Cold-Process Soap Lye Calculator
Enter your recipe’s oils by weight to get the exact lye and water amounts.
Before you mix — lye safety:
- Always add lye to water, never water to lye — adding water to lye can cause a violent, boiling reaction.
- Weigh lye and water by mass (a scale), never by volume — this calculator’s numbers are all masses.
- Never use aluminum containers or utensils — lye reacts with aluminum to release flammable hydrogen gas. Use stainless steel, or heat-safe HDPE/PP plastic (not ordinary glass, which lye can etch and weaken over repeated use).
- Wear sealed splash goggles (not just eyeglasses) and gloves, and mix in a well-ventilated area — avoid breathing the fumes given off in the first moments after mixing.
- Lye solution can reach 200°F/93°C or higher within seconds — use cool water, not hot, and keep it away from children and pets.
- Freshly poured soap is still caustic until saponification finishes — wear gloves when unmolding, and allow the full cure time before handling bare-handed.
- If lye contacts skin or eyes: flush with cool running water for at least 15 minutes, remove any contaminated clothing, and seek emergency medical care for eye contact.
Recipe oils
Total oils: 800 g
NaOH: 117.8 g
Water: 304 g
Total batch weight: 1221.8 g · Lye concentration: 27.9%
SAP values are sourced and cited in this project’slib/oil-sap-reference.ts(see also docs/domain-reference.md for the full review) — this tool is a reference aid, not a substitute for double-checking your own oils and lye against a trusted calculator before a large or unfamiliar batch.
Frequently asked questions
- Is it safe to add water to lye instead of lye to water?
- No — always add lye to water, never water to lye. Adding water to solid lye can cause a violent, boiling, splattering reaction. Pour the lye slowly into the water while stirring, in a well-ventilated area.
- What superfat percentage should I use?
- 5% is a common default for a balanced bar. Lower (0-3%) uses more of the oils for cleansing; higher (8-20%) leaves more unsaponified oil for a more moisturizing, softer bar. This calculator accepts 0-20%, the typical safe range for cold-process soap.
- Why does this calculator ask for water as a percent of oil weight?
- It's one common convention (a typical starting point is around 38%). If your recipe instead gives a water:lye ratio or a lye concentration percentage, use the water:lye ratio converter to translate it first.
- Where do the SAP values come from?
- See the oil SAP value reference chart for sources and confidence notes on every oil. Always double-check an unusual or less common oil against your supplier's own data sheet before a large batch.
- Why does it ask for lye purity?
- Pure NaOH is close to 100%, but commercial KOH commonly ships around 90% pure — treating it as 100% would under-deliver actual alkali. Check your product's label; if it states a purity, enter it here so the weighed-out amount is corrected for it.
- Why did it reject my recipe with a water-amount error?
- Sodium and potassium hydroxide can't fully dissolve much above about 50% concentration in water — a recipe with too little water for its lye amount asks for a physically unrealistic solution, and undissolved lye risks a caustic pocket in the finished bar. Increase the water percentage.