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Mushroom Substrate Calculator
Wet substrate weight, flush yield, pasteurization hours by method and a colonization finish date for a dry-weight recipe.
Mushroom Substrate Calculator
Chlorova · updated for 2026
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Field-capacity and biological-efficiency figures are the midpoints of commonly published cultivation ranges, and your results move with chop size, bag density, strain, temperature and how much water you actually squeeze out. Contamination is the real variable no calculator models: pasteurization is not sterilization, and a rich supplemented substrate treated like straw will usually mold rather than fruit. Follow the temperature and time your grain or block supplier specifies, work clean, and treat the colonization date as the earliest plausible one rather than a promise.
Mushroom growing is cooking with someone else's mold, and the two decisions that decide a crop are how wet the substrate ends up and whether it was heat-treated long enough. Dry weight is the only number you can trust at the scale, so every recipe starts there and multiplies into a wet weight that then determines flush size and how many bags you actually fill. This calculator takes species, substrate and method, and returns the wet pounds, the yield estimate, the hours at temperature and a colonization finish date to plan the fruiting space around.
Planning the Batch
- Choose the species, which sets water-holding and yield.
- Pick the substrate you have actually bought or gathered.
- Select the pasteurization or sterilization method planned.
- Weigh the dry substrate before anything gets wet.
- Set the dry weight each grow bag will hold.
- Enter the day you intend to run the heat or the bath.
- Read the wet weight and squeeze-test against it before filling.
- Count the bags so lids, filters and spawn are bought together.
- Hold the flush estimate back until colonization actually finishes.
- Plan fruiting space for the printed colonization date.
Dry Weight In, Wet Pounds Out
Substrate at field capacity carries a multiple of its dry weight in water, roughly two times for a dense sawdust block and three times for a loose straw bed, and the species table supplies that factor. Ten dry pounds of master's mix at 2.4 becomes 24 wet pounds, which is the weight you will actually be carrying and the number yield is measured against.
Yield follows biological efficiency expressed on wet substrate, so a king oyster at 0.25 first-flush factor on 24 wet pounds expects about six pounds from the first flush, and the season line multiplies that by 1.7 to account for the smaller second and third flushes. Pasteurization time arrives from a two-key table of substrate and method, is converted to whole days for the schedule, and then adds the species' colonization length to your treatment date.
Worked Example: Ten Dry Pounds of King Oyster
- You choose king oyster on master's mix with hot-water pasteurization.
- The scale reads ten dry pounds, and bags hold five each.
- Field capacity for king oyster is 2.4 times dry weight.
- So the wet substrate total comes out at 24 pounds.
- Each five-pound bag will weigh near 12 pounds when full.
- Ten dry divided by five fills two grow bags.
- Hot-water treatment of master's mix takes two hours.
- Two hours rounds up to one day on the calendar.
- King oyster colonizes in about fourteen days.
- March 1 plus 15 days prints a finish near March 16.
- The first flush estimate is 24 times 0.25, or six pounds.
- The season line adds the later flushes for 10.2 lb total.
Pasteurization and Sterilization Hours
The time-at-temperature table behind the calculator, by substrate and method, counting only the hold time once the core of the material is at temperature. Cold lime baths are a chemical pasteurization at high pH, while supplemented substrates are too rich to pasteurize safely and want full pressure sterilization.
| Substrate | Cold lime (h) | Hot water (h) | Steam (h) | Pressure (h) |
|---|---|---|---|---|
| Straw | 24 | 1.5 | 2 | 2.5 |
| Master's mix | 24 | 2 | 2.5 | 2.5 |
| Sawdust supplement | 48 | 2.5 | 3 | 2.5 |
| Coffee grounds | 12 | 1 | 1.5 | 2 |
Key Terms
- Field capacity
- The wetness at which substrate holds water without dripping when squeezed.
- Biological efficiency
- Fresh mushroom yield expressed as a percentage share of the wet substrate weight it grew from.
- Pasteurization
- Heat or chemistry that reduces competitors without sterilizing the substrate.
- Sterilization
- Full killing of all organisms, required for rich supplemented substrates.
- Colonization
- The run of days until mycelium has consumed the substrate and is ready to fruit.
Growers Who Have Lost Batches
- Wet is a failure mode: substrate packed past field capacity starves mycelium of oxygen and grows contamination in the middle of the bag rather than at the edges.
- Dry weight is the only honest unit in a recipe, because a bucket of chopped straw weighs absurdly different depending on chop size and how hard you shoved it in.
- Richness buys yield and buys risk in the same transaction, which is why soy-hull and supplement mixes demand pressure sterilization while plain straw gets by on a hot bath.
- Biological efficiency on wet weight always falls as flushes continue, so a 1.7 multiplier for the season is generous for a third flush and stingy for a very vigorous oyster.
- The colonization date is the day to have fruiting space ready, not the day to start looking for one, because a fully colonized block begins pinning wherever it gets a draft and a window.
Common Mistakes & Fixes
- Measuring substrate by volume and then wondering why the wet weight missed.
- Pasteurizing a supplemented block and inviting green mold to win.
- Counting hold time from when the water was lit rather than when the core reached temperature.
- Squeezing for a drip instead of a few drops at field capacity.
- Stacking bags with no air exchange and losing the whole shelf to CO2 stall.
- Planning fruiting space after colonization finishes instead of before.
- Expecting the first flush number to repeat on flush three.
When to Use This (and When Not To)
Open it with the dry material weighed and the method chosen, which is the moment a recipe becomes a schedule. It covers substrate and timing rather than room conditions, so use the grow-room fan CFM calculator for air exchange, the hydroponic reservoir calculator for humidity fogging plumbing, and log flush weights afterward to correct the efficiency figure for your own strain.
Mushroom Substrate Calculator: Frequently Asked Questions
How wet should mushroom substrate be?
At field capacity, which for most substrates means two to three times the dry weight in water and a squeeze test that yields a few drops rather than a stream. Too wet is worse than too dry, because waterlogged substrate suffocates mycelium and breeds contamination inside the bag.
How many mushrooms will a pound of substrate give?
Roughly 0.15 to 0.30 fresh pounds per pound of wet substrate depending on species, so a 12 lb wet bag of oyster substrate can deliver three to four pounds in its first flush. Later flushes are progressively smaller, and a whole season usually lands near 1.7 times the first flush.
Do I need a pressure cooker or is pasteurization enough?
Plain straw and other woody materials can be pasteurized with hot water or a lime bath, but supplemented substrates containing grain or bran need full sterilization at 15 psi. Rich food pasteurized rather than sterilized is the single most common cause of a lost block.
How long does colonization take?
About ten days for pearl oyster at room temperature, two weeks for king oyster and lion's mane, and a month for shiitake blocks. Temperature, bag density and spawn rate all move the number, so treat the printed date as the earliest plausible one.
Can I use coffee grounds as substrate?
Yes, used grounds are a free, mildly pasteurized material that oysters colonize quickly, but they are dense and drain poorly, so they perform best blended with straw rather than packed alone. Coffee also holds a lower pH, which suits most gourmet species well.
What is the cold lime method?
It is a chemical pasteurization, soaking chopped straw in a hydrated-lime bath at high pH for a day or two without applying any heat at all. It is cheap and works notably well on straw, and it is not a substitute for sterilization on rich supplemented substrates.
Sources & Further Reading
- Growing Gourmet and Medicinal Mushrooms — Penn State Extension. Basis for field-capacity and biological-efficiency ranges.
- Mushroom Cultivation: Substrate Pasteurization — University of Florida Extension. Source of the method-by-substrate hold times.
- Small-Scale Mushroom Production for Market — Kerr Center Sustainable Agriculture. Supports the flush-decay and contamination guidance.
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Formula verified against published references · Updated