Free tool
Mushroom Substrate Calculator
Enter the weight of your dry substrate and the moisture you're aiming for. You'll see how much water to add and how much spawn to mix in.
For this batch
Always finish with the squeeze test: a handful should release only a few drops of water.
How the calculation works
Moisture content here means the share of water in the wet substrate. The calculator keeps the dry matter the same and adds water until the mix reaches your target. For example, 1 kg of dry material at 0% moisture needs 1.5 kg of water to reach 60%.
- Hardwood sawdust and Master's Mix: about 60% moisture is a common target.
- Straw: usually soaked and drained to field capacity rather than weighed. Use the calculator as a starting point.
- Spawn rate: 5–20% of the wet substrate weight is common. Higher rates colonize faster and resist contamination better.
Values are typical guidelines from growers. Materials vary, so adjust with the squeeze test. 1 kg of water is about 1 liter.