Lye Calculator
Work out how much sodium hydroxide (NaOH) and water a cold-process soap recipe needs, from your oils and fats.
Recipe
Water (% of oil weight) / Superfat / lye discount
Bar shows oils, lye, and water proportional to their weight in the finished batch.
Safety first
- Wear safety goggles and chemical-resistant gloves — lye splashes are a serious eye and skin hazard.
- Always add lye TO water, slowly, while stirring — never water to lye. Adding water to lye can cause a violent, boiling eruption.
- Work in a well-ventilated area or outdoors; lye dissolving in water releases hot, irritating fumes.
- Store lye in a clearly labelled, child- and pet-proof container, away from food and drink containers.
How this is calculated
Each oil needs a different amount of lye to fully turn into soap — its saponification (SAP) value, in grams of NaOH per gram of oil. Multiplying every oil's weight by its SAP value and summing the results gives the lye needed for 100% saponification. A superfat percentage is then subtracted, leaving that percentage of oils unreacted on purpose — this makes the soap milder and, importantly, guarantees no free (unreacted) lye remains once the oils fully react. Water is calculated separately, as a percentage of total oil weight, and simply dissolves the lye — it doesn't affect saponification chemistry.
Common questions
Where do the SAP (saponification) values come from?
They're cross-checked against two independently published soap-making charts (The Soap Kitchen's SAP value guide and SoapCalc's oil list) — standard reference figures used across the soap-making community, not something invented for this calculator. Where the two sources disagreed (only coconut oil: 0.183 vs 0.190), the more widely published value was used.
Why does the superfat percentage matter for safety, not just texture?
A 0% superfat recipe uses exactly enough lye to react with every gram of oil — but real measuring and mixing always have some imprecision, so a 0% target risks leaving trace free lye in the finished soap. A 5-8% superfat leaves a deliberate safety margin of unreacted, skin-safe oil, which is why virtually every published cold-process recipe uses one.
Is this a substitute for learning cold-process soap making before starting?
No. This calculator gets the lye/water math right for a recipe you already have in mind, but it doesn't teach technique, trace, mold prep, or cure time. If you're making soap for the first time, follow a complete, tested recipe and tutorial from an established soap-making resource before working with lye.
Part of Hobby & Craft Calculators