What Is a Soap Making Lye (Saponification) Calculator?
Cold-process soap is made by reacting fats and oils with sodium hydroxide (lye) in a chemical process called saponification. Every oil requires a different, well-documented amount of lye to fully convert to soap — too little lye leaves the bar greasy, too much leaves it caustic and harsh on skin. This calculator uses standard published saponification (SAP) values to get the lye amount right for your specific recipe.
Enter up to three oils by weight, choose your superfat percentage, and get the exact amount of NaOH your batch needs.
How to Read Your Results
Sodium Hydroxide (NaOH) Needed
The precise weight of lye to use for this batch, already adjusted for your chosen superfat percentage. This is the number you weigh out on a scale.
Total Oil Weight
The combined weight of every oil entered — useful for cross-checking your recipe and container sizing.
NaOH Before Superfat Discount
The full chemical requirement to saponify 100% of the oils, before the superfat reduction is applied — shown so you can see exactly how much lye the superfat percentage is holding back.
Suggested Water
An estimated water amount using a common 38%-of-oil-weight default ratio. Many recipes use a different lye concentration or water discount — adjust to your preferred method.
Real-World Example
A simple three-oil recipe: 400 g olive oil, 300 g coconut oil, 300 g palm oil, at 5% superfat.
| Oil | Weight | SAP | NaOH (g) |
|---|---|---|---|
| Olive oil | 400 g | 0.134 | 53.6 |
| Coconut oil | 300 g | 0.190 | 57.0 |
| Palm oil | 300 g | 0.141 | 42.3 |
The full requirement is about 152.9 g of NaOH; at 5% superfat, the final amount to weigh out is about 145.3 g.
Tips for Safe Soap Making
- Always add lye to water, never water to lye, and work in a well-ventilated space wearing gloves and eye protection.
- Double-check any recipe against a second established lye calculator before mixing a batch.
- Use a digital scale for both oils and lye — volume measurements aren't accurate enough for safe soap making.
- A 5–8% superfat is a common, skin-friendly range for most cold-process recipes.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team