How the numbers on this site are computed

What this page covers

Fermentcalc is a calculator-first reference for lacto-fermentation. Every number on the site — salt mass for a sauerkraut batch, brine percentage for a jar of pickles, fermentation time at a given kitchen temperature — comes from one of three input sources and one of three formulas. This page documents both, so a reader, food-safety extension agent, or fellow fermenter can audit and reproduce the math.

Source hierarchy

1. Government and academic food-safety authorities. The US National Center for Home Food Preservation (NCHFP, University of Georgia), the FDA pH 4.6 threshold for low-acid fermentation safety, USDA Cooperative Extension publications, and peer-reviewed food-microbiology research (Journal of Food Protection, IFT). These are the primary source for salt-percentage ranges, brine ratios, and safety thresholds.
2. Practitioner references with cited methodology. Sandor Katz's The Art of Fermentation, the Noma fermentation guide, and Kirsten Shockey's vegetable fermentation books — used for vegetable-specific timing ranges and style variants where extension data is silent.
3. Practitioner consensus where authoritative sources disagree or are silent. A small number of style-specific ratios (e.g. some hot-pepper-mash percentages) come from a consensus of practitioner sources. Where this is the case the page footer says so explicitly rather than implying a single authoritative source.

The formulas

The whole site rests on three formulas.

1. Dry-salt percentage

Used for sauerkraut, kimchi, pepper mash, and other ferments where salt is added directly to chopped or shredded vegetables that release their own brine.

salt_grams = vegetable_grams × (salt_percent ÷ 100)

The accepted safe range is 1.5–3.0% by weight of vegetable. We default to 2.0% for sauerkraut and 2.5% for higher-risk ferments (kimchi, mash) and let you adjust within the safe range. Below 1.5% the brine takes too long to acidify; above 3% the ferment slows materially.

2. Brine percentage

Used for submerged vegetables — pickles, peppers, garlic, cauliflower, green beans.

salt_grams = water_grams × (brine_percent ÷ 100)

Note this is salt over water, not salt over the total weight including vegetables. Brine percentages are typically 3.0–5.0% for most vegetables (vs the lower 1.5–3% range for dry-salted ferments) because the brine has to do the work the vegetable's own released water does in a dry-salted ferment.

3. Temperature-adjusted timing

Lactic-acid bacteria roughly double in metabolic rate every 10 °C (Q10 ≈ 2). The site uses a Q10 = 2 adjustment anchored at 68 °F (20 °C): a ferment at 78 °F (26 °C) completes roughly 1.4× faster than at 68 °F; at 58 °F (14 °C) roughly 1.4× slower. This is a first-order approximation — final readiness is determined by taste and pH, not by the calendar.

Safety thresholds

• Finished pH must drop below 4.6 (FDA threshold for low-acid shelf-stable fermentation). Most successful lacto-ferments reach pH 3.4–4.0.
• Surface yeasts (kahm) are usually harmless but indicate insufficient submersion or oxygen exposure. Coloured (pink, red, black) mould — discard the ferment.
• Pickling-salt or non-iodised sea salt only. Table salt with iodine or anti-caking agents can inhibit bacteria and discolour the brine. The site assumes non-iodised salt throughout.

Review cadence

Every reference page shows a Last verified date. Primary-source pages (NCHFP recommendations, FDA thresholds, peer-reviewed citations) are re-checked at least twice per year and immediately on any update notice from the source. Per-vegetable timing references are reviewed at least annually.

Corrections

If you spot a wrong number, a broken citation, or a safety threshold that has moved, email the address on the contact page and we verify within 7 days. If the correction holds, the page is updated and the Last verified date refreshes.

What we do not claim

• Fermentcalc is a reference, not medical or food-safety advice. If you are fermenting commercially or for an immunocompromised household, you need a process-authority review beyond what this site can give.
• We do not test individual batches. The math gives you a starting point that aligns with published safe ranges; pH and taste are the final arbiter.
• We do not collect user accounts, emails, or batch logs. There is no "save your ferments" product behind the calculator.

More common questions are answered on the FAQ page.