Wet basis and dry basis are not the same number
Substrate moisture is quoted on the wet basis. Sixty per cent moisture means that in every 100 pounds of finished mix, 60 pounds are water and 40 pounds are solid. It does not mean 60 pounds of water added to 100 pounds of material — that would be 37.5 per cent on the wet basis and a very different mix.
The dry basis is the other convention, and it is the one wood people use. On the dry basis you divide the water by the bone-dry weight, which is why a moisture content above 100 per cent is possible for green timber and impossible on the wet basis. The conversion runs both ways:
dry basis = wet basis ÷ (100 − wet basis) × 100
So 60 per cent wet basis is 150 per cent dry basis. 65 is 186. 55 is 122. If someone quotes you a moisture figure and does not say which basis it is on, the difference between the two readings is larger than any tolerance you were working to. The lumber drying water weight calculator does the same arithmetic on the dry basis for timber, and is a useful sanity check if you have both numbers in front of you.
Why the supplement percentage does not add to the moisture percentage
Supplement rates are quoted on dry weight, because that is the only weight that means anything — bran that has been sitting in a damp shed is not the same amount of bran as bran off a pallet. But the material you weigh out on the day has water in it, and the two percentages are then measured against different things.
There is a second ambiguity inside the supplement rate itself. Ten per cent supplement can mean ten per cent of the dry base material, or ten per cent of the finished dry mix. Against the base, 100 pounds of dry sawdust takes 10 pounds of supplement and the mix is 110 pounds. Against the finished mix, the same 100 pounds of sawdust takes 11.1 pounds and the mix is 111.1 pounds — because the supplement is being counted in its own denominator. Eleven per cent more supplement is not a rounding difference, and the selector on this page exists because both conventions are in use.
Measuring the moisture you are actually starting with
Sawdust off a pallet is not bone dry, straw out of a bale is not bone dry, and the water already in the sack is water you should not add again. Weigh a sample, dry it at low heat until it stops losing weight, and weigh it again. The loss divided by the original weight is the wet-basis moisture of the material as you received it.
Doing that once per supplier per season is enough. It moves with the weather and with how the material has been stored, but not by much, and being 8 per cent out on the incoming moisture of a 50 pound sack is 4 pounds of water in the wrong place.
| Quoted as | Water in 100 lb of mix | Solids in 100 lb of mix | Same figure, dry basis |
|---|---|---|---|
| 50% wet basis | 50 lb | 50 lb | 100% |
| 55% wet basis | 55 lb | 45 lb | 122% |
| 60% wet basis | 60 lb | 40 lb | 150% |
| 65% wet basis | 65 lb | 35 lb | 186% |
What this page will not tell you
It has no formula, no supplement rate, no heat treatment schedule and no opinion about whether a mix is right for anything. Those come from your own tested process or from a cultivation reference, and the fields on this page are where you put them. What the page does is stop the arithmetic being wrong once you have them.
For the next step, the spawn rate calculator takes this batch and works out the spawn. If you also mix growing media in containers, the growing media and container volume calculator works from container geometry instead of from mass, and the compost and amendment calculator handles anything spread by depth over a bed.
Questions people ask
Is 60 per cent moisture 60 pounds of water added to 100 pounds of sawdust?
No, and this is the single most common error in the topic. Wet basis means water as a share of the finished total, so 60 per cent moisture is 60 pounds of water in every 100 pounds of mix, alongside 40 pounds of solids. Sixty pounds of water added to 100 pounds of bone-dry material gives 160 pounds of mix that is 37.5 per cent water. The same figure on the dry basis would be quoted as 150 per cent, which is why bases have to be stated.
Should the supplement rate be against the base material or the finished mix?
Both conventions are in real use, which is why the page asks. Ten per cent against 100 pounds of dry base is 10 pounds; ten per cent against the finished dry mix is 11.1 pounds of supplement in a 111.1 pound mix. That is eleven per cent more supplement for the same stated number. Use whichever your own process was tested at, and write down which one it was.
Why does the calculator ask what moisture is already in my sawdust?
Because water already in the sack is water you must not add twice. Air-dry material typically carries some, and on a 50 pound sack an eight per cent reading is four pounds. Weigh a sample, dry it to constant weight in a low oven and weigh again; the loss over the starting weight is the figure.
Does the mixing loss field change the recipe?
It changes the batch size, not the proportions. Everything is scaled up so that after the share you lose on the paddles and the table, the wet substrate left still fills the bags you asked for. Set it to zero and the batch shrinks by that percentage.
What does this page say about the process itself?
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.