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Koji Incubation Calculator

Enter your rice weight, incubation length and chamber temperature to get the spore dose, the times to break and turn the rice, and the peak temperature to watch for.

kg

Polished rice, steamed and cooled to about 35 C before inoculating; short-grain rice is the classic choice.

h

40-48 hours is the standard window for a full batch; shorter runs give sweeter, sake-style koji, longer runs deepen the enzyme load.

°C

Around 30 C grows koji evenly; the rice pile heats itself by several degrees on top of this setting.

Results

Spores needed
2g
Peak core temperature
35 °C
Expected koji yield
2 kg

Turn schedule

  • 6 h - Break and turn the rice
  • 12 h - Break and turn the rice
  • 20 h - Break and turn the rice
  • 28 h - Break and turn the rice
  • 36 h - Break and turn the rice

Keep humidity around 70-80% and check the rice when it smells chestnut-sweet and grains crumble like short vermicelli - that is the moment to pull or refrigerate it.

How the koji schedule works

Spores are dosed at about 1 g per kilogram of rice - a safe working rate for home batches (commercial shops often use half of that). The rice is broken up and turned at roughly 6, 12, 20, 28 and 36 hours to release heat and even out growth, so the schedule lists the checkpoints that fall inside your batch. Because the mold generates its own heat, the core of the pile runs about 5 C above the chamber - keep it under 38 C by turning, spreading or dropping the room temperature.

spores (g) = rice (kg) x 1; peak core = chamber + 5 C; turns at 6, 12, 20, 28, 36 h

The miso salt ratio calculator turns the finished koji into a miso batch, and the fermentation temp and time calculator shows how temperature reshapes any ferment's schedule.

Frequently asked questions

Why do I have to turn the rice so often?

Koji mold respiration generates real heat, and a stationary pile overheats and dries unevenly. Breaking and turning the rice releases trapped heat and moisture and exposes fresh surfaces, which keeps the mold growing evenly instead of clumping.

How do I keep humidity right without special equipment?

A covered tray or a box with a damp cloth inside holds 70-80% humidity easily. Too dry and the grains crust over white and powdery; too wet and they mat together and invite bacteria. Briefly airing the tray during each turn usually corrects both.

When exactly is koji finished?

Look and smell rather than the clock: the grains are covered in white fuzz (green dust means sporulation has started), the aroma is sweet and chestnut-like, and a grain snaps cleanly with a crumbly, vermicelli-like core. At that point chill or dry it to stop growth.