How much water to add to mushroom substrate?

To reach field capacity (about 60% moisture), add water equal to roughly 1.5x the dry substrate weight, so dry substrate plus water ends near 2.5x the dry weight. Water = M/(1-M) x dry weight, where M is the target moisture fraction.

What is biological efficiency for mushrooms?

Biological efficiency (BE) is fresh mushroom yield divided by dry substrate weight, as a percentage. Oyster mushrooms commonly hit 75-100%+, while king oyster and lion's mane are often 50-75%.

What spawn ratio should I use?

A common spawn rate is 10-30% of the hydrated substrate weight. 20% equals a 1:5 spawn-to-substrate ratio. Higher spawn rates colonize faster and resist contamination but cost more.

Mushroom Grow Calculator — substrate, spawn ratio, yield & profit

1 Substrate batch

Hydrate dry substrate (sawdust, soy hull / "masters mix", straw pellets) to field capacity.

Dry substrate per block Dry weight before adding water.

Target moisture % Field capacity is ~58–62% for masters mix.

Number of blocks / bags In this batch.

Water to add per block–

Hydrated weight per block–

Total dry substrate–

Total water–

Total hydrated substrate–

2 Spawn

Grain (or sawdust) spawn as a percentage of the hydrated substrate weight. New to this? See the spawn ratio guide.

Spawn rate % 10–30% typical · higher = faster colonization, less contam.

Spawn-to-substrate ratio For your spawn rate.

Spawn per block–

Total spawn needed–

3 Expected yield

Estimated fresh harvest across all flushes, from biological efficiency (fresh weight ÷ dry substrate).

Species (sets a typical BE) Typical mid-range — edit BE to match your strain.

Biological efficiency (BE) % Oyster 75–100%+ · King/Lion's Mane 50–75% · Shiitake 75–125%.

Expected fresh yield per block–

Total expected fresh yield–

4 Cost & profit

Your real cost per block and per pound — so you can price wholesale with confidence.

Dry substrate cost

Spawn cost

Other cost per block ($) Bag, filter, labor, energy, etc.

Sale price

Cost per block–

Total batch cost–

Cost per lb produced–

Batch summary

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Fresh yield

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Batch cost

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Expected revenue

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Profit · – margin

Want this to remember every batch for you? Try the free Grow Log — it tracks each batch from inoculation to harvest and works out your biological efficiency automatically. Mycro is the full version: synced across devices, with contamination patterns and real BE and cost numbers for your farm. Join the early-access list:

Ready to back us early? Become a Founding Grower — $99 once, lifetime founder pricing, fully refundable before launch.

How the math works

No black box — here's exactly what the calculator does.

Substrate hydration (field capacity)

To hit a target moisture content M, water added = M ÷ (1 − M) × dry weight. At 60% moisture that's 1.5× the dry weight in water, so a hydrated block ends around 2.5× its dry weight. Assumes a near-dry starting substrate (sawdust, soy hull pellets, dried straw). "Field capacity" means fully hydrated but not dripping when squeezed firmly.

Spawn ratio

Spawn rate here is a percentage of the hydrated substrate weight. 20% equals a 1:5 spawn-to-substrate ratio. More spawn colonizes faster and crowds out contamination, at higher cost.

Yield & biological efficiency

Biological efficiency is fresh mushroom weight ÷ dry substrate weight × 100%, summed across flushes. We estimate fresh yield as dry substrate × BE. BE varies widely with species, strain, substrate, and conditions — treat the presets as starting points and dial in BE from your own results (which is exactly what Mycro tracks).

Cost & profit

Batch cost = dry substrate cost + spawn cost + other per-block costs. Revenue = expected fresh yield × sale price. Cost per pound produced = total cost ÷ total fresh yield — the number to beat when you quote a restaurant.

Typical biological efficiency by species

Species	Typical BE	Notes
Blue / Pearl Oyster	75–100%+	Fast, forgiving, high yield
Pink / Yellow Oyster	65–90%	Fast; short shelf life
King Oyster	50–75%	Loves supplemented sawdust
Lion's Mane	50–75%	Sensitive to fresh-air / humidity
Shiitake	75–125%	Long colonization; multiple flushes
Chestnut / Pioppino / Nameko	50–75%	Clustered, attractive to chefs
Reishi	~30–50%	Grown for medicinal use, not weight

Ranges are general guidance gathered from common cultivation practice, not guarantees. Your mileage will vary. Mycro is for legal culinary & medicinal mushrooms only.