Grain Spawn Jar Calculator
Calculate grain spawn jars from substrate volume and spawn rate.
Covers rye, wheat, oat, and corn for oyster, shiitake, and gourmet mushroom cultivation.
What is grain spawn?
Grain spawn is sterilized grain (rye, wheat, millet, oats, or corn) that has been colonized with mushroom mycelium. It serves as the inoculum that you mix into your pasteurized substrate (straw, sawdust, coco coir, etc.) to start the fruiting process. Each grain kernel acts as an individual inoculation point, so more spawn means faster colonization.
Spawn rate formula:
Spawn needed (g) = Wet substrate weight (g) × Spawn rate (%)
Spawn jars needed = Spawn needed / Spawn per jar
Standard spawn rates:
| Spawn Rate | Speed | Risk Level | Best For |
|---|---|---|---|
| 5% | Slow (3–4 weeks) | Higher contamination risk | Experienced growers, cost saving |
| 10% | Medium (2–3 weeks) | Standard risk | General use, recommended |
| 15% | Fast (1.5–2 weeks) | Lower risk | Beginners, difficult substrates |
| 20% | Very fast (1–1.5 weeks) | Lowest risk | Maximum speed, competition |
Higher spawn rates mean faster colonization, which reduces the window for contamination. The tradeoff is cost — grain spawn is the most expensive consumable in mushroom cultivation.
Grain types and their properties:
| Grain | Dry Weight/Liter | Hydrated Weight/Liter | Inoculation Points/L | Notes |
|---|---|---|---|---|
| Rye berries | 680 g | 1,000 g | ~3,000 | Gold standard, excellent nutrition |
| Wheat berries | 700 g | 1,050 g | ~2,800 | Similar to rye, slightly cheaper |
| Millet | 650 g | 950 g | ~8,000 | Most inoculation points per volume |
| Oats (whole) | 450 g | 750 g | ~2,500 | Easy to prepare, slightly sticky |
| Corn (cracked) | 700 g | 1,100 g | ~1,500 | Large kernels, fewer inoculaton points |
| Wild bird seed | 600 g | 900 g | ~5,000 | Cheap mix, good for beginners |
Jar size reference:
| Jar Size | Hydrated Grain Weight | Covers Substrate (at 15% → 10%) |
|---|---|---|
| Half-pint (250 ml) | ~200 g | 1.3–2 kg |
| Pint (500 ml) | ~400 g | 2.7–4 kg |
| Quart (1 L) | ~800 g | 5.3–8 kg |
| Half-gallon (2 L) | ~1,600 g | 10.7–16 kg |
Read those ranges as a rate, not a tolerance: the low figure is what one jar covers at a 15% spawn rate and the high figure at 10%. A quart jar covers exactly 8 kg at 10%, because 800 g is a tenth of 8 kg.
About the grain weights above. The dry and hydrated columns in the grain table are densities of two different volumes, so do not divide one by the other to get a dry-to-hydrated conversion. What matters for buying grain is moisture: dry berries sit near 12% moisture and you hydrate them to roughly 45%, which works out at about 1.6 kg of prepared grain for every 1 kg you buy. That is the ratio this calculator uses when it tells you how much dry grain to order.
Example calculation:
Growing oyster mushrooms in 5 × 2.5 kg straw bags:
- Total substrate: 12.5 kg wet weight
- Spawn rate: 10%
- Spawn needed: 12,500 × 0.10 = 1,250 g
- Using quart jars (800 g each): 1,250 / 800 = 1.6 → 2 quart jars
Sterilization protocol:
Grain jars must be pressure sterilized at 15 PSI (121°C) for:
- Half-pint jars: 60 minutes
- Pint jars: 75 minutes
- Quart jars: 90 minutes
- Half-gallon jars: 120 minutes
Let jars cool completely (12–24 hours) before inoculating. Opening the pressure cooker early creates temperature shock that can crack jars and introduce contaminants.
Grain preparation:
- Soak grain in water for 12–24 hours
- Simmer for 15 minutes (until grain is hydrated but not burst)
- Drain and dry surface moisture (spread on towels for 30 minutes)
- Load into jars with modified lids (injection port + filter patch)
- Sterilize in pressure cooker
- Cool and inoculate with liquid culture or agar transfer
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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