Grain Spawn Calculator
Work out how much dry grain and water your spawn jars need — or how many jars a bag of grain will fill — hydrated to the moisture that gives clean, fast-colonizing spawn.
Grain spawn is the engine of a grow: dry grain is soaked, simmered and drained, then jarred and sterilized. Get the moisture right and it colonizes quickly without pooling water — the enemy of clean spawn. Pick your grain and jar size, and the calculator handles the hydration and the jar count.
Grain is soaked or simmered, then drained so there's no standing water before it goes in the jar. Target is a moist grain that colonizes fast without pooling — the enemy of clean spawn.
How to use it
- Choose a mode. Start from how many jars you want, or from the grain you already have.
- Pick grain and jar size. Rye, millet, wheat, oats or popcorn, and pint, quart or half-gallon jars.
- Enter the figure. The number of jars, or the dry grain weight.
- Read the result. Dry grain, water absorbed and jars filled, with a note to simmer, drain hard, and sterilize at 15 psi.
Questions
What grain is best for spawn?
Rye is the classic — big, nutritious and easy to hydrate. Millet is dense for more grains per jar; wheat and popcorn are cheap and forgiving. All work well.
How do I hydrate grain for spawn?
Soak it, then simmer or steam, then drain hard so there's no standing water before it goes in the jar. The goal is moist grain, not wet grain.
How much grain goes in a jar?
Roughly 350 g hydrated in a pint, 650 g in a quart and 1300 g in a half-gallon, leaving headspace to break up the grain.
Why should there be no standing water in the jar?
Pooled water invites bacteria and sour-smelling contamination. Drain well so the grain is moist but not wet.