This mushroom substrate calculator turns the size of your tub, bag or bucket into an exact shopping list: how much coco coir, vermiculite, gypsum, sawdust or soy hulls to buy, and how much water to add to hit field capacity. Pick a recipe, enter one number, and the amounts scale for you.
Every figure below comes from published ratios, not from guesswork. The source for each recipe is named underneath it so you can check the arithmetic yourself before you commit a batch.
Ratio source: the standard CVG recipe of 650 g coco coir : 2 L vermiculite : 240 ml gypsum : 4.25 L water, as published by EZMushroom and corroborated by Out-Grow, which puts finished CVG at 60 to 70 percent moisture. One 650 g brick yields about 9.5 L (10 US quarts) of finished substrate; that is the figure used to scale everything above.
Start by deciding what you are filling. If you are growing in a monotub or a shoebox, switch the CVG tab to Container dimensions and enter the inside length, inside width and how deep you want the substrate. Four to five inches is the usual depth. The calculator converts that to volume and hands back the ingredient list.
If you already know you want, say, twenty quarts, use A finished substrate volume instead. And if you have already bought bricks and just want to know what else to add, use A number of coir bricks. All three routes run the same ratio, they just start from different ends of it.
For wood-loving gourmet species such as oyster, shiitake and lion's mane, use the Masters Mix tab instead. That tab works from the number of bags and the dry weight per bag, because sawdust and soy hulls are sold and measured by weight rather than by volume. The straw tab is for the cheap outdoor route: bulk straw, hot water, and a generous spawn rate to outrun contamination.
CVG stands for coco coir, vermiculite and gypsum. The coir supplies bulk and water retention, the vermiculite adds aeration and holds moisture in reserve, and the gypsum supplies calcium and sulfur while stopping the mix from compacting into a brick. The published ratio is one 650 gram coir brick to two litres of vermiculite to 240 millilitres of gypsum, hydrated with roughly 4.25 litres of water.
Hydrate with boiling water in a sealed bucket and leave it overnight. The heat does double duty: it breaks the brick apart and it pasteurises the mix as it cools. That is why CVG is the friendliest bulk substrate for anyone without a pressure cooker. If you want the full method rather than just the numbers, read our guide to sterilising mushroom substrate.
Masters Mix is fifty-fifty hardwood sawdust and soy hulls by dry weight. It is nutrient dense, which is exactly why it must be pressure sterilised rather than pasteurised: everything else in the room wants to eat it too. The target is about sixty percent moisture content.
Moisture content is defined as water divided by total wet weight, so the water you add is dry weight multiplied by m divided by one minus m. At sixty percent that comes out to one and a half times the dry weight. Ten pounds of dry ingredients wants about fifteen pounds of water. That is a calculation, not folklore, and you can check it on paper. Add roughly ninety percent of it, mix, then bring it the rest of the way by feel.
Field capacity is the point where the substrate is holding every drop it can without any water pooling loose. Grab a fistful and squeeze hard. A few drops should appear between your fingers. A trickle means too wet, and a wet block goes sour and anaerobic instead of colonising. Bone dry with no drops at all means the mycelium will crawl instead of run.
Water is the single most common thing beginners get wrong, closely followed by fresh air exchange. Both are covered in our list of the mistakes that kill most batches. If you are still deciding what to inoculate with, start with what mushroom spawn is and how much of it you need.
You do not need much. One honest recommendation per item, and nothing here is exotic:
Coco coir bricks. Buy the standard 650 gram bricks, because that is the unit every published recipe including this one is written around. Coco coir bricks on Amazon
Coarse vermiculite. Coarse grade, not the fine horticultural dust, or you lose the aeration you were buying it for. Coarse vermiculite on Amazon
Gypsum. A single bag lasts most home growers a very long time. Gypsum on Amazon
Filter patch grow bags. Only if you are going the Masters Mix route, since those blocks have to be sterilised in a sealed bag. Filter patch grow bags on Amazon
A pressure cooker. Required for Masters Mix and any other supplemented substrate. Not needed for CVG or straw. Pressure cookers on Amazon
Want the whole method in one place? Our book The Beginner's Guide to Growing Mushrooms at Home walks you through substrate prep, spawn, colonisation and a full four-week first-grow plan. Written by us, and it says so plainly.
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Measure the inside length and width of the tub in inches and multiply by your target substrate depth, usually 4 to 5 inches. Divide that number by 57.75 to get US quarts. A 6-quart shoebox typically takes about 5 to 6 quarts of finished substrate, and a 54-quart tub filled to 4 inches takes roughly 18 to 22 quarts. More on the format in what a monotub is.
The widely published CVG ratio is 650 grams of coco coir to 2 litres of vermiculite to 240 millilitres of gypsum, hydrated with about 4.25 litres of water. That is one standard coir brick and it yields roughly 9.5 litres, about 10 US quarts, of finished substrate.
Masters Mix is 50 percent hardwood sawdust and 50 percent soy hulls by dry weight, hydrated to about 60 percent moisture content. The water weight equals the dry weight multiplied by 1.5. Ten pounds of dry ingredients therefore needs about 15 pounds of water, which is roughly 1.8 US gallons.
Field capacity is the moisture level where the substrate holds all the water it can without pooling. Squeeze a handful hard and only a few drops should run out. A steady stream means it is too wet and will go anaerobic. No drops at all means it is too dry and the mycelium will stall.
Yes. This calculator gives you quantities, not sterility. Coir hydrated with boiling water in a sealed bucket is pasteurised as it cools. Straw needs a hot water soak at about 71 C for an hour, covered in how to pasteurise straw. Nutrient rich substrates such as Masters Mix must be fully sterilised in a pressure cooker at 15 psi for around 2.5 hours.
The full beginner walkthrough, start to first harvest.
Pressure cooking, pasteurising, and when each one is enough.
Grain, sawdust and plug spawn, and how much you need.
The cheapest substrate there is, done properly.
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