How Much Lye for Cold-Process Soap?
Soap is the chemical reaction between fats and lye (sodium hydroxide, NaOH, for solid bars). Every oil needs a specific amount of lye to fully saponify, described by its SAP value. This calculator uses an average SAP figure across common soaping oils to give a quick estimate from your total oil weight — useful for planning batch size and mold fill, but not a substitute for a precise per-oil calculation.
Why superfat matters
Superfatting means using slightly less lye than would react with all the oil, leaving a small percentage of unreacted fat in the finished bar. That extra fat makes soap gentler and more moisturizing, and it's your safety margin against a lye-heavy, skin-irritating bar. A superfat of 5% is a common default; the calculator reduces the lye accordingly.
Always verify, and work safely
Because different oils have different SAP values, a recipe of mostly coconut oil needs more lye than one of mostly olive oil at the same weight. So treat this estimate as a starting point and run your exact oil blend through a dedicated SAP calculator before mixing. And respect the chemistry: lye is caustic. Always add lye to water rather than water to lye, work in a ventilated space, and wear goggles and gloves. This tool is for planning, not a safety authority.
Frequently asked questions
- Is this calculator exact?
- No — it uses an average SAP value. Verify your specific oil blend in a full SAP calculator before making soap.
- What does superfat do?
- It leaves a little unreacted oil in the bar for a gentler, more moisturizing soap and a safety margin against excess lye.
- Can I use this for liquid soap?
- No. Liquid soap uses potassium hydroxide (KOH); this estimates sodium hydroxide (NaOH) for solid bars.