The mirror is the thing you hit
Shelving gets planned against the width of the car, and the width of the car is not what arrives at shelf height. A vehicle 73 inches across the doors is commonly 84 across the mirrors, which puts each mirror five and a half inches outboard of the bodywork, at a height between roughly 40 and 55 inches from the floor. Shelving units are typically 16 to 24 inches deep and their shelves land squarely in that band. The result is a bay where the car parks perfectly, the doors open, and one mirror gets folded back by an angle bracket every time somebody is in a hurry.
So the side clearance you have to reserve is not the door swing figure on its own. It is whichever is larger of the door swing figure and the mirror overhang plus however close you are prepared to park to a shelf edge. In a two-car bay the middle gap is worse again, because it has two mirrors facing each other and the overhangs add.
Reserve first, then see what is left
The arithmetic only works in one direction. Add up the body widths, add the gaps each door needs, add whatever the mirrors demand on top of that, and whatever remains of the bay width is the shelving budget. Split that between the walls you intend to shelve. It is usually a smaller number than people expect, and on a tight two-car bay it is frequently zero.
| Bay width | Two vehicles at 73 and 71 in body | Left for shelving | Depth per wall, both sides |
|---|---|---|---|
| 18 ft | 144 in body, 72 in of 24 in gaps | 0 in | None |
| 20 ft | same | 24 in | 12 in |
| 22 ft | same | 48 in | 24 in |
| 24 ft | same | 72 in | 36 in |
Two feet of extra bay width buys one foot of shelf on each wall, and that is the whole exchange rate. It is worth knowing before a shelving order goes in, because a 24 inch unit in a 20 foot two-car bay is not a tight fit, it is an impossible one.
Height is where the depth lives
Everything above roof height is exempt from all of the arithmetic above, because nothing up there has to clear a mirror or a door. That is the reason the useful pattern in a narrow garage is shallow at hand height and deep above the vehicle, rather than a uniform depth all the way up.
A run over the bonnet at the back wall is the easiest version, because it uses depth the vehicle was never going to occupy and it is reachable from a step. A run along a side wall above roof height is the more valuable one, because it is long, and it is also the one that needs the most care: it has to start above the tallest vehicle that will ever park there, including one with a roof box, and it has to be reachable from somewhere that is not between two parked cars.
Reach and load then become the constraints instead of width. A shelf at 76 inches is above head height for most people. What goes there is light and bulky, it wants to be visible from the floor, and the fixings holding it are carrying a cantilever load that a wall may or may not be built to take.
Depth is half the question
Nothing on this page is a fixing schedule or a load rating. What a wall or a ceiling will carry depends on how it is built, what it is built from and how the fixings land in it, none of which can be worked out from a depth in inches. Overhead platforms in particular carry far more than people estimate, and the load per member is the number that decides whether it is a reasonable idea — that is the overhead storage load calculator. For a wall rack carrying long stock rather than boxes, the lumber rack load calculator covers the per-arm and overturning arithmetic.
The clearance side of this connects straight back to the garage vehicle fit calculator, which is where the door swing and mirror numbers come from in the first place, and to the material storage space calculator if the bay is doubling as a workshop and sheet goods are involved. If the shelving is going in around a bay that will also be coated, do the coating first: the garage floor coating calculator works out the kits, and painting under a full shelving unit is a job nobody does twice.
Questions people ask
How deep can garage shelving be if I park two cars?
Take the bay width, subtract both body widths, subtract the gap each door needs, subtract any extra the mirrors demand, and split what is left between the walls you are shelving. On a 20 ft bay with two vehicles at 73 and 71 inches and 24 inch door gaps, that leaves 24 inches total, which is 12 inches per wall if you shelve both sides. A 12 inch shelf holds paint tins, tools and small boxes and does not hold a stack of storage totes, which is the usual disappointment. Shelving one wall only doubles the depth available on that wall, and lifting the run above roof height removes the constraint entirely.
Why does the calculator use mirror width instead of body width?
Because it uses both, for different parts of the problem. Body width sets where the door swings from and therefore how much gap the door needs. Mirror width sets what the shelf edge actually meets, because the mirror is the widest part of the vehicle and it sits at roughly the height of a shelf. When the mirror overhang plus your parking clearance exceeds the door swing gap, the mirror becomes the binding constraint and the shelf has to be shallower than the door arithmetic alone suggested. In a two-car bay the middle gap has two mirrors pointing at each other, so the overhangs add together.
Is it better to shelve one wall deeply or both walls shallowly?
One wall deeply, for most people, because shelf usefulness is not linear with depth. A 12 inch shelf takes one row of most things. A 24 inch shelf takes two rows, or one row of storage totes, which is the item that actually determines whether a garage feels organised. Both walls shallow gives you the same total area with worse access on both sides and two sets of uprights to walk past. The exception is when the two walls have genuinely different jobs, such as tools on one side and garden equipment on the other, in which case splitting is worth the depth penalty.
How high can I start a shelf so a car parks underneath it?
Above the tallest vehicle that will ever be in the bay, plus a clearance you are comfortable with, and the answer to what that tallest vehicle is should include a roof box, a roof rack and a bike carrier, because those are the things that turn a comfortable gap into a repair. The calculator asks for the vehicle height and the clearance separately for that reason. Bear in mind that a run this high is above head height for most people, which changes what should go on it: light, bulky, infrequently used items rather than anything heavy or anything you will want twice a week.
Does the calculator tell me what the shelves will hold?
No, and it deliberately stops short of that. Depth and volume are geometry; load is a function of how the wall is built, what it is built from, how the fixings land in it and how the shelf itself is made, and none of that can be derived from dimensions. What it does give you is the volume, which is the number to sanity-check the plan against: a run producing 90 cubic feet of storage is roughly 25 totes, and 25 full totes is a lot of weight on a wall. If the weight is going overhead rather than on a wall, work out the load per ceiling member before anything goes up there.