🧪 Bulk Substrate Calculator

Calculate precise hydration ratios for hardwood pellets, masters mix, and custom blends.

Base Substrate

Hardwood Fuel Pellets (HWFP)

100% hardwood, 6% moisture. The most common base for gourmet mushroom blocks.

Pre-Mixed Pellets (80/20)

80% hardwood, 20% wheat bran already mixed. 8% moisture.

Masters Mix Pellets (50/50)

50% soy hulls, 50% hardwood. 7% moisture. Great for shiitake and lion's mane.

Custom Blend

Set your own starting moisture content for any substrate material.

Final Block Weight

0.5 kg2.3 kg standard25 kg

Target Moisture Content

62%

50% Dry60-65% Ideal75% Wet

62% moisture: Ideal range for most gourmet mushrooms (oyster, lion's mane, maitake).

Additives

Percentages are calculated by dry substrate weight. Adjust to 0% to exclude.

Gypsum (calcium sulfate)2%

Improves structure and provides calcium. 1-3% is typical.

Wheat bran supplement10%

Boosts nutrition for faster colonization. 5-15% is common.

Soy hull supplement0%

Alternative protein source. Often used with hardwood pellets.

Coffee grounds0%

Can boost yields for oyster mushrooms. Use fresh, cooled grounds only.

Spawn Rate

10%of total block weight

5% (slow colonization)10% typical25% (fast colonization)

Your Substrate Recipe

Hardwood Fuel Pellets (HWFP) (raw pellets)836.81 g

Water to add1.23 L

Gypsum15.73 g

Wheat bran78.66 g

Spawn needed230 g

Final Block Weight

2.3 kg

Moisture Content

62%

Substrate Prep Tips

1.Weigh your pellets dry, then add water. Pellets will absorb water and expand — wait 20-30 minutes before mixing in additives.
2.The squeeze test: Grab a handful and squeeze firmly. A few drops should drip out — if water streams, it is too wet; if nothing drips, add more.
3.Sterilize at 15 PSI for 2.5 hours for supplemented substrates.
4.Cool to room temperature before inoculating with spawn.

About This Calculator

Getting the right moisture content is the single most important factor in mushroom substrate preparation. Too dry and your mycelium will stall; too wet and you invite bacterial contamination and green mold. This mushroom substrate calculator takes the guesswork out of hydration ratios, giving you precise water-to-substrate measurements for any batch size.

Whether you are using standard hardwood fuel pellets, a 50/50 masters mix recipe, or a custom blend, the calculator adjusts for starting moisture content and additive ratios. For a deeper dive into substrate science, see our complete substrate guide.

Need supplies? Browse our grow bags, substrates, and cultivation supplies — everything ships fast across Canada from our facility in Brantford, Ontario.