Mushroom Substrate Calculator

How this is calculated

WORK FROM DRY WEIGHT, BECAUSE IT IS THE ONLY FIGURE THAT DOES NOT MOVE. Straw, sawdust and other substrates arrive at whatever moisture the supplier's shed happened to have, so a wet weight tells you very little about how much material you actually have. Weigh it dry, add water to a target, and the recipe is repeatable from batch to batch. Recipes written in wet weight are not, which is why two people following the same one get different results. THE HYDRATION ARITHMETIC IS EXACT ONCE YOU FIX THE TARGET. At a target moisture of M percent, the dry material is (100 − M) percent of the finished weight - so the wet weight is the dry weight divided by (1 − M/100), and the water to add is the difference. At 60% moisture, dry material is 40% of the total, so 5 lb of dry substrate becomes 12.5 lb wet and needs 7.5 lb of water. Note that this is moisture on a WET basis, which is the convention substrate recipes normally use; a dry-basis figure of the same number means something different. ⚠️ BOTH THE MOISTURE TARGET AND THE SPAWN RATE ARE INPUTS, BECAUSE BOTH BELONG TO YOUR SUPPLIER AND YOUR SPECIES. They differ by substrate, by species and by method, and your spawn supplier publishes the rate for what they sold you. A tool that assumed either would be confidently wrong for most readers. SPAWN RATE HERE IS A PERCENTAGE OF THE FINISHED WET SUBSTRATE, which is the common convention - but not the only one. Some suppliers quote it against dry weight instead, and the two differ by a large factor: 10% of wet is 25% of dry at 60% moisture. Check which your supplier means before ordering, because it is the difference between a bag that colonises quickly and one that stalls. ⚠️ THE FIELD-CAPACITY SQUEEZE TEST BEATS ARITHMETIC AS A FINAL CHECK. Squeeze a handful hard: a few drops should come out, not a stream and not nothing. Substrate that drips is too wet and will go anaerobic and sour; substrate that yields nothing is too dry and colonises slowly. Use the calculation to get close and your hand to confirm, because absorbency varies between batches of the same material. THIS IS A QUANTITY TOOL AND NOTHING ELSE. Pasteurisation and sterilisation, sterile technique, incubation temperature and contamination control decide whether the batch works at all, and none of them are arithmetic. Getting the water right and the technique wrong produces a bag of mould. WATER IS ROUGHLY A PINT PER POUND, so weights and volumes are close enough to interchange when you are hydrating. Weighing is still more repeatable than measuring by jug.

Common questions

Why work from dry weight?

Because it is the only figure that stays put. Straw, sawdust and other substrates arrive at whatever moisture the supplier's shed happened to have, and that varies with the weather and how long it sat. So a wet weight tells you very little about how much material you actually have, and a recipe written in wet weight is not repeatable - two people following the same one get different results, and so does the same person in a different month. Weigh dry, hydrate to a target, and the batch is reproducible.

How does the hydration maths work?

Moisture here is on a wet basis, meaning water as a share of the FINISHED weight, which is the convention substrate recipes normally use. So at a 60% target the dry material makes up the other 40% of the total - which means the wet weight is the dry weight divided by 0.4, and the water is the difference. Five pounds of dry substrate becomes 12.5 pounds wet and needs 7.5 pounds of water. If a recipe quotes moisture on a dry basis instead, the same number means something quite different, so it is worth knowing which you are reading.

Is my spawn rate against wet or dry weight?

Check, because the difference is large. This tool takes it as a percentage of finished WET substrate, which is the common convention, but some suppliers quote against dry weight instead - and at 60% moisture, 10% of wet is 25% of dry. The tool reports your entered rate both ways so you can compare directly with whatever your supplier states. Getting this wrong in the low direction gives a bag that colonises slowly and is more vulnerable to contamination, which is the expensive kind of mistake.

Should I trust the calculation or the squeeze test?

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. Substrate that drips freely is too wet and tends to go anaerobic and sour; substrate that yields nothing at all is too dry and colonises slowly. 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 and note what you did for next time.

What happens if it is too wet?

It tends to go anaerobic in the low parts of the bag, which turns sour, smells wrong and gives contaminants an advantage over the mycelium you wanted. Excess water also displaces the air that mycelium needs. Too dry is a different failure and generally the less catastrophic of the two - colonisation is slow and the flush is poor, but the bag is often recoverable. Neither is fixed easily once the bag is sealed, which is why the check happens at mixing.

Does getting the water right mean the batch will work?

No, and it is worth being clear about that. 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 is entirely possible to get the hydration exactly right and open a bag of mould two weeks later. The calculation removes one variable from a process that has several.

Can I measure the water by volume instead?

Yes, near enough. Water is roughly a pint per pound, so the weights and volumes are close enough to interchange while hydrating - a jug is perfectly workable. Weighing is still more repeatable, particularly at batch scale where small errors compound across ten or twenty bags, and a scale removes any question about whether the jug was filled to the same line each time. If you are working to a recipe you intend to repeat, weigh.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Mushroom Substrate Calculator at https://www.bfcbrilliance.com/tools/mushroom-substrate-calculator.

What I entered:
- Number of bags or blocks (bags): ___
- Dry substrate per bag (lb): ___
- Target moisture (%): ___
- Spawn rate (% of wet substrate): ___

What it calculated:
- Water to add per bag: ___
- Finished wet weight per bag: ___
- Spawn per bag: ___
- Dry substrate for the batch: ___
- Water for the batch: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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