Miso Fermentation Calculator
Calculate soybean, koji, and salt ratios for homemade miso by style and batch size.
Covers white, yellow, and red miso with fermentation timelines.
What is miso?
Miso is a fermented paste made from soybeans, koji (grain inoculated with Aspergillus oryzae mold), and salt. It originated in Japan and has been produced for over 1,300 years. The fermentation transforms simple ingredients into a deeply complex umami-rich paste.
The three key ratios:
Miso recipes are defined by three ratios:
- Koji ratio (koji-buai): Weight of koji relative to soybeans
- Salt percentage: Salt as a percentage of total weight
- Moisture content: Controlled by the soybean cooking liquid (tane-mizu)
Standard formulas by miso type:
| Miso Type | Koji : Soybeans | Salt % | Ferment Time | Color |
|---|---|---|---|---|
| Shiro (white) | 2:1 | 5–7% | 1–3 months | Light |
| Shinshu (yellow) | 1.2:1 | 10–12% | 4–8 months | Golden |
| Aka (red) | 1:1 | 12–13% | 1–2 years | Dark red |
| Hatcho (pure soy) | 1:1 (soy koji) | 10–11% | 2–3 years | Very dark |
| Mugi (barley) | 1.5:1 | 10–12% | 6–12 months | Medium |
Weight formula:
Dry soybeans (g) = Target batch weight ÷ (1 + koji ratio + salt ratio + water ratio)
For a typical shinshu miso:
- 1 part dry soybeans (which double in weight when soaked and cooked)
- 1.2 parts rice koji
- Salt at 12% of total weight
- Tane-mizu (seed water) as needed for consistency
Worked example (1 kg batch of shinshu miso):
Starting with 250g dry soybeans:
- Soaked and cooked soybeans: 250 × 2.2 = 550g
- Rice koji: 250 × 1.2 = 300g
- Tane-mizu: 63 mL of the soybean cooking liquid (about a quarter of the dry bean weight)
- Everything before salt: 550 + 300 + 63 = 913g
- Salt (12% of the finished paste): 913 × 0.12 ÷ 0.88 = 125g
- Total batch: ~1,038g, of which the salt is 12.0%
Include the tane-mizu in that base. It is easy to work out the salt from the beans and koji, then pour the cooking liquid in afterwards and quietly dilute yourself down to 11%. On a two-week pickle nobody would notice. On an eight-month room-temperature ferment it is the margin you are relying on.
Fermentation conditions:
- Temperature: Room temperature (60–75°F / 15–24°C) is standard
- Warmer temperatures speed fermentation but can produce sharper flavors
- Cooler temperatures slow fermentation and produce rounder, more complex flavors
- Weight: Press miso with 20–30% of its weight on top (stone, bag of salt) to eliminate air pockets
- Salt layer: Sprinkle salt on the surface to prevent mold
Tamari bonus:
During fermentation, liquid pools on top of the miso. This is tamari — a rich, complex soy sauce. It is a natural byproduct and a prized condiment. Do not discard it; either mix it back in or collect it separately.
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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