Natto Yield
1,045g
Servings
26
Ferment
24 hr
1,250 ml
1,100g
0.11g
24 hr
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Inputs
Result
500g dry small soybeans × 2.2 yield factor = 1,100g cooked. Starter: 1.1kg × 0.1g/kg = 0.11g powder. Natto yield: 1,100 × 0.95 = 1,045g. Soak water: 500 × 2.5 = 1,250 ml.
Inputs
Result
1,000g dry medium × 2.0 yield = 2,000g cooked. Backslop: 2.0kg × 50g/kg = 100g existing natto. Yield: 2,000 × 0.95 = 1,900g. Soak: 1,000 × 3.0 = 3,000 ml.
The amount depends on the starter type. Powdered B. subtilis natto spores require about 0.1g per 1 kg of cooked soybeans. A single spore packet (0.3g) can inoculate up to 6 kg of beans. Backslopping with existing natto requires about 50g per 1 kg of cooked beans.
| Starter Type | Amount per 1kg Cooked | Reliability | Cost |
|---|---|---|---|
| Powder | 0.1g | High | $8–15 (100+ batches) |
| Spore Packet | 0.05g | High | $3–5 per packet |
| Existing Natto | 50g | Medium | Free (from previous batch) |
Standard natto fermentation runs at 100–115°F (38–46°C). The sweet spot is 105°F (41°C) for 22–24 hours. Lower temperatures (100°F) produce milder natto in 30–36 hours. Higher temperatures (115°F) finish faster (14–18 hours) but may produce stronger ammonia notes.
Small soybeans (natto-specific varieties) produce the best natto with more even fermentation and traditional texture. Standard medium soybeans work well too. Large soybeans can be used but take longer to cook and may ferment unevenly. Avoid split or old soybeans.
Fresh natto keeps 5–7 days refrigerated and 2–3 months frozen. After fermentation, let natto cool to room temperature, then refrigerate for at least 24 hours before eating. This aging step mellows the flavor and improves the sticky strings. Freeze in single-serving portions for convenience.
Read our guide
Read our guide
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Last Updated: Mar 19, 2026
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