Soap Making Lye Calculator
Build your oil blend, set your superfat percentage, and get the exact amount of NaOH or KOH to weigh out — plus the water amount and INS hardness value. 16 oils from coconut to shea. Nothing uploaded.
Learn more: lye calculation for cold-process soap
Why exact lye measurement is critical
Too little lye and the soap never finishes saponifying - it stays soft and greasy or goes rancid. Too much lye and the finished soap is caustic and dangerous to skin. The lye amount must be calculated from the specific SAP (saponification) value of each oil in your blend - different oils require different amounts of lye per gram. The calculator handles multi-oil blends: add up to 10 different oils by weight or percentage (it auto-normalises to 100%), set your superfat and KOH purity, and get the exact NaOH or KOH weight and water amount to use.
Lye calculation in three steps
First, build your oil blend by adding oils from the 16-oil database with weight or percentage - percentages auto-normalise to 100%. Second, set superfat (0-10%) and soap type: bar (NaOH) or liquid (KOH), with KOH purity at 85%, 90%, or 100%. Third, read the exact grams for lye and water, INS hardness value, iodine value, and estimated cure time note.
FAQ
What is superfat in soap making?
Superfat (or lye discount) is the percentage of oils left unsaponified in finished soap. A 5% superfat means 5% of your oils remain as free fatty acids, making the soap more moisturising and providing a safety margin so there is no unreacted lye left in the finished bar. 5-8% is typical for cold process soap.
What is the difference between NaOH and KOH for soap?
Sodium hydroxide (NaOH) produces bar soap because it creates harder soap molecules. Potassium hydroxide (KOH) produces liquid soap because it creates softer, water-soluble soap molecules. The calculator supports both with the correct SAP values.
What is INS value in soap making?
INS (Iodine-Naught-SAP) is a calculated score that predicts soap hardness, lather, and skin feel from the oil blend. A well-balanced soap typically has an INS value between 136-165. The calculator shows your oil blend's INS value automatically.