Mushroom Spawn & Spawn, Bags & Yield
Sizes oyster
Enter your substrate weight, spawn rate and biological efficiency to get the kilograms of spawn to order, the number of bags and the expected fresh yield.
Plan your spawn run
Next: buy 4 kg of fresh spawn, mix evenly into pasteurised substrate, fill 20 bags, and maintain humidity/temperature for the species.
Spawn rate and biological efficiency vary by species, substrate quality and management; use fresh, contamination-free spawn.
Mushroom spawn — key facts
- Spawn
- substrate × spawn rate%
- Bags
- substrate ÷ bag size
- Yield
- substrate × BE%
- Oyster rate / BE
- ≈ 4% / ≈ 70%
- Button rate / BE
- ≈ 0.75% / ≈ 35%
- Milky rate
- ≈ 5%
- Spawn
- fresh, contamination-free
- Privacy
- Runs in your browser; nothing uploaded
From a sack of substrate to a forecast crop
Mushroom growing is a numbers game before it is a craft: order too little spawn and your run is slow and prone to contamination; over-order and you waste money. Once the substrate is pasteurised and inoculated, the yield is set by the strain's biological efficiency — how well it converts substrate into mushrooms — so you can forecast a harvest before the first flush even appears. Getting the spawn rate, bag count and expected yield right is what turns a hobby into a reliable enterprise.
This tool takes your substrate weight, spawn rate and biological efficiency and returns the spawn to order, the number of bags to fill, the total expected yield and the yield per bag. Use the oyster, button and milky presets or your own figures, always inoculating with fresh, clean spawn into properly pasteurised substrate. Pair it with the Value Addition Profit, Crop Yield Estimator and Seed Rate tools to plan the whole enterprise.
Order the right spawn
Spawn kg straight from your substrate weight.
Plan your bags
Bag count and yield per bag at a glance.
Forecast the harvest
Expected fresh yield from biological efficiency.
Use the presets
Oyster, button and milky rates built in.
Frequently Asked Questions
How does mushroom growing from spawn work?+
You pasteurise or sterilise a substrate — straw, sawdust or compost — then mix in grain or sawdust spawn at the species' inoculation rate, pack it into bags or beds, and incubate in the dark until the white mycelium colonises it. Once fully run, you trigger fruiting and harvest flushes of fresh mushrooms over the following weeks.
How is the spawn quantity calculated?+
Spawn = substrate × spawn rate%. Oyster mushrooms run at about 4% of wet substrate, button (Agaricus) at roughly 0.75% of compost, and milky (Calocybe) at about 5%. So 50 kg of oyster substrate needs about 2 kg of spawn. A higher spawn rate colonises faster and crowds out contaminants but costs more spawn.
How is the expected yield calculated?+
Yield = substrate × biological efficiency (BE). BE is the fresh mushroom weight per unit of dry substrate, but as a practical wet-substrate figure oyster runs around 70% and button around 35%. So 50 kg of oyster substrate at 70% BE gives roughly 35 kg of fresh mushrooms across its flushes — the tool also splits this per bag.
What is biological efficiency (BE)?+
BE compares fresh mushroom harvested to the dry substrate used, expressed as a percentage — 100% BE means a kilogram of dry substrate yielded a kilogram of fresh mushrooms. It captures how well a strain and substrate convert into crop, so growers use it to compare recipes, strains and management and to forecast yield.
How many bags will my substrate fill?+
Bags = substrate ÷ bag size. If you fill 2 kg bags from 50 kg of substrate, that's 25 bags. Bag size affects colonisation speed and handling — smaller bags run faster and are easier to move, larger bags are more efficient on labour and plastic. The tool gives both the bag count and the yield per bag.
What spawn rate should I use?+
Follow the species and your conditions. The presets — oyster ~4%, button ~0.75%, milky ~5% — are typical starting points. Raise the rate in warm, contamination-prone conditions or for faster colonisation; you can lower it slightly with very clean, fast strains. More spawn means a quicker, safer run but a higher input cost per bag.
Why does spawn quality matter so much?+
Spawn carries the living mycelium that colonises your substrate, so weak, old or contaminated spawn either fails to run or introduces moulds that out-compete the mushroom. Always use fresh, vigorous, contamination-free spawn from a trusted source, keep it cool until use, and inoculate quickly in clean conditions to protect the whole batch.
Does the substrate need to be pasteurised?+
Yes — raw substrate is full of competing microbes. Oyster straw is usually pasteurised (heated to about 65–80 °C for an hour or so) to knock back competitors while leaving helpful ones; sawdust and grain are fully sterilised. Proper pasteurisation or sterilisation is the single biggest factor in a clean, high-yielding run.
How many flushes will I get?+
Most species fruit in two or three flushes a week or two apart, with the first flush the largest. The BE figure here covers the total across flushes, not a single pick. Keep humidity and fresh air right between flushes; once yields drop off sharply the spent substrate is exhausted and is best composted.
Are these figures exact?+
They're solid planning numbers. Real spawn needs and yields shift with strain, substrate quality, moisture, temperature, humidity and hygiene, so treat the output as a target. Keep records of spawn used and mushrooms harvested per batch, work out your own BE, and feed it back in — the calculator is for planning, not a guarantee.