Loft floor area is a misleading number
A 12 by 6 loft is 72 square feet, and that is the figure it gets described by. What you can put on it depends entirely on how much clear height sits over each part of it, and under a pitched roof that varies from nothing at the walls to the full ridge height at the centre. This calculator walks across the width in small steps, works out the clear height at each one, and sorts the result into three bands: enough room to work, enough to shove something in and never see it again, and nothing.
Under a 6 in 12 gable on a 12 foot span, the ridge is 3 feet above the plate. If the loft floor is at the plate, no part of that loft has four feet of headroom and the middle two feet have three. It is a shelf for flat objects. The same span as a gambrel puts the ridge 6 feet up and holds most of that height out to within a couple of feet of the walls, which is a genuinely different room.
The three bands, and why the middle one is halved
| Band | Default | Counted as | What it actually stores |
|---|---|---|---|
| Working height | 48 in and over | All of it | Stacked totes, bulky items, anything you retrieve regularly |
| Low band | 24 to 48 in | Half of it | Things pushed in from the tall band and forgotten |
| Dead space | Under 24 in | Nothing | Nothing, because you cannot reach into it |
The low band gets halved rather than counted or discarded because it is real volume that is genuinely hard to use. You can fill it, but only by unloading the tall band first, which means in practice it holds the things you put there once. If you build proper low drawers or pull-out trays under the slope, count it fully and raise the packing efficiency.
Packing efficiency is where the optimism lives
Sixty-five percent is the default and it is not conservative. Rectangular boxes in a triangular space waste volume at every edge, sizes never match, and any loft you actually use needs a path to reach the back of it. A loft packed to eighty percent is a loft where nothing at the back can be retrieved without emptying the front, which is a warehouse problem people rediscover in their own sheds every spring.
The tote count that comes out is an upper bound in the same way. It assumes the entire usable volume is filled with identically sized boxes, which is what nobody has. Treat it as a scale check — is this a twenty-tote loft or a sixty-tote loft — rather than an inventory.
What all that weight does
The load figures are the part most people never calculate. Forty totes of boxed household goods at eight pounds per cubic foot is over a thousand pounds sitting on a loft floor that was framed with whatever was left over. Spread across 72 square feet that is around 15 pounds per square foot of contents, which sounds modest until you remember that real loft storage is not spread evenly: it is stacked against the end wall and in the corners, and the heavy things end up wherever they were easiest to slide.
The per-joist figure exists to make that concrete. What it does not do is tell you the joists are adequate. A loft floor is a floor, and if anyone will climb onto it the design load comes from the adopted code rather than from the weight of the boxes. The overhead storage load calculator follows the load path further, the floor joist calculator counts the framing, and headers and beams covers what happens where the loft lands on a wall with an opening in it.
The lever nobody uses
Everyone tries to fix a cramped loft by changing the roof pitch. The stronger lever is usually the loft floor height. Dropping the loft floor by six inches adds six inches of clear height across the entire loft width — not just at the peak, everywhere — because the whole roof profile lifts relative to the floor. It costs six inches of head height in the room below, which is a real cost, but it is a linear trade against a gain that applies to every square foot up there.
The second lever is wall height, which does the same thing without the cost below. A shed with eight foot walls and a loft at seven feet has a foot of full-height wall above the loft floor before the roof even starts, and that foot is worth more usable volume than a pitch change. Both of those are decisions made before the walls go up, which is why this page is worth running before the material takeoff rather than after it.
Questions people ask
How much can you store in a shed loft?
Less than the floor area suggests, and the roof shape decides how much less. A 12 by 6 loft under a 6 in 12 gable with the floor at the top plate has around 100 cubic feet above it and almost none of it with four feet of headroom. The same loft under a gambrel has roughly three times the volume and most of it usable. Run your own numbers, because the answer moves enormously with the loft floor height, the wall height and the roof shape, and those three matter more than the footprint.
How high should a shed loft floor be?
It is a trade rather than a number. Every inch you lower the loft floor adds an inch of clear height across the whole loft and takes an inch off the head height below. Most people set it just under the top plate so the room below keeps full height, then discover the loft is a crawl space. If the loft is the point of the shed, taller walls solve the trade properly: eight foot walls with the loft at seven leave a foot of full-height wall up there before the roof starts.
What does a loft full of boxes weigh?
Boxed household goods run roughly 6 to 12 pounds per cubic foot, so a hundred usable cubic feet is somewhere between 600 and 1,200 pounds. That is a wide band because contents vary enormously — books, paint, hardware and tools are several times the density of clothes and Christmas decorations. Use your own figure if you know it. The number matters because it is going onto framing that was almost certainly sized by what the plan said rather than by what you intend to put on it.
Does a gambrel really give that much more loft than a gable?
At the same span and the same ridge height, the gambrel has about 41 percent more cross-section, but the useful comparison is width at a headroom you can work in. On a 12 foot span with the ridge 6 feet up, at five feet of headroom the gambrel is nearly 5 feet wide and the gable is 2 feet. The gambrel holds its height out toward the walls instead of pinching to a point, and that is where loft storage actually lives. The full comparison is on the gambrel roof calculator.
Can I use the loft as a room rather than for storage?
That is a different question with a local answer. Habitable space has requirements for ceiling height, floor loading, egress, stairs and often for the building being permitted as something other than an accessory structure, and none of those are decided by a calculator. What this page gives you is the volume and the clear heights, which will tell you quickly whether the idea is even geometrically plausible before you ask anyone about the rest of it.