Spawn calculator

Enter your hydrated substrate weight, species, and spawn type. Returns the recommended spawn weight, the spawn-to-substrate ratio, and a rough Canadian-dollar cost estimate so you can plan a batch before ordering.

Why spawn ratio matters

Every percentage on this page is a share of hydratedsubstrate weight — the weight after soaking and draining to field capacity, not dry weight. The two differ by roughly three times on straw, so the basis matters more than the number.

The more spawn you add, the faster the substrate colonizes and the smaller the window for contamination. Growers commonly use 10–20% on raw or pasteurized straw, because the substrate isn't sterile: the high end colonises faster, the low end costs less. Sterilized supplemented sawdust runs as low as 3–5% because the only mycelium present is what you put in.

• Too low — mould reaches the substrate before your mycelium does. Loss of the batch is common below 3% on non-sterile substrate.
• Too high — wasted spawn and money; yields don't improve much past ~15% on sterilized blocks.
• Grain vs. sawdust spawn — grain colonizes faster (more nutrients) but is more contamination-prone if your substrate is non-sterile. Sawdust spawn is slower but more forgiving and is the standard for outdoor logs and bulk straw.

Inoculation tips

• Break up grain spawn — shake the bag thoroughly so every grain separates before adding it to the substrate. Each loose grain becomes an inoculation point.
• Layer or mix evenly — for bags, mix top-to-bottom by hand (clean gloves, in front of a still-air box). For straw buckets, layer spawn between handfuls of substrate.
• Work clean — wipe down surfaces with isopropyl, wear a fresh pair of nitrile gloves, and minimize the time the substrate is open.
• Incubate at species-appropriate temperatures — most oysters at 21–24 °C, shiitake and lion's mane at 21–25 °C. Cold incubation slows colonization and widens the contamination window.
• Spore safety — work in a ventilated space and wear an N95 mask when opening bags of grain spawn or breaking up colonized substrate. See our mushroom worker's lung guide.

Cost figures are estimates. Canadian grain-spawn prices typically land between $20 and $40 CAD per kg from established suppliers, with bulk orders and DIY grain prep both bringing the effective price down. Sawdust spawn is usually priced similarly per kg but covers a smaller inoculation volume because it carries less nutrient density.