Mushroom substrate recipe calculator
Enter the finished weight of one block, and get the sawdust, supplement and water by weight.
FreeNo sign-upRuns in your browser
Enter a block weight and press Calculate.
How the math works
Start from the finished block weight. The dry mix is that weight times (1 minus the water share), and the water is the rest. The dry mix then splits by your sawdust share, and what is left is the supplement.
As a check, Grocycle's example of a five-pound Master's Mix block uses 1 lb of sawdust and 1 lb of soy hulls. Enter 5 lb at 60% water and 50% sawdust and the calculator gives 1 lb of sawdust, 1 lb of soy hulls and 3 lb of water.
Where the recipe comes from
Grocycle describes Master's Mix as a 50:50 mix of hardwood sawdust and soybean hulls, hydrated to 60% and sterilized in a pressure cooker. Mushroom Media, another grower supply site, gives a target of about 50 to 60% moisture for its Master's Mix, which is why the water field is editable.
The 80/20 preset is for shiitake-style blends. Mushroom Media lists a shiitake mix of 40 lb of oak hardwood pellets to 10 lb of wheat bran, which works out to 80% and 20% by weight. We have only that source for it, so treat the preset as a starting point and check it against your own recipe.
What this does not cover
The calculator gives block amounts only. It does not give spawn amounts, sterilization times or pasteurization steps, because we have not tested them and do not want to hand you untested numbers. For the wider picture, see how to grow mushrooms without a kit, and how to grow mushrooms at home for the kit method.
Related pages
Common questions
What is Master's Mix?
Grocycle describes Master's Mix as a 50:50 mix of hardwood sawdust and soybean hulls by weight, hydrated to 60% and sterilized in a pressure cooker.
How much water should the substrate have?
Grocycle hydrates Master's Mix to 60%. Mushroom Media gives about 50 to 60%, so the calculator lets you change it.
Does this calculator include spawn?
No. It gives the sawdust, supplement and water for the block. We do not give spawn amounts because we have not tested them.
Where does the 80/20 shiitake preset come from?
Mushroom Media lists 40 lb of oak hardwood pellets to 10 lb of wheat bran for shiitake, which is 80% to 20%. It is a single source, so check it against your own recipe.
Sources
- What is Master's Mix + How to Make It?, Grocycle. Checked November 11, 2026.
- Make Your Own Master's Mix, Mushroom Media. Checked November 11, 2026.
How this page is checked
Cedrick Reese built this calculator from Grocycle's Master's Mix recipe and checked it against Grocycle's own five-pound example. The shiitake preset rests on one grower site, which the page says. The tool only does the weight math, so it carries no spawn rates or sterilization times, and we have not made substrate blocks for this page.