Soap Lye (Saponification) Calculator
Calculate the exact amount of NaOH or KOH lye needed to saponify various oils for cold-process soap making, with superfat adjustment.
Results
What is it?
Saponification is the chemical reaction between lye (sodium hydroxide for bar soap, potassium hydroxide for liquid soap) and oils to produce soap and glycerin. The SAP value (saponification value) for each oil specifies exactly how many grams of NaOH are needed to fully react with 1 gram of that oil. Superfat leaves a percentage of unreacted oil in the finished soap for a conditioning, skin-friendly bar.
How to use
Select your oil, lye type, and enter the oil weight and desired superfat percentage. The calculator outputs the exact lye weight and distilled water needed. Scale your recipe by changing the oil weight — the lye amount scales proportionally.
Example scenario
500g olive oil, NaOH, 5% superfat: lye needed = 500 x 0.134 = 67g NaOH full saponification; with 5% superfat: 67 x 0.95 = 63.7g NaOH. Water = 63.7 x 2.5 = 159.2g distilled water.
Pro tip
SAFETY FIRST: Always add lye TO water — never add water to lye (this causes violent boiling and splashing). Lye solution heats to 90°C immediately — work in a well-ventilated area, wear gloves, goggles, and long sleeves. Use a digital scale for lye — never measure by volume. Lye is caustic and will cause chemical burns. Always use distilled water (tap water minerals can affect saponification). Fresh soap needs 4-6 weeks curing time before use.