |
| h | Hung or shelved | Shelf area only — no floor at all | Totes, boxes, ladders, hoses, small tools |
Moving one line from w to h is the cheapest square footage available to anyone. An extension ladder stored on the floor along a wall takes seventeen feet of wall run and returns nothing; the same ladder on two brackets takes zero floor and zero wall run at working height. Run the list once with everything on the floor and once with the obvious candidates hung, and the difference is usually a whole shed size.
The door is the decision you cannot undo
Every other constraint on this page can be worked around after the fact. You can add shelves, hang things higher, buy narrower totes, and rearrange until it works. You cannot easily widen a door, because widening an opening in a finished wall means a new header, new framing, new siding and new trim, and on a shed that is most of a wall rebuilt.
The tightest-item check exists for this. It takes the narrowest way each item can be turned — its smaller of width and depth — and finds the largest of those, because that is the item that decides the opening. A mower is usually it, and it is why sheds holding mowers almost always have double doors. Size the opening for the item you will own in five years, not the one in the garage now, and check the height as well as the width. The rough opening that produces the clear width you want is on the rough opening calculator.
Where the growth allowance comes from
Twenty percent is the default and it is an observation rather than a safety factor. A shed that exactly fits the list on the day it is built is full within a season, because the existence of the shed changes what you keep. Things that were thrown away because there was nowhere to put them now have somewhere to go. Nobody has ever reported that their shed turned out to be too large.
The floor packing efficiency does a different job. Free-standing items do not tile neatly, they have handles and spouts and pedals that stick out, and you have to be able to get between them, so seventy percent of the raw footprint sum is a realistic figure for how much floor they actually consume.
Once the footprint is settled
The footprint is the input to nearly everything else. It sets the material list on the shed material takeoff, it sets the base on the foundation comparison, and it sets the volume that has to be ventilated on the shed ventilation calculator. If the list is close to fitting and a loft would resolve it, check whether the roof shape actually delivers the loft on the loft calculator before counting on it.
For the layout itself rather than the size, the garage shelving planner covers shelf depth against the space you have to leave, and the workshop layout planner handles the case where machines rather than storage are the constraint — because a shed with a table saw in it is a different problem, governed by the run a workpiece needs rather than by wall length.
Questions people ask
What size shed do I need?
It depends on wall run far more than on area, so the honest answer comes from listing what goes in it. As a rough orientation, a mower plus garden tools plus a few bins is usually an 8 by 10; add a bench, bikes and bagged material and it becomes a 10 by 12; add anything workshop-shaped and you are at 12 by 16. Those are starting points, not answers. Put your own list in and the difference between what you assumed and what the list demands is often two sizes.
Is a 10x12 shed big enough for a mower and a workbench?
Usually yes, and the constraint is not the area but the width. A 10 foot width with a two foot deep storage band down both long walls leaves six feet clear in the middle, which takes a walk-behind mower with room to get past it. Put the bench on a long wall and the mower parks opposite. Where it breaks down is with a riding mower, which needs both more floor and a much wider door, and at that point the width matters more than the length.
How wide should shed doors be?
Wider than the widest thing you will ever push through them, measured across its narrow way, and then wider again. Double doors giving a five foot clear opening are the common answer for a shed holding a mower; single doors around three feet are fine for a pure garden store. The check on this page finds the item that governs. What no calculator can tell you is what you will buy later, which is the actual reason people regret door width, so treat the result as a minimum.
Should I go longer or wider if I need more room?
Wider, in most cases. Length adds wall run, which is cheap to gain elsewhere by hanging things. Width adds clear floor down the middle, which is the thing you cannot recover by any amount of organising, and it is what decides whether a mower can stand in the middle while you get to the bench. The exception is when your list is dominated by long items against walls and nothing free-standing, in which case length is what you actually need.
Does a loft change the footprint I need?
It can, and the calculator will show you by how much if you switch the loft option on: everything marked as shelved moves upstairs and that wall run comes out of the requirement. The catch is that a loft is only storage if it has usable height, and on a shallow gable roof it mostly does not. Check the volume the roof shape actually delivers on the shed loft calculator before you size the shed down on the strength of it.