Grow Bag Calculator
Work out exactly what goes into a mushroom grow bag — how much dry substrate, how much water to reach field capacity, and how much grain spawn to mix in — scaled to the number of bags you're filling.
Pick your species and the calculator pulls its substrate and moisture targets from our cultivation database, so the recipe fits what you're actually growing rather than a generic guess. Figures are the ranges real cultivators work within; treat them as a starting point to tune for your strain.
Your batch
Pick a species for sensible defaults, then adjust anything.
Recipe
Everything to mix and fill.
How to use it
- Choose your species. Substrate and moisture defaults load from the database — straw holds more water than sawdust, and supplemented mixes default to added bran.
- Set bags and fill weight. Enter how many bags and the wet fill per bag; the 3, 5 and 10 lb presets cover standard filter-patch bags.
- Adjust the mix. Tune target moisture, spawn ratio and bran supplement if your setup differs from the defaults.
- Read the recipe. You get dry substrate, water, spawn and supplement per bag and for the whole batch, with warnings when a value drifts into risky territory.
Questions
How much spawn should I use per grow bag?
A spawn ratio of 1:5 (about 20% of substrate weight) is the usual default. More spawn colonizes faster and resists contamination; less stretches your grain further but colonizes more slowly.
What moisture should mushroom substrate be?
Around 60% for sawdust and up to about 68% for straw. The squeeze test is the final check: a firm squeeze should yield a few drops, not a stream.
Does the calculator work for any mushroom?
It's tuned for the species in our database — oyster, shiitake, lion's mane, king oyster, reishi and more — pulling each one's substrate and moisture targets. For a species not listed, pick the closest substrate match and adjust.
Why does it show ranges instead of one number?
Cultivation figures shift with strain, substrate and conditions. A range is honest about that; a single number would imply a precision that doesn't exist.