How to Work Out Mushroom Substrate and Spawn

Dry weight is the only figure that stays put

Straw, sawdust and other substrates arrive at whatever moisture the supplier's shed happened to have — which varies with the weather and how long it sat.

So a wet weight tells you very little about how much material you actually have. A recipe written in wet weight isn't repeatable: two people following it get different results, and so does the same person in a different month.

Weigh dry, hydrate to a target, and the batch reproduces.

The hydration arithmetic

Moisture here is on a wet basis — water as a share of the finished weight, which is the convention substrate recipes normally use.

So at a 60% target, the dry material is the other 40%:

wet weight = dry ÷ (1 − moisture)
water      = wet − dry

Five pounds of dry substrate becomes 12.5 lb wet and needs 7.5 lb of water.

A useful check on the maths: at a 50% target, the water added exactly equals the dry weight. If a recipe quotes moisture on a dry basis instead, the same number means something different — worth knowing which you're reading.

Check whether your spawn rate is wet or dry

This is the one that costs money.

The tool takes spawn as a percentage of finished wet substrate, which is the common convention. But some suppliers quote against dry weight — and the two differ by a lot:

At 60% moisture, 10% of wet = 25% of dry.

That spawn rate against DRY weight reports your entered figure both ways, so you can compare directly with whatever your supplier states.

Getting it wrong in the low direction gives a bag that colonises slowly, which leaves a longer window for contaminants to get established. That's the expensive direction.

The squeeze test is the real check

Use the calculation to get close and your hand to confirm.

Squeeze a handful hard. A few drops should come out — not a stream, and not nothing.

• Dripping freely: too wet. It tends to go anaerobic in the low parts of the bag, turns sour, and hands contaminants an advantage. Excess water also displaces the air the mycelium needs.
• Nothing at all: too dry. Colonisation is slow and the flush is poor — generally the less catastrophic of the two, and often recoverable.

Absorbency varies between batches of the same material, so the arithmetic gets you to roughly the right place and the squeeze test tells you whether you actually arrived. Adjust at mixing, because neither is fixable once the bag is sealed.

Both targets belong to your supplier

Target moisture and Spawn rate are inputs because both depend on your substrate, your species and your method. Your spawn supplier publishes the rate for what they sold you.

A tool that assumed either would be confidently wrong for most readers.

Getting the water right doesn't mean it works

Worth being blunt: this is a quantity tool and nothing more.

Pasteurisation or sterilisation, sterile technique when spawning, incubation temperature and general contamination control all decide whether a batch succeeds — and none of them are arithmetic.

It's entirely possible to hit the hydration exactly and open a bag of mould two weeks later. The calculation removes one variable from a process that has several.

A practical note on measuring

Water is roughly a pint per pound, so weights and volumes interchange closely enough while hydrating. A jug works.

Weighing is still more repeatable, especially at batch scale where small errors compound across ten or twenty bags — and a scale removes any question about whether the jug hit the same line each time. If you intend to repeat the recipe, weigh it.