Grain Spawn Jar Calculator
Calculate how many grain spawn jars to prepare based on substrate volume, spawn rate, and grain type.
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 |
|---|---|---|
| Half-pint (250 ml) | ~200 g | 1.3–2 kg at 10% |
| Pint (500 ml) | ~400 g | 2.7–4 kg at 10% |
| Quart (1 L) | ~800 g | 5.3–8 kg at 10% |
| Half-gallon (2 L) | ~1,600 g | 10.7–16 kg at 10% |
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