Shoyu: salt weight calculator

On shoyu moromi saltiness

What is a brine?
A brine is water + sea salt at a target strength (12%, 15%, 18%, 20%, etc.), expressed by weight (w/w).

What we do not do
We do not calculate salt as “a percent of (water + koji)”. Koji is not part of the brine denominator.

Why this matters for shoyu

• Salt sets water activity, selects which microbes thrive, and governs enzymatic reactions speed.
• Using the w/w definition keeps results consistent across batch sizes and temperatures.
• Using “% of water” or including koji in the denominator can under- or oversalt, affecting safety, flavor, and timing.

Practical assumption (common practice)
Most shoyu makers use water ≈ koji volume (1:1).
→ Choose your water (equal to your available koji volume), then compute salt from water only.

So now we have:

$$ \frac{\text{salt}}{\text{salt} + \text{water}} = \text{target \% (as a decimal)} $$ So, for example 300 ml water (with any amount of koji alongside) and a 20% brine: $$ \frac{x}{x+300} = 0.20 \Rightarrow x = 75\ \text{g of salt} $$