Mushroom Substrate Calculator
Get the exact shopping list and prep procedure for any mushroom fruiting container. Pick your substrate recipe and container size — the calculator tells you how much of each ingredient, how much water for field capacity, how much grain spawn to mix in, and whether to pasteurize or pressure-sterilize.
Your substrate shopping list
Supplementation forces sterilization
Adding bran means you MUST pressure-sterilize at 15 psi — pasteurization won't kill the contaminants that thrive on extra nutrients. Plan for sterile technique at inoculation too.
Your recipe
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Pick what you're growing and your container to see the shopping list.
Common substrate mistakes
- Guessing hydration by eye — field capacity has a specific squeeze test: 1–2 drops when firmly squeezed. Eyeballing it usually drowns the substrate or leaves it too dry to fruit.
- Pasteurizing supplemented substrate — bran and other nutrients feed contaminants as fast as mycelium. Any supplementation needs pressure sterilization at 15 psi, not pasteurization.
- Wrong spawn ratio for species — wood-lovers colonize fastest at 1:1; cubensis is standard at 1:2. Going 1:4 on cubensis invites contam before mycelium establishes.
- Wrong container for species — cubensis fruits poorly in deep bulk bags (no surface area). Lion's mane pins poorly in wide monotubs (too much air movement).
Total substrate volume
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qt
Shopping list
| Ingredient | Weight | Volume |
|---|---|---|
| Configure inputs to see recipe | ||
Water
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Grain spawn
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Colonization
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Hydration
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Preparation
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Pairing note
Sterilization required
Pick your recipe, prep your substrate
Start with what you're growing
Species is the most important input — it drives every recommendation downstream. CVG works for P. cubensis but starves wood-lovers; Master's Mix fruits lion's mane beautifully but invites contamination on dung-loving species. If you're experimenting or not sure, pick Other / not sure and the calculator defaults to the cubensis + CVG + monotub path — the most forgiving route for first-time growers.
Confirm the container matches your setup
The container default matches the species — monotub for cubensis, bulk bag for wood-lovers — but swap it if you have something different on hand. Shoebox (6qt) is good for a first test batch. 32qt monotub is the beginner's real grow. 54qt is the sweet spot once you've got your routine. 66qt is for when you can keep up with fresh-air exchange. Custom takes any container — just enter length × width × depth in inches.
Use the shopping list exactly
The right-hand column scales every ingredient, water amount, and spawn weight to your container size. Weigh dry ingredients on a kitchen scale — measuring cups work for vermiculite but lie about coco coir density (hydrated coco weighs 3–4× its dry weight). Underfilling is the number-one cause of slow colonization and disappointing yields, so don't round down to save a few dollars on vermiculite.
Match your prep method to the recipe
Non-supplemented substrates (CVG, straw, coco-only, manure) pasteurize at 160–170°F for 60–90 minutes — enough to kill molds and bacteria that can't compete with mycelium, but light enough to leave beneficial microbes that help the grow. Master's Mix and any bran-supplemented substrate must pressure-sterilize at 15 psi (250°F) for 2.5 hours, because bran feeds contaminants as readily as mycelium. The Preparation card in the output tells you exactly which method your recipe needs — don't substitute.
Squeeze test, not eyeballing
Field capacity looks deceptively dry. Grab a fistful, squeeze as hard as you can — 1–2 drops of water should fall. Too wet and mycelium drowns. Too dry and colonization stalls. Adjust with a spray bottle and re-squeeze until the test passes. Do this at room temperature, not right after pouring boiling water (hot substrate reads differently).
Don't let grain spawn age
Freshly colonized grain (white, fluffy, under 2 weeks old) runs fastest when mixed with substrate. Grain that's sat in the fridge for a month loses vigor and carries more contamination risk. If your spawn is older than 2 weeks, tighten the ratio (1:1 instead of 1:2) to compensate — more spawn means more mycelial mass competing with contaminants from the start.
How mushroom substrates actually work
A substrate is the food mushrooms eat. Different species evolved on different food sources — coprophilous (dung-loving) species like P. cubensis thrive on low-nutrient, coco-based mixes that mimic wet manure, while wood-loving species like lion's mane and king trumpet need nutrient-dense supplemented substrates that mimic decaying hardwood. Choosing the wrong substrate doesn't just lower yields — it can make colonization impossible. Start with your species; the right substrate follows. For deeper reading on each substrate type, see our Substrate learning module.
68%
Field capacity — CVG
Hydration target for standard cubensis substrate. Below 62%, mycelium stalls from dehydration. Above 74%, free water drowns air pockets and anaerobic contaminants take over.
1:2
Standard spawn ratio
Classic cubensis spawn-to-substrate ratio. Colonizes in 10–14 days and balances grain cost against time-to-pin. Wood-lovers typically run 1:1 for speed over economy.
15 psi
Sterilization pressure
Minimum pressure to kill bacterial endospores in supplemented substrates. Below this (including atmospheric pasteurization at 165°F) endospores survive and outcompete mycelium.
Why hydration matters
Water is how mycelium moves nutrients and expands. Hyphae can't extend into dry pockets, so below field capacity the mycelium stalls out in patches. Above field capacity, free water fills air gaps and creates anaerobic conditions — where trichoderma (green mold), bacillus, and pseudomonas outcompete mycelium. Field capacity is the sweet spot: the substrate holds all the water it can while still having air spaces between particles.
The squeeze test is the practical measure. Grab a fistful of prepared substrate, squeeze as hard as you can — 1–2 drops of water should fall. No drops means too dry; a stream of water means too wet. Different recipes target different hydration levels because the ingredients absorb water differently:
| Recipe | Target hydration | Why that level |
|---|---|---|
| Master's Mix | 60% | Hardwood pellets swell dramatically and release water during colonization — start drier. |
| Straw | 65% | Straw drains water easily; drier optimum avoids puddling at the bottom of the tub. |
| CVG | 68% | Coco + vermiculite compromise — forgiving middle ground for first-time growers. |
| Coco-only | 70% | Coco absorbs heavily and dries out fast; run wetter to buffer against evaporation. |
| Manure-based | 72% | Coprophilous species evolved on fresh wet dung — they thrive at hydration levels that would drown cubensis on CVG. |
Spawn-to-substrate ratio: the colonization math
Mycelium colonizes outward from each spawn point. The more spawn points (higher ratio), the faster the whole mass colonizes. That speed matters — contamination risk compounds over time, so a substrate that colonizes in 10 days has less than half the exposure window of one that takes 21 days. The tradeoff is cost: grain spawn is expensive to produce, substrate is cheap. Wood-lovers run 1:1 by convention because their substrates are nutrient-dense and harder to re-make than grain. For more detail on spawn types and production, see our Spawn module.
7–10
1:1 Ratio
Fastest. Uses 2× the grain but slashes contamination exposure. Standard for wood-lovers on Master's Mix.
10–14
1:2 Ratio
The cubensis standard. Best balance of grain cost against colonization speed for dung-loving species on CVG.
14–21
1:3 Ratio
Budget option. Works if your sterile technique is tight — otherwise the longer window invites contam.
21–28
1:4 Ratio
Minimum viable ratio. Real growers rarely use this — the contamination risk isn't worth the grain savings.
Colonization times shown are for P. cubensis at 75°F. Lion's mane runs ~1.5× longer; reishi ~2× longer; king trumpet ~1.3× longer. Oyster matches cubensis pace on its preferred substrate.
CVG vs Master's Mix: two opposite recipes
The two most common mushroom substrates solve different problems. CVG (Coco · Vermiculite · Gypsum) is low-nutrient and cubensis-optimized — the low nutrient density is a feature, not a limitation. It means contaminants have little to eat, so pasteurization (which doesn't kill every organism) is sufficient. Master's Mix (hardwood pellets + soy hulls) is high-nutrient and wood-lover-required — the soy hulls provide the nitrogen that wood-loving mycelium needs to fruit, but that same nitrogen feeds bacterial contaminants, which is why the recipe demands pressure sterilization.
| CVG | Master's Mix | |
|---|---|---|
| Main ingredients | Coco coir + vermiculite + gypsum | Hardwood pellets + soy hulls (50/50 dry) |
| Target hydration | 68% | 60% |
| Nutrient density | Very low (essentially no N) | High (soy hulls provide ~3% N) |
| Prep method | Pasteurize at 165°F for 60 min | Sterilize at 15 psi for 2.5 hrs |
| Ideal species | Cubensis, coprophilous spp. | Lion's mane, oyster, king trumpet, reishi |
| Contamination risk | Low — nutrient-poor, less food for contam | High — demands sterile technique at inoculation |
| Typical yield | ~0.5–1.0 lb wet per qt substrate | ~1.5–2.5 lb wet per qt substrate |
Pasteurization vs sterilization
These are two different thermal treatments with two different goals — not interchangeable. Pasteurization (160–180°F for 60–90 min, atmospheric pressure) kills molds and mesophilic bacteria but leaves thermophilic bacteria like Bacillus alive. That's a feature: thermophiles are beneficial on low-nutrient substrates, where they outcompete contaminants without interfering with mycelium. Sterilization (250°F at 15 psi for 2.5+ hours) kills everything, including bacterial endospores. Supplemented substrates need sterilization because the added nutrients feed every microbe pasteurization leaves behind — beneficial thermophiles become competitors when they have soy protein to eat.
| Pasteurization | Sterilization | |
|---|---|---|
| Temperature | 160–180°F | 250°F |
| Pressure | Atmospheric | 15 psi above atmospheric |
| Duration | 60–90 minutes | 150–180 minutes |
| Equipment | Large pot, turkey roaster, or insulated cooler | Pressure cooker or autoclave |
| Use for | CVG, straw, manure, coco-only | Master's Mix, any bran-supplemented substrate |
| What it kills | Molds, mesophilic bacteria, yeast | Everything including bacterial endospores |
| What survives | Thermophilic bacteria (beneficial on low-N substrates) | Nothing |
Container depth and fruiting surface area
Monotubs fruit from the surface of the casing layer, not the volume underneath. Deeper substrate doesn't produce bigger flushes — it produces the same flush size with more colonized mass holding moisture underneath. The sweet spot is 3–4 inches of colonized mass (spawn + substrate): deep enough to buffer moisture loss across a 7–10 day flush, shallow enough that fresh-air exchange reaches the casing layer without over-drying.
Bulk bags are different. A 5lb block fruits from its entire surface — top, sides, and through slits cut in the bag. Wood-lovers like lion's mane and king trumpet benefit from the higher surface-to-volume ratio of bulk bags, which is why they fruit poorly in wide flat monotubs where most of the block's surface is buried. Container choice isn't just about how much substrate fits — it's about whether the geometry matches your species' fruiting pattern.
Common substrate questions
What's the difference between substrate and spawn?
Spawn is colonized grain — rye, oats, millet, or similar — inoculated with mycelium from a liquid culture or agar wedge. Substrate is the bulk food mushrooms eat to fruit: coco, vermiculite, straw, or hardwood pellets. You always mix the two together at inoculation. The spawn carries mycelium into the fresh substrate, and the mycelium grows outward from the grain, colonizing the bulk material until it's ready to pin. Ratios vary — 1:1 to 1:4 depending on species and how fast you want colonization.
Do I have to pressure-sterilize, or is pasteurization enough?
It depends entirely on the substrate. Non-supplemented substrates (CVG, straw, plain coco, unenriched manure) can be pasteurized at 160–170°F for 60–90 minutes — a large pot, turkey roaster, or insulated cooler works fine. Supplemented substrates (Master's Mix, anything with bran or added nutrients) must be pressure-sterilized at 15 psi for 2.5 hours. The added nutrients feed bacterial contaminants that pasteurization doesn't kill, so skipping the pressure cooker means a near-guaranteed contamination. If you don't own a pressure cooker, stick to unsupplemented recipes.
How do I know if my substrate is at field capacity?
Use the squeeze test. Grab a fistful of prepared substrate at room temperature. Squeeze as hard as you can. You should see 1–2 drops of water fall, no more. If a stream of water runs out, it's too wet — spread it out and let it air-dry for 30 minutes, then re-test. If nothing drips at all, it's too dry — spray with distilled water (not tap — chlorine kills mycelium), mix thoroughly, wait 10 minutes for absorption, and re-test. Field capacity is a feel, not a number — the squeeze test is more accurate than any weight-based calculation because ingredient water content varies batch to batch.
Can I use the same substrate recipe for all mushroom species?
No, and trying to will waste your time and money. Mushroom species evolved on specific substrates and reject others. P. cubensis and other coprophilous species thrive on CVG or manure-based mixes but barely fruit on Master's Mix. Lion's mane, reishi, king trumpet, and other wood-lovers need the high nitrogen and lignin content of Master's Mix — they'll colonize CVG slowly and produce almost no fruits. Oysters are the most flexible and grow on straw, Master's Mix, coffee grounds, and cardboard, but straw is the cheapest and most reliable home option. Use the calculator's species selector — it prevents mismatched pairings.
How much grain spawn do I need for a 54qt monotub?
At the standard cubensis ratio of 1:2 (one part spawn to two parts substrate), a 54qt monotub holds about 8qt of CVG substrate and needs 4qt of colonized grain spawn — roughly 6 lb of rye. For a 32qt monotub use ~2.5qt of spawn (~3.75 lb); for a 66qt use ~5qt (~7.5 lb). If you want faster colonization, run 1:1 (doubles spawn usage but cuts time by ~30%). If you're tight on grain, 1:3 works but expect 14–21 days of colonization and watch for contamination. The calculator gives exact weights for any container.
Why does my substrate keep getting contaminated?
Most contamination comes from four sources, in order of frequency. First: skipping sterilization on supplemented substrate — Master's Mix pasteurized instead of pressure-sterilized fails almost every time. Second: spawn that's too old or weak — grain older than 2 weeks loses vigor and imports contaminants. Third: too wet — free water in the substrate creates anaerobic pockets where bacillus and pseudomonas thrive. Fourth: poor sterile technique at inoculation — opening the container in a dusty room, not wiping down surfaces, not wearing gloves. Fix these in order: confirm your prep method, use fresh spawn, pass the squeeze test, and inoculate in a still-air box or flow hood.
Can I reuse spent substrate?
Mushroom-wise, not really — spent substrate has given up most of its nutrients after 2–3 flushes and the mycelium has largely exhausted itself. You might get a weak fourth flush but the effort/yield math doesn't work. Garden-wise, it's gold. Spent CVG or Master's Mix breaks down into excellent compost amendment with active mycelial residue that continues to decompose organic matter in your soil. Many gardeners specifically grow mushrooms for the spent blocks. Outdoor mushroom beds (wine caps, oysters) can also be started from spent indoor substrate — especially with wood-lovers, which will keep fruiting outdoors on fresh wood chips for years.
Does the calculator account for evaporation during pasteurization?
Yes, partially. Every recipe has a built-in waste factor (5–10%) that scales the ingredient totals up to account for handling loss, including evaporation during heat treatment. However, if your pasteurization container isn't sealed well or you're running in a very dry environment, you may lose more water than expected — open the lid after cooling and squeeze-test before inoculating. If it's dry, spray distilled water and mix. The calculator's water estimate is a starting amount; the squeeze test is the final authority.
Everything in your shopping list
The ingredients your recipe calls for, shipped from our Minnesota warehouse. Everything here is contamination-screened and graded for mushroom growing — no pesticide residues in the coco, no accelerants in the pellets, no fine-grade vermiculite that compacts when wet.
Coco Coir Bricks
The anchor ingredient of every CVG recipe. One 650g brick hydrates into ~8 qt of substrate — enough for a 54qt monotub with room to spare. Low-nutrient, pH-stable, and free of the contaminants that off-brand compressed coco sometimes carries.
Coarse Vermiculite & Amendments
Vermiculite holds moisture between waterings without compacting — coarse grade outperforms fine for mushrooms because larger particles keep air pockets open. Gypsum sits on the same shelf for the calcium your CVG and manure recipes need.
Monotubs & Fruiting Chambers
The fruiting container is as critical as the substrate that fills it. 32qt–66qt monotubs with FAE hole patterns pre-drilled, plus shotgun fruiting chambers for humid-loving species that don't thrive in tubs.
Pre-Mixed Substrates & Pellets
Hardwood fuel pellets for Master's Mix — 100% hardwood, no binders or accelerants. Plus pre-mixed CVG and manure-based substrates if you want to skip the measuring step entirely.
Grow Bags & Lab Supplies
5lb unicorn grow bags with 0.2-micron filter patches — the standard for Master's Mix and wood-loving species. Pressure-cooker rated. Still-air boxes, injection ports, and syringes round out your lab setup.
Ready-to-Use Grain Spawn
Skip the grain prep step entirely. Pre-sterilized, pre-inoculated rye, oats, or millet — inject your liquid culture or mix straight into your substrate. 5lb bags work for one 54qt monotub at the standard 1:2 cubensis ratio.