Free tool
Spawn Rate Calculator
Inoculation rate is the share of the total substrate weight that spawn makes up. Higher rates colonise faster but cost more; lower rates are economical but slower and riskier. This calculator converts a rate into a weight.
How to use these numbers
Tip
Beginners do well at 20–25%, which colonises quickly and outruns contaminants. Experienced growers often drop to 10% for economy once sterile technique is reliable.
Note
Beyond about 35% you are mostly paying for spawn rather than gaining speed, and the extra nutrition can make substrate more attractive to mould.
Caution
Below 5% colonisation is slow enough that contamination usually wins. If you want to stretch spawn, use it in a smaller number of larger blocks instead.