Free mushroom grow bag calculator. Enter grow bags to find the substrate and grain spawn you need for a home mushroom growing batch.
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Enter the number of grow bags in your batch. This mushroom grow bag calculator uses a simple planning baseline of 2 kilograms of substrate and 100 grams of grain spawn per bag, then scales both amounts by the number of bags. Real bags vary a lot by type, so read the all in one, bulk and oyster sections below and follow your kit or spawn supplier instructions for the batch you are actually growing.
For this mushroom grow bag calculator, one bag means 2 kilograms of substrate and 100 grams of grain spawn as a planning baseline, so 4 bags need 8 kilograms of substrate and 400 grams of grain spawn. That baseline is a simple batch planning model, not a recipe for every species or bag type. How much substrate per grow bag really depends on the bag size and system, as the sections below show, so check your kit instructions before ordering.
The spawn to substrate ratio is grain spawn weight divided by substrate weight, usually given as a percentage. The best spawn to substrate ratio for bulk growing is commonly 10 to 20 percent by weight, which colonises faster than a very low ratio while keeping spawn costs sensible. A grain spawn calculator helps here: at 10 percent, a 5 kilogram substrate batch needs 500 grams of spawn, and at 20 percent it needs 1,000 grams. Higher ratios speed colonisation but do not automatically raise yield, especially on rich substrate.
An all in one grow bag already contains grain and substrate in one bag, so its amounts are fixed by the maker. A popular 4 lb all in one grow bag holds about 1.75 lb of grain and 2.25 lb of substrate, while a 6 lb XL bag holds about 2 lb of spawn and 4 lb of substrate. The 4 lb vs 6 lb all in one grow bag choice is therefore about batch size and space: the 4 lb bag suits a first grow, and the 6 lb bag suits a larger batch in the same footprint. Mushroom grow bag size also affects fruiting space, so check the bag dimensions against your fruiting area.
For bulk growing, the common planning rule is one pint of fully colonised grain spawn per 5 lb of bulk substrate, which sits near the 10 to 20 percent range above. How much grain spawn per 5 lb substrate bag matters for ordering: at 10 percent you need about 0.5 lb of spawn per 5 lb of substrate, and at 20 percent about 1 lb. Monotub guides often use a higher ratio, such as 2 lb of spawn per 5 lb of compost, which colonises fast but costs more per tub. Always match the ratio to your species and substrate, not just to speed.
How much spawn for a 10 kg oyster bag is very different from a bulk bag: commercial oyster straw bags use only about 300 to 350 grams of grain spawn per 10 to 11 kg bag, because straw is a poor substrate and the mycelium spreads well through it. That means 1 kilogram of spawn inoculates about 3 oyster bags. Oyster yield is often split 60 to 70 percent on the first flush and 30 to 40 percent on the second flush, so plan harvest space for the first flush to be the largest.
When should I break and shake a grow bag? At roughly 50 to 75 percent colonisation, break up the grain gently through the bag and mix it through the substrate, which spreads the mycelium and speeds the rest of the colonisation. How long does grain spawn take to colonize? Usually 2 to 3 weeks at 70 to 75 degrees Fahrenheit for grain spawn, with bulk bags taking longer depending on ratio and temperature. Do not break and shake an all in one bag before the grain is well colonised, and do not open a bag that shows green or black mould.
For 4 grow bags, this calculator gives 8 kilograms of substrate and 400 grams of grain spawn. Compare that with a bulk plan at 10 percent: 4 bags of 5 lb substrate each (20 lb total) would need about 2 lb of spawn, which is more than the simple baseline here. That difference is exactly why the page shows the ratio sections: use the calculator for a quick batch estimate, then adjust to your bag type and supplier recipe before you buy.
Use grain spawn within 2 to 4 weeks where possible, stored cool and dark at about 4 degrees Celsius, and never freeze it. Green mould usually means Trichoderma and black mould usually means Aspergillus; discard a contaminated bag rather than trying to save it, and clean the area before starting the next batch. Grain spawn must be sterilised properly, usually at 15 PSI for 90 minutes in a pressure cooker, and most Instant Pots do not reliably hold 15 PSI for this job.
It depends on the bag type. This calculator plans 100 grams of grain spawn per bag as a baseline, but bulk bags commonly use 10 to 20 percent spawn by weight, and a 10 kg oyster straw bag uses only about 300 to 350 grams. Check your kit or supplier recipe for the exact amount.
For bulk growing, 10 to 20 percent spawn by weight is the common range: 10 percent is economical and reliable, while 20 percent colonises faster. All in one bags use their own fixed recipe, and oyster straw bags use a much lower ratio, so there is no single ratio for every system.
At 10 percent, a 5 lb substrate batch needs about 0.5 lb of grain spawn, and at 20 percent it needs about 1 lb. Many guides also describe this as one pint of colonised grain spawn per 5 lb of bulk substrate.
Break and shake at about 50 to 75 percent colonisation, when the grain is well colonised but the bag is not yet fully white. Mix gently through the bag, then leave it to finish colonising before moving it to fruiting conditions.
Grain spawn usually takes 2 to 3 weeks to fully colonise at 70 to 75 degrees Fahrenheit. Cooler temperatures slow it down, and bulk bags take longer than grain jars because the mycelium must spread through more substrate.
Not necessarily. More spawn speeds colonisation and can help on poor substrate, but on rich substrate it does not guarantee a higher yield. Yield depends more on substrate nutrition, moisture, fresh air, humidity and genetics than on spawn amount alone.
As a guide, all in one bags often take 4 to 5 cc of spore solution or 5 to 10 cc of liquid culture per bag, depending on the maker. Follow the bag instructions, because too much liquid can make grain too wet and slow colonisation.
Store grain spawn cool and dark at about 4 degrees Celsius and use it within 2 to 4 weeks for best results. Do not freeze it, and discard any bag that shows green or black mould rather than trying to rescue it.