Two different square feet
Every propagation house has a floor area and a bench area, and they are not interchangeable. The floor is what you heat, cover, insure and pay rent on. The bench top is the only part that grows anything. The ratio between them is set by one decision — how wide the aisles are — and it is usually made by whoever assembled the benches rather than by anyone doing arithmetic.
On the numbers this form opens with, a 5 ft bench with one 2.5 ft aisle charged to it occupies 7.5 ft of house width for 5 ft of bench. That is a growing area ratio of 66.7 percent. Switch to the two-aisle case, where every bench has a walkway on both sides and nothing is shared, and the same bench occupies 10 ft for 5 ft of bench — 50 percent. The house has not changed size. You have simply thrown away a sixth of it, and you will keep paying for that every winter the heater runs.
Trays do not tile a bench
The second loss is smaller and much more annoying. A bench 20 ft long and 5 ft wide is 240 by 60 inches. Take 5 percent off the length for the wet end and the fittings and you have 228 by 60 inches to fill. A tray with a 21.5 by 11 inch footprint goes five across the 60 inch width with 5 inches left over, and ten along the 228 inch length with 13 inches left over. That is 50 trays. Turn the leftover 13 inch strip and two more trays go in sideways, which is why the calculator reports 52 rather than 50.
Fifty-two trays of 128 cells is 6,656 cells on one bench, or 66.6 cells for every square foot of bench top. Once the aisle is charged in, the same bench delivers 44.4 cells per square foot of floor. That second figure is the one to carry into any conversation about whether a house pays for itself.
| Measured against | Area | Cells per sq ft |
|---|---|---|
| Bench top under trays | 85.4 sq ft | 77.9 |
| Whole bench top | 100.0 sq ft | 66.6 |
| Floor, one shared aisle | 150.0 sq ft | 44.4 |
| Floor, two aisles | 200.0 sq ft | 33.3 |
The spread between the top and bottom rows is a factor of 2.3, and every one of those numbers describes exactly the same 52 trays.
The rounding at the end
Twenty thousand cuttings at 6,656 cells a bench is 3.005 benches. That rounds to four, and the fourth bench carries 6,624 empty cells all season — nearly a whole bench of nothing. It is worth seeing that before the steel is ordered rather than after. Six inches more bench width, a slightly different tray, or accepting 19,968 sticks instead of 20,000 all remove the fourth bench entirely, and any of them is cheaper than building it.
The packing routine here tries four layouts: the tray both ways round, and each of those with a turned strip at the far end. It is a shelf packer and not an optimal one. A real bench can occasionally take one more tray than this finds, by mixing orientations in a pattern no simple rule produces. It will never claim a tray that does not fit, so read the answer as a floor rather than a ceiling.
Where the inputs come from
Measure the tray you own rather than trusting a trade name. Nominal 1020 flats from different mouldings differ by enough to change whether five fit across a 60 inch bench. Measure the bench top including any lip, and decide honestly how much of the length disappears to the hose reel and the end post before you type the usable share.
Once you have the cells per bench, the next questions are what those cells become. The potting-up stage calculator walks the crop forward through container sizes and shows where the space peaks, which is almost never here. The stock plant calculator works backwards to the block that has to supply the cuttings. For the house itself, hoop house sizing gives the floor area to type into the last field, and greenhouse heat loss tells you what heating that floor costs.
This page holds no horticultural information. It does not know what you are sticking, how long it takes, or what share of it roots. Those come from your own records or from your extension service, and the calculator only counts rectangles.
Questions people ask
Why does the calculator sometimes report more trays than length divided by tray length times width divided by tray width?
Because it also tries turning the leftover strip. On the default bench the straight grid gives 10 by 5, or 50 trays, and leaves a 13 inch strip across the end. A 21.5 by 11 inch tray turned sideways is 11 inches deep, so two more fit in that strip standing on their long edge across the bench. That is 52. The routine tries four arrangements in total and reports whichever holds most, along with a description of which one it was.
Should the aisle be counted once per bench or twice?
It depends entirely on the layout, which is why it is a select rather than an assumption. In a house where benches run in parallel rows down the length, each aisle serves the bench on either side, so one aisle width per bench is the right charge. A single bench standing alone in the middle of a floor, or a bench that has to be walked all the way round, carries two. Run it both ways and the difference on the default numbers is 50 square feet of floor per bench, which over four benches is a fifth of a small house.
How wide should the benches and aisles actually be?
That is not a question this page answers, and there is no general figure. It depends on how far the people working can comfortably reach, whether carts or a boom carriage have to pass, whether trays are moved by hand or on a conveyor, and what the building allows. What the calculator does is show you the cost of each choice in cells and square feet, so the decision gets made with a number attached rather than by eye.
Does leaving the outer row of a tray empty change much?
Set the cell share field and see. On a 128 cell tray, sticking 100 percent gives 6,656 cells per bench; the arithmetic is linear from there, so any share you enter scales the per-bench figure directly and then the bench count rounds up on top of it. The reason growers do it is edge drying and grading, which is a horticultural question this page has nothing to say about — it just counts whatever share you tell it.
Can I use this for whole flats rather than cells?
Yes. Put 1 in the cells field and every cell figure becomes a flat figure. That is the right way to use it for anything sold or moved by the flat, and it also works for counting pots directly if you enter the pot footprint as the tray size, although for containers standing on ground rather than in flats the offset spacing on the overwintering space calculator will fit more in the same area.