Liquid Soap Lye Calculator (KOH)
Enter the weight of each oil, then set the superfat, your KOH's purity and the lye concentration. The calculator gives the KOH and water for the paste stage of liquid soap.
The formula
Worked example. 1,000 g of olive oil (KOH SAP 0.189) at 3% superfat and 90% purity needs 204.1 g of KOH; at 25% concentration that is 612.4 g of water.
How to use it
- Enter the weight of each oil. Liquid soap recipes often lean on olive, coconut, castor and sunflower oil.
- Keep the superfat low, 0–3%. Excess oil tends to separate out of liquid soap as a cloudy layer.
- Enter your KOH's purity from the supplier's label; about 90% is typical. The calculator increases the KOH to make up for the impurity.
- Use a 25% lye concentration (3 parts water to 1 part KOH) for a paste that is easy to stir, then cook and dilute the paste separately.
Questions
Why does purity matter for KOH but not NaOH?
Potassium hydroxide absorbs moisture and is commonly sold around 90% pure, so 100 g of it contains only about 90 g of KOH. Sodium hydroxide for soapmaking is usually close enough to 100% that calculators ignore the difference.
How do I dilute the paste?
After the paste has cooked and passes a clarity test, dissolve it in hot distilled water, often one to two times the paste weight to start, and add more until the soap is as thin as you like.
Can I use this for bar soap?
No. Bar soap uses sodium hydroxide (NaOH); use the soap lye calculator for that. Some cream soaps use both, which this calculator does not cover.