The denominator is the whole argument
Spawn rate is expressed as a percentage, and the percentage is of something. Growers who work in wet weight quote one number; growers who think in dry substrate quote another; and both write it down as "five per cent" without qualification.
At 60 per cent moisture, a hundred pounds of wet substrate contains 40 pounds of solids. Five per cent of the wet figure is 5 pounds of spawn. Five per cent of the dry figure is 2 pounds. That is a factor of two and a half, and the conversion is simply one divided by the solids fraction — at 60 per cent moisture, 1 ÷ 0.40, which is 2.5. At 65 per cent it is 2.86. At 70 per cent it is 3.33.
Neither convention is wrong. What causes trouble is copying a rate out of one source and applying it against a weight measured the other way, which either doubles the spawn or halves it.
Working backwards up the ladder
Spawn is made from spawn. If you need 30 units to inoculate a batch, and one unit transfers into ten new ones, you needed 3 units a generation earlier and one the generation before that. The arithmetic is a division rounded up, repeated once per step — the same shape as compound interest run in reverse, with the rounding-up doing something interest never does.
Losses change the ratio rather than the shape. If ten per cent of units do not carry through a step for whatever reason, the useful expansion is not ten but nine, and the ladder needs a slightly wider base. The page asks for that share as a plain number and draws no conclusion at all about the cause; it is bookkeeping, not diagnosis.
| Step | Units needed | Ready by |
|---|---|---|
| Final generation | The spawn you inoculate with | Substrate mixing day |
| One step back | Final ÷ good units per parent, rounded up | One generation earlier |
| Two steps back | Same division again | Two generations earlier |
The lead time is the part people underestimate. Two generations at fourteen days is 28 days before the substrate is even mixed, and a step that has to be repeated costs a whole generation rather than a few days, because there is nothing to transfer from in the meantime.
Rounding up is not free
Spawn comes in whole units. If the rate asks for 4.2 units you make five, and the real rate applied to the batch is higher than the one you wrote down. On a small batch this is a large distortion — a single unit against a rate that wanted two thirds of one is fifty per cent over. On a batch of forty units it is noise. The page shows the rate you asked for and the rate you actually get side by side for exactly that reason.
Where this sits
The substrate itself comes from the substrate batch calculator, which produces the wet weight and moisture this page starts from. What comes out the far end is handled by the biological efficiency and yield calculator. If the units are going onto shelves while they colonise, the grow room staging calculator works out how much shelf that ties up and for how long.
Questions people ask
Is a spawn rate a percentage of wet or dry substrate?
Both conventions are in use and the source usually does not say. At 60 per cent moisture the two readings are 2.5 times apart, because the conversion factor is one divided by the solids fraction. Use whichever basis your own process was tested at, set the selector to match, and write the basis down next to the rate.
How do I work out how many generations I need?
Divide the units you need by the good units one parent yields, round up, and repeat. Ten-fold expansion with a tenth lost gives nine useful units per parent, so thirty final units need four one step back and one the step before that. The page walks it back for you and shows what each step has to be ready by.
Why does the calculator round the unit count up?
Because spawn comes in whole jars or bags and half a colonised jar is not a unit. Rounding up means the rate you actually apply is a little higher than the one you entered, which the results show explicitly. On small batches the gap is large enough to matter.
Does the loss percentage mean contamination?
The page does not say and does not care. It is whatever share of units does not carry through a step in your own records, for any reason at all. Nothing here diagnoses a cause or issues a verdict on any unit, and no figure on this page should be read as one.
What process figures does this page provide?
None at all, deliberately. The numbers that decide a growing process — the formula, the supplement rate, the spawn rate, any heat treatment, the colonisation time, the room conditions — are inputs you supply from your own tested process or a cultivation reference. This page multiplies and divides them. It cannot tell you whether a batch is clean, whether a process worked, or whether anything is safe to eat, and it does not try.