Soap Making Lye Calculator (Cold Process)
Calculate the perfect lye amount for your cold process soap. Get NaOH, KOH, and water measurements.
Total oils in your recipe
Water as % of lye solution (typical: 33-38%)
Extra fat for conditioning (typical: 5-8%)
How much lye do I need for soap making?
Lye amount depends on your oils - each oil has a different SAP (saponification) value. Use our calculator for accurate measurements. Never guess with lye - too much lye causes harsh, irritating soap. Too little lye leaves greasy, mushy soap. Always run your exact recipe through a calculator before making soap. The SAP value represents grams of NaOH to saponify 1 gram of oil.
What is superfat in soap making?
Superfat (or "superfatted") is extra oil beyond what the lye can saponify, typically 5-8%. This ensures all lye is used, leaving conditioning excess oils. More superfat = more conditioning but softer soap. Recipes below 5% may be harsh. Above 8% yields soft, speedy-dissolving soap. Olive oil soap (Castiglione) traditionally uses 0% superfat - the long cure handles it. Start at 5% superfat.
Can I use potassium hydroxide (KOH) instead of NaOH?
KOH makes soft/tliquid soap. NaOH makes hard bar soap. They are not interchangeable. KOH creates paste that dissolves for liquid soap. Both work - choose based on desired product. For bar soap: use NaOH. For liquid soap: use KOH. Use online calculators that support both. Some soap calculators do both automatically. Double-check which your recipe requires.
How long does cold process soap take to cure?
Cold process soap needs 4-6 weeks cure time minimum. The water evaporates and lye finishes neutralizing. Longer cure = harder, longer-lasting bar. 8-12 weeks is ideal for most recipes. Hard oils (coconut, palm) need full cure to not be drying. Castile soap (100% olive) needs 6-12 months for best lathering. Cure on a rack in ventilated area, flipping weekly. Hardness tested: bar should feel firm.