The pitch is four numbers, and only one of them is negotiable
A tier pitch is the tray and its structure, the crop, the gap up to the fixture, and the fixture body. The tray depth is fixed once the system is bought. The fixture depth is fixed by the fixture. The crop height is fixed by what you grow and when you cut it. That leaves the fixture clearance as the only number anyone actually argues about, and it is the one with the least reliable answer, because the mounting height in a photometric report is a measurement condition rather than a recommendation.
The defaults on the form — 4 in of tray, 8 in of crop, 10 in of clearance and 3 in of fixture — make a 25 in pitch. In a 12 ft room with 8 in kept clear underneath and 12 in above, that is 124 in of usable height and four tiers, with 24 in spare and exactly one inch short of a fifth.
One inch, one tier, a quarter of the room
That one inch is the whole point of the page. Four tiers becomes five and the canopy on six 8 x 4 racks goes from 768 sq ft to 960. Nothing else about the building changes. It is also why the measurements deserve more care than they usually get: room height taken to the lowest sprinkler head rather than to the deck, tray depth measured on a built tier with water in it, crop height taken on the tallest plants in the tray on harvest day.
The failure runs the other way too. A crop that finishes two inches taller than last time, because it sat an extra four days waiting for a delivery slot, does not cost you two inches of headroom. It costs a tier.
What the canopy per square foot number is for
Dividing the canopy by the room floor gives a single number for what the stacking bought. With the defaults it is 1.28 sq ft of canopy for every square foot of building, which sounds unimpressive until you notice the racks only stand on 32 percent of the floor. The other 68 percent is aisle, plant room, wash-down and the space to get a trolley past, and none of it goes away when you add tiers.
That is the honest version of the vertical farming arithmetic. Tiers multiply the canopy over the rack footprint, not over the building. Getting the footprint share up is a layout problem — carriages, narrower aisles, moving the plant room out of the grow space — and it is usually harder than adding a tier.
What this cannot see
Structure, airflow, heat and reach. A tier of flooded trays is a serious load and the rack either carries it or does not, which is a question for whoever designed the frame. Air has to move between tiers or the canopy sits in its own boundary layer. A fixture radiates downward onto its own tier and upward onto the tray above it. And a top tray at 11 ft is worked off something, in a wet room, which is a real hazard rather than a design detail. A pitch that divides beautifully and fails any of those four is not a design.
Questions people ask
What crop height should I put in?
The one you measured, on the tallest plants in the tray, on the day you actually cut. Not the average and not the figure from a seed packet. The tallest plants are the ones that hit the fixture, and the day you actually cut is often later than the day you planned to. If you have never measured it, measure it once at the next harvest and then keep the number.
Is the fixture clearance a safety figure?
No, and this page treats it purely as a dimension. How close a fixture can run to a canopy depends on the fixture, on what it does to the light distribution across the tray, and on the crop. The photometric report is written at a stated mounting height, and readings taken with your own quantum meter at the height you actually intend to use are the only thing that settles it. Nothing here says any clearance is acceptable.
Why does the leftover height matter?
Because it tells you how close you are to the step. With the defaults there is 24 in left over and the next tier needs 25, so you are one inch away — which is inside the error of most of the measurements that went in. Twenty-four inches of leftover is also worth spreading across the tiers as crop headroom rather than leaving stacked at the top doing nothing.
Does this account for the aisles?
Only if you fill in the room dimensions, and then only as an outcome rather than a plan. It divides the rack footprint by the room floor and tells you what share the racks cover. It does not lay racks out, check that an aisle is wide enough to work in, or know anything about doors, drains or where the panel is.
Can I use this for movable carriage racks?
Partly. The tier arithmetic is the same, and the canopy per rack is the same. The floor share is not, because carriages share one aisle between many racks, so the calculation will report a floor share above 100 percent and flag it. Take the tier count and the canopy figures and work the floor out separately.