How Lye Is Calculated for Cold Process Soap
Soap is the product of a chemical reaction called saponification: fats and oils react with lye (sodium hydroxide, NaOH) to form soap and glycerin. Every oil reacts differently, so each has its own saponification value (SAP) that tells you how many grams of lye it takes to fully turn one gram of that oil into soap. Olive oil has a SAP of about 0.134, coconut oil 0.183, palm oil 0.141, shea and cocoa butter around 0.128 to 0.137, and castor oil about 0.129. To size a batch, you multiply each oil weight by its SAP value and add the results together to get the total lye needed at 0% superfat.
Superfat and Water: The Two Dials That Matter
You almost never want to use 100% of that calculated lye. A superfat (also called a lye discount) holds back a small percentage of the lye so a little oil stays unsaponified, making the bar gentler and insuring against measuring errors so no harsh excess lye remains. Five percent is the standard sweet spot. Water does not change the chemistry; it only dissolves the lye and controls how fast the soap thickens and how long it cures. This tool sizes water as a percentage of total oil weight, defaulting to a safe 33%.
lye = sum(oil weight x SAP value) x (1 - superfat%) ; water = total oils x water%
Why You Weigh Everything
Soap making is chemistry, not cooking. A 500-gram oil batch at a 5% superfat needs roughly 70 to 75 grams of lye depending on the oil blend, and being off by even a few grams shifts the bar from gentle to harsh. Always weigh oils, lye, and water on a digital scale in grams, and never measure lye by volume.
Frequently Asked Questions
What is superfat and what percentage should I use?
Superfat is the percentage of oils left unsaponified by holding back that much lye, which makes the finished bar milder and more moisturizing. Five percent is the recommended default for most recipes; go lower (3%) for a harder, more cleansing bar, or higher (7 to 10%) for an extra-gentle bar that may be slightly softer and shorter-lived.
How much water do I use for cold process soap?
A common starting point is water equal to about 33% of your total oil weight, which gives a safe, workable lye solution. Using more water (around 38%) extends working time but lengthens cure, while a water discount (around 28%) speeds up trace and hardens the bar faster but leaves less room for error, so beginners should stick near 33%.
Why can't I just measure lye by the scoop?
Lye must be measured by weight on a digital scale, never by volume, because the amount is calculated precisely from your oil weights and saponification values. A scoop varies with packing and humidity, and too little lye leaves greasy, oily soap while too much leaves caustic, skin-burning excess in the bar.
Is this calculator for NaOH (bar soap) or KOH (liquid soap)?
It uses sodium hydroxide (NaOH) saponification values, which are for solid bar soap made by the cold process method. Liquid and paste soaps use potassium hydroxide (KOH), which has different SAP values and a different purity factor, so do not use these numbers for a KOH recipe.
Practical Guide for Cold Process Soap Lye Calculator
Run your recipe twice before you ever touch lye. Saponification values come from oil databases and assume pure, fresh oils, so it pays to double-check your numbers and confirm every oil is in the recipe at the weight you intend. A single transposed digit or a forgotten butter can swing the lye amount enough to ruin a batch, so treat the calculation as the most important step, not the mixing.
Respect the chemistry and the safety gear. Sodium hydroxide is caustic and generates heat and fumes the instant it hits water, so always add lye to water (never water to lye), work in a ventilated space, and wear gloves and eye protection. Keep vinegar nearby only for cleanup of spills on surfaces, mix at moderate temperatures around 95 to 110 F, and blend just to a light trace so you have time to pour and add fragrance.
Cure is where good soap becomes great soap. Even a perfectly calculated bar is soft and high in residual water the day it is unmolded. A full four to six week cure lets excess water evaporate and the crystalline soap structure firm up, producing a harder, milder, longer-lasting bar with better lather. Space bars on a rack with airflow, flip them weekly, and label each batch with its superfat and date so you can dial in future recipes.
Quick Checklist
- Weigh every oil, the lye, and the water in grams on a digital scale.
- Confirm your superfat is between 3% and 8% for a balanced bar.
- Always add lye to water, in a ventilated space, wearing gloves and goggles.
- Cure finished bars 4 to 6 weeks on an airflow rack before use.