How Do You Calculate Lye for Handmade Soap?
Cold process soap is made through saponification, the chemical reaction between oils and lye (NaOH). Since each oil has a different fatty acid profile, the amount of lye needed per gram — its saponification (SAP) value — varies by oil. Coconut oil has a high SAP value of about 0.183, needing more lye than the same weight of olive oil at about 0.135. When blending multiple oils, multiply each oil's weight by its own SAP value and add them together.
Superfat is the percentage of oil left over that doesn't react with the lye. It's usually set around 5%, and this leftover oil adds moisturizing properties to the soap while making sure no excess unreacted lye remains to irritate skin. A higher superfat percentage means less lye is needed, but going too high (above 15%) can make the soap soft and prone to spoiling.
Lye is dissolved in water before use — typically about twice the weight of the lye, creating a roughly 33% sodium hydroxide solution. Too little water and the lye won't fully dissolve; too much and trace (the point where the mixture thickens, signaling the saponification reaction is underway) takes longer to reach, extending your working time. Lye is highly hazardous to skin and eyes, so always wear gloves and eye protection and work in a well-ventilated area.
Frequently Asked Questions
Superfat is the leftover percentage of oil that doesn't react with the lye. It's usually set around 5% to leave moisturizing oils in the soap and prevent skin irritation from excess unreacted lye.
Each oil has a different fatty acid profile, giving it a different saponification (SAP) value. Coconut oil has a high SAP value and needs more lye per gram than olive oil, which needs relatively less.
Water is typically used at about twice the weight of the lye, creating a roughly 33% lye solution. Too little water makes the lye hard to dissolve; too much delays trace.