Mushroom Substrate, Spawn Rate & Yield Calculator
Plan a mushroom grow: dry substrate and water to hydrate to the target moisture, bran or soy-hull supplement and gypsum, grain spawn by spawn rate or spawn-to-substrate ratio, number of grow bags or blocks, expected fresh yield from biological efficiency by species (oyster, king oyster, shiitake, lion's mane, wine cap, button), a flush-by-flush harvest calendar, and cost, revenue and cost per kilogram or pound.
How it works
Bone-dry = weight × (1 − moisture%) (+ supplement × 90%); wet weight = (bone-dry + gypsum) ÷ (1 − target moisture); water = wet − everything added dry; spawn = wet × spawn rate (or wet ÷ N for a 1 : N ratio); yield = bone-dry × biological efficiency; flushes ≈ 50/30/20%
Mushroom growers plan by biological efficiency: fresh mushroom weight as a percentage of the dry substrate weight. Mushrooms are about 90% water, and the dry matter of the substrate is what the fungus actually converts. Oysters on straw routinely return 100% BE, a kilogram of mushrooms per kilogram of dry straw over several flushes, and shiitake and lion's mane give 50–100% on supplemented sawdust. Supplements such as wheat bran or soy hulls add dry matter and nitrogen, which raises yield but also the contamination risk, so supplemented substrates are sterilised rather than pasteurised. The substrate is hydrated to 60–70% moisture and inoculated with grain spawn at 5–20% of its wet weight, or by a spawn-to-substrate ratio such as 1:4. A higher rate buys faster colonisation and fewer contaminations at the cost of more spawn. The calculator works out the water, spawn, supplement and gypsum, how many bags or blocks to fill, the expected harvest and a typical calendar from inoculation through three flushes. It also gives the material cost per bag and per kilogram and, if you enter a selling price, the margin. Half the yield typically comes in the first flush, and the rest tails off over two more.
Worked example
10 kg of chopped straw at 10% moisture is 9 kg bone-dry. Bringing it to 65% moisture means adding about 15.7 L of water for 25.7 kg of wet substrate, plus 2.6 kg of grain spawn at 10%, for 28.3 kg in total, or twelve 2.5 kg blocks. At 100% BE that should give about 9 kg of oyster mushrooms: 4.5 kg in the first flush about 25 days after inoculation, then 2.7 kg and 1.8 kg about 12 days apart. Materials cost roughly $26.57, about $2.95 per kilogram. Swapping to sawdust with 20% wheat bran (2 kg) for shiitake raises the dry matter to 10.8 kg, needs about 3.1 L more water, and at 75% BE yields about 8.1 kg of shiitake.
Sources: Penn State Extension — Mushroom production · Biological efficiency (Wikipedia: Fungiculture)
Frequently asked questions
Why is my yield far below the quoted BE?
Moisture outside 60–70%, poor fresh-air exchange during fruiting (high CO₂ gives long stems and small caps), low humidity, or contamination. Weigh a block before and after fruiting to compute your own BE and adjust one variable at a time.
How much spawn do I need?
Commercial growers use 5–10% of the wet substrate weight, so 50–100 g of grain spawn per kilogram of hydrated substrate. Home monotubs often go much richer, 1:2 to 1:4 spawn to substrate, to colonise faster and resist contamination. Switch the spawn input to a ratio if that is how your recipe is written.
Should I add bran or other supplements?
For shiitake, lion’s mane and king oyster on sawdust, yes: 10–20% wheat bran or a 50/50 Master’s Mix with soy hulls can double the yield. Supplemented substrates must be sterilised at 121 °C, though; on pasteurised straw a supplement mostly feeds moulds.
When will I harvest?
Oysters colonise straw in about two to three weeks and pin about a week later, with flushes 10–14 days apart. Shiitake needs 6–12 weeks of colonisation and browning first. Set the inoculation date under More options to see a calendar; actual timing depends on temperature and strain.
Can I reuse spent substrate?
Spent oyster blocks can be broken up as 10–20% "spawn" for outdoor beds, or composted; a second indoor run gives little. Spent shiitake blocks make good garden mulch.
Related calculators
Rules of thumb from extension-service and grower literature (UVI aquaponics, Cornell/Penn State mushroom guides, hydroponic nutrient manuals). Crops, cultivars, water chemistry and climate change the numbers; start conservatively and adjust from your own measurements.