Miso & Shio-Koji Salt Percentage Calculator
Enter the soybeans you are starting with and the strength you are after, and it weighs out the salt, the rice culture and the finished batch, then tells you how long comparable batches take to be ready. Recipes disagree about what the salt percentage is a percentage of, so it shows all three readings side by side. Nothing uploaded.
What goes in the tub
The same salt, read three ways
One weight of salt, three percentages. This is the whole reason a recipe number copied out of context goes wrong.
How long it needs
Every published salt level, weighed for this batch
The middle column is the salt you weigh out at that level. The right-hand one is what the finished batch works out at once the salt is counted in its own total.
Will it fit your vessel
Shio-koji instead
What the published recipes actually say
Take it to the kitchen
Copies the whole batch as plain text, assumptions included, so the numbers travel with the reasoning behind them.
Learn more: miso salt percentages and koji ratios
Which weight the percentage is a percentage of
Every miso recipe quotes its salt as a percentage, and almost none of them say clearly what that percentage is measured against. There are three plausible answers and they give three different amounts of salt.
Koji House is explicit about it: take "the weight of the koji and cooked soy beans" and "calculate 10% of that total weight". Nanase Sukeno uses the same base, recommending "a minimum salt amount of 9% for room temperature fermentation" calculated on koji plus cooked beans.
Preserved does the same thing in different words, giving each style as a percentage "of the total weight of legumes + koji", from 4.5% for its sweetest formula up to 16% for its saltiest.
The other camp measures against the finished batch, with the salt counted inside its own denominator. Fermented Food Recipes writes the formula out in full: "salinity concentration＝weight of salt÷(weight of dry soybeans×2.5＋weight of rice Koji＋adding beans broth＋weight of salt)×100".
That is a real difference, not a rounding one. Take 240 g of cooked beans and 100 g of koji, and twelve percent on the beans-and-koji base is 40.8 g of salt.
Twelve percent of the finished batch is 46.4 g instead, because on that base the salt has to cover itself as well.
Read the other way round, that first batch of 40.8 g is a 10.7% miso on the finished-batch base. The tool shows both numbers at once so you can match whichever one your recipe meant.
The dry weight is the trap
The mistake that actually spoils batches is not choosing the wrong one of those two. It is using the dry bean weight, because that is the number you wrote down at the shop.
Sukeno's own worked pair is the clearest illustration: 100 g of dried beans "weigh 240 g" after cooking. Add 100 g of koji and the correct base is 340 g, so a 12% miso needs 40.8 g of salt.
Run the same sum on the dry weight and the base is 200 g, which gives 24 g. That batch is really sitting at 7.1% salt, under the 9% Sukeno names as the room-temperature minimum, and you would have no reason to suspect it.
The error scales with how much the beans swell, which is the one figure nobody agrees on. Koji House says "the beans will double in weight once soaked and cooked", Sukeno's pair works out at 2.4 times, and Fermented Food Recipes uses 2.5 in its formula while stating elsewhere that boiled soybeans "become 2.4 - 2.6 times as heavy as when it's dry".
That is why the cooked factor is an editable input rather than a constant. Weighing your own beans after they drain removes the guess entirely, and the tool takes a cooked weight directly if you do.
Koji ratio sets the style, and it gets quoted two ways
More koji means more enzyme and more of the starch turned to sugar, so a high-koji batch tastes sweeter and is ready sooner. Preserved runs the ratio from koji at three times the legumes for its sweetest style down to half the legumes for its saltiest.
Sukeno gives the workable span as anywhere "from a 1:1 ratio (equal parts beans and koji) up to 3:1 (three times more koji than beans)". Fermented Food Recipes puts it at 0.8 to 2.3 times the weight of the soybeans.
The catch is the same as with the salt. Koji House quotes its ratio against the cooked beans, at "2 parts cooked soy beans : 1 koji", while most home recipes quote it against the dry weight.
This tool counts the ratio against the dry weight and prints the cooked-basis equivalent next to it in the mix panel. At 500 g of dry beans and a 1:1 ratio, the 500 g of koji is 0.42 times the cooked bean weight, which looks like a completely different recipe until you see both figures together.
Reading the fermentation window honestly
The window this tool reports is looked up from published guidance for comparable batches rather than modelled. Nothing about your kitchen is in it.
It does that because the two sources that give times disagree about what drives them. Preserved times a batch by its salt, from two to three weeks at 4.5% up to 24 months at 16%. Sukeno times it by koji instead, saying "triple koji miso in 2 months", about four months for double koji, and "1:1 koji miso takes 6 months or more".
At the default settings those two land on different answers, so the tool shows the span between them and names which source gave which end. A batch at 12% salt with a 1:1 koji ratio is 12 months by Preserved's salt table and six months or more by Sukeno's koji one.
Temperature moves both. Koji House says miso is "usually done in 6-12 months when fermented at room temperature", and that holding it at 90 °F can "cut this time down by as much as a third". Sukeno's summer advice runs the other way, raising salt to 11-13% because warmth speeds the fermentation up more than you want.
Taste it rather than trusting the calendar. The window tells you when to start checking, not when it is done.
Shio-koji is the same sum with one ingredient fewer
Shio-koji is koji, salt and water, and its percentage is quoted against the finished batch. Briefer gives the target band as 8 to 10% and suggests aiming for 9% to start.
Its worked example is the one the shio-koji panel reproduces: 200 g of koji in a 500 g batch at 9% takes 45 g of salt, leaving 255 g of water. Read against the koji and water alone that same 45 g is 9.9%, which is the same two-base gap the miso side has.
Briefer is also the source of the tool's caution about thin batches, warning that "too little salt creates a breeding ground for undesirable organisms" and that going "lower than 11% increases the risk of spoilage". That last figure is stricter than the floors the other sources give, which is worth knowing rather than averaging away.
What the tool deliberately does not do
It does not tell you whether a batch is safe. Salt percentage is one input to that and the tool has no access to the others, including how clean the vessel was and whether the paste stayed under its weight.
It also does not model pH, water activity or the enzyme activity of your particular koji, all of which move the result and none of which can be inferred from weights.
The vessel figure is the softest number here. It converts the batch weight to a volume with a packing density of 1.2 g per millilitre, which is an estimate rather than a published constant, so treat it as a rough check on whether the jar is big enough.
FAQ
Is the miso salt percentage based on dry or cooked soybeans?
Cooked. Koji House calculates 10% of "the weight of the koji and cooked soy beans", and Sukeno recommends a 9% minimum "calculated based on the total weight of koji + cooked beans". Taking the percentage against the dry weight instead is the common mistake, and it always comes out short: 100 g of dry beans that cook up to 240 g, mixed with 100 g of koji, need 40.8 g of salt for a 12% batch, while the same sum run on the dry weight gives 24 g, which is a real salinity of 7.1%.
How much salt do I need for 1 kg of soybeans?
For 1 kg of dry beans cooked to 2.4 kg and mixed with 1 kg of koji, a 12% batch on the beans-plus-koji base takes 408 g of salt. If the recipe means 12% of the finished batch with the salt counted inside it, the same batch takes 464 g instead. The two readings of the same number differ by about 56 g of salt on that one batch.
What is the minimum salt percentage for miso?
The published floors do not agree, which is worth knowing before you pick one. Sukeno gives 9% of koji plus cooked beans as a room-temperature minimum and raises it to 11-13% for a batch started in summer, Fermented Food Recipes says miso salinity "has to be between 5 to 20%", and Briefer says anything "lower than 11% increases the risk of spoilage". Sweet white styles genuinely do sit under those floors, at 4.5% in Preserved's sweetest formula, but they ferment for two to three weeks rather than a year and are kept cold.