Area, not trays
A nine tray machine with 13 inch round trays and a 2.5 inch centre hole has about 128 square inches of mesh per tray. Take off the rim and the edge you leave for air and you are loading about 109 of them. Nine trays of that is 6.79 square feet in total.
Now weigh what you actually put on one. Sliced fruit in a single layer, not touching, comes out surprisingly light — three quarters of a pound to the square foot is a normal figure and half a pound is common for anything thick. So the machine takes about five pounds a run, and twelve pounds of fruit is three runs.
That is the whole reason this calculator exists. The tray count on the box tells you nothing until it is multiplied by an area and divided by a loading rate you measured yourself.
Measuring the loading rate
Load one tray the way you like it loaded. Weigh what went on. Divide by the tray area in square feet. That is your figure for that crop at that thickness, and it will hold up year after year. Loading it heavier is possible and slows everything down; loading it lighter wastes a run. The number is a habit rather than a specification.
The other end is easy, which is the surprise
Most of what you handled was water and it leaves. Twelve pounds of fruit at twenty per cent remaining is 2.4 pounds of product and 9.6 pounds of water — well over a gallon of it, at 8.34 pounds to the gallon of water. If a quart of the finished slices weighs four ounces, that 2.4 pounds is 9.6 quarts, and the whole harvest is a modest row on a shelf.
| Stage | 12 lb of sliced fruit |
|---|---|
| Tray area needed at 0.75 lb/sq ft | 16 sq ft |
| Trays at 109 sq in of loadable mesh each | 22 |
| Runs on a 9 tray machine | 3 |
| Dried weight at 20% remaining | 2.4 lb |
| Water removed | 9.6 lb, about 1.15 gal |
| Storage at 4 oz to the quart | 9.6 quarts |
The bulk density is the input that decides the shelf, and it is not the weight. Whole slices, broken pieces and powder made from the same fruit fill wildly different volumes. Fill a jar with what you actually store and weigh it once.
What this does not tell you
Nothing here is a drying time, a temperature, a test for done, or a keeping time, and nothing here says whether a result is dry enough or safe to eat. Those come from a tested process — the USDA, the National Center for Home Food Preservation, your state extension service — and from the manual for your machine. This page divides pounds by square feet.
Questions people ask
How many trays do I need for ten pounds of fruit?
It depends entirely on how heavily you load a tray, which is why that is an input rather than an assumption. At three quarters of a pound per square foot and 0.75 square feet of loadable mesh per tray, ten pounds is about 18 trays, which is two runs on a nine tray machine. Load it thicker and it is fewer trays and a longer run.
How do I measure the loading rate?
Load one tray the way you normally would, weigh what went on it, and divide by the loadable area of that tray in square feet. Once measured it holds for that crop at that thickness and you never have to guess again. It is the single input that decides the whole tray count.
Why is the finished amount so small?
Because most of what you handled was water and it left. At twenty per cent remaining, five pounds in is one pound out and four pounds of water gone, which is nearly half a gallon. That is why the drying problem is a tray problem and never a shelf problem.
Does the same arithmetic work for a freeze dryer?
The tray area, the runs and the shrink all work the same way, because they are geometry and weights. What differs is the loading rate you measure and the weight remaining, both of which you enter yourself, and the run time, which belongs to your own machine and its manual.
How do I know when it is dry enough?
It gives none, on purpose. This page is quantity and container arithmetic. Process times, pressures, headspace, altitude adjustments and anything about acidity belong to a tested recipe from the USDA, the National Center for Home Food Preservation or your state extension service, and those are the only places to get them. Where the calculator asks for one of those numbers, it is because you are copying it in from the recipe you are already following.